Doug Tallamy – Conservation Sense and Nonsense

Going Toe to Toe with Doug Tallamy

In June 2023, Washington P ost p ublished an opinion piece advocating for the use of herbicides to kill non-native plants, in which Doug Tallamy was quoted as saying that spraying herbicide on non-native plants is “chemotherapy,” equating non-native plants with cancer and pesticides with medical therapy. Tallamy. and more broadly his viewpoint, received some blowback from Conservation Sense and Nonsense and others.

Thomas Christopher and Doug Tallam y collaborate on their shared mission of promoting the use of native plants and the closely related goal of eradicating non-native plants they consider a threat to native plants and insects. In October 2023, Tom Christopher (TC) gave Doug Tallamy (DT) an opportunity to respond to criticism of native plant dogma on his Growing Greener podcast that is available HERE. Christopher also invited listeners to send him feedback on the podcast. Professor Art Shapiro, whose work was central to the interview, has responded separately and his response is available as a footnote. Conservation Sense and Nonsense (CSN) sent Christopher an email, which I hope he shared with Tallamy. The following is an excerpt from that email.

Hi Tom, Thanks for the air time for opposition to eradicating non-native plants in your interview with Doug Tallamy and for this opportunity to respond. I’m flattered that criticism of native plant dogma has attracted some attention on the East Coast. I’ve transcribed most of your interview with Doug Tallamy as best I can and provided some feedback to Tallamy’s viewpoint. I sent Art Shapiro the podcast and he has responded separately.

TC: Some people say that non-native plants are just as effective as natives in supporting food webs. For example, buddleia that is spreading throughout the East and West is used by butterflies.

CSN: Buddleia davidii is on California’s list of invasive plants, but it is not considered invasive in California. It was put on California’s list because it is considered invasive elsewhere, making the point that invasive plant behavior varies depending on local conditions, such as climate. Sweeping generalizations about invasiveness are rarely accurate. If gardeners are concerned about the potential for invasive behavior, they can plant a cultivar of buddleia that does not reproduce.

DT: We shouldn’t call all insects pollinators. Just because an insect visits a flower for nectar doesn’t mean it’s pollinating that flower. There are more visitors to flowers than there are pollinators. Butterflies visiting buddleia are just there to sip nectar.

Euphydryas chalcedona — Variable checkerspot. Photo by Roger Hall

CSN: Buddleia davidii is native to Central China. Non-native buddleia is used by a butterfly species that is native to California and other states in the Western US.

The first actual observation of checkerspot butterflies breeding spontaneously and successfully on buddleia was in Mariposa County, California in the Sierra Nevada foothills. Checkerspot bred there successfully on buddleia in 2005 and in subsequent years. This colony of checkerspot on buddleia was reported in 2009: “We conclude that buddleia davidii [and other species of buddleia] represents yet another exotic plant adopted as a larval host by a native California butterfly and that other members of the genus may also be used as the opportunity arises.” (1)

In 2017, a gardener in Mendocino County, California also reported the use of buddleia as the host plant of checkerspot: “By now I am questioning how it was that butterfly larvae were using my butterfly bush as a host plant, completely against everything I’d ever heard. How was this possible? I emailed Art Shapiro, a very well-known butterfly expert and author, sending him a pic. He wrote back to confirm they were butterfly larvae, but added, ‘These are not mourning cloak butterflies. They are checkerspots. And the only time I’m aware this has happened [like, ever, except one in a lab in 1940…] is in Mariposa County.’” (2)

Buddleia is available as the host plant of checkerspot butterflies with a native range from Alaska south along the Pacific Coast through California and Arizona to Baja California and Mexico; east to Montana, the Dakotas, Wyoming, Colorado, New Mexico. This is a clear case of a widespread native butterfly choosing a non-native plant as its host.

Arthur M. Shapiro and Katie Hertfelder, “Use of Buddleia as Host Plant by Euphydryas chalcedona in the Sierra Nevada foothills, California,” News of the Lepidopterists’ Society, Spring 2009
http://plantwhateverbringsyoujoy.com/never-pull-up-and-discard-what-you-cannot-identify/

DT: Most bees that people see in their gardens are honeybees that are there to get pollen and sometimes nectar. These are generalist bees but specialist bees that require pollen from particular plants (always native plants) can’t be supported by those at all.

CSN: Specialization of insects is exaggerated by Tallamy. For example, he would probably call a squash bee a specialist. As its name implies, its host plant is squash plants in the squash family, with 98 genera and 975 species. The squash bee is considered an excellent pollinator of zucchini and butternut squash, both native to Central and South America. However, they do not usually visit melon plants, according to Wikipedia. Again, we are reminded to avoid broad generalizations when describing the complex and diverse natural world.

Likewise, the native alkali bee is a particularly effective pollinator of alfalfa, which is native to the Mediterranean region. Alkali bees also pollinate members of the large legume family with over 16,000 species that are native all over the world. If you are interested in such associations, you can find an exhaustive list of native butterflies and their many non-native host plants in Art Shapiro’s butterfly guide for Central California and the Bay Area. It is not true that bees Tallamy considers “specialists” require pollen from only native plants.

DT: Sometimes butterflies adopt a new host plant as a caterpillar host. For example, black swallowtail butterflies caterpillars eat carrots or parsley or dill. What’s going on? There are two different kind of hosts: 1) The caterpillar has not adopted a new host at all because it was already adapted to that particular host. 2) Actual host switching from one plant to another is very rare. It happens on a time-scale of thousands of years. It requires a mutation or an adaptation to chemical defenses of new host plants.

CSN: Tallamy tries to make a distinction to avoid acknowledging that insects make use of introduced plants because they are chemically similar to the native plants they have used in the past, which in some cases are no longer available. The butterfly has, in fact, adopted a new host, a plant that wasn’t there before and is now hosting the caterpillar. There are many cases of rapid evolution that enable such transitions, but both cases are clearly transitions from native to non-native plants. If the original native host is still available, it isn’t necessarily abandoned in favor of a non-native. Such transitions are useful because they increase the population of available insect hosts and are essential if the original native host is no longer available.

TC: Pushback from California cites research of Professor Art Shapiro reporting that spontaneous spread of non-native plants has benefited native butterflies. He reports that 82 of 236 California native butterfly species (34%) are laying their eggs on introduced plant taxa, so caterpillars feed on them and many more butterflies use introduced plants as nectary sources.

DT: Great! These are host range expansions. Agriculture in California has eliminated the host plants of a lot of butterflies and it’s a good thing we had close relatives of natives so butterflies could expand their host range and use them. But if 34% of native butterflies are using introduced plants that means 66% are not. If all plants were introduced, we would lose 66% of butterflies in California. This is not the direction I want to go. I would choose 60% rather than 34%.

CSN: Christopher and Tallamy seem to have read one sentence in the abstract of Shapiro’s study without reading subsequent sentences: “Interactions with introduced plant taxa are not distributed evenly among butterfly species. Alpine and desert butterflies interact with relatively few introduced plants because few exotic plant species have reached and successfully colonized these habitats. Other California butterfly species are specialists on particular plant families or genera with no exotic representatives in California and have thus far failed to recognize any introduced plants as potential foodplants. Some California butterflies have expanded their geographic ranges and/or extended their flight seasons by feeding on exotic plants.” In other words, where there are more introduced plants and some are closely related to native plant hosts, more native butterflies use introduced plants.

TC: What do you say to the claims that introduced plants stay greener longer than native plants adapted to wet or dry seasons so that introduced plants give rise to extra generations of caterpillars?

DT: This is only true if caterpillars can use those plants and in host range expansions they can. Shapiro is also right about extending availability of nectar. For example, monarchs that migrate need forage along the way. The minus is that we’ve been so hard on native flora. These insects were doing just fine before we brought in non-native plants. It’s a Band-Aid we’re putting on an environment that has been ravaged by taking out native species that were here before. Let’s put native species back too.

CSN: The claim that non-native plants are driving native plants to extirpation or extinction goes to the heart of the controversy. Native plant advocates believe that accusation, although there is little evidence to support it. The greatest threat to native plants and insects is habitat loss, particularly converting wildlands to agricultural fields. The second greatest threat is the pesticides that are used by agriculture. Remember that Tallamy is an enthusiastic promoter of herbicides to eradicate non-native plants. He calls it “chemotherapy” in a recent opinion column in the Washington Post. Pesticides kill both plants and the animals that feed on them, they are anathema to biodiversity and the food web that Tallamy believes he is supporting.

Marcel Rejmanek (UC Davis) is the author of the most recent report on plant extinctions in California, published in 2017. At that time there were 13 plant species and 17 sub-species native to California known to be globally extinct and another 30 species and sub-species extirpated in California but still found in other states. Over half the globally extinct taxa were reported as extinct over 100 years ago. Although grassland in California had been converted to Mediterranean annual grasses by grazing domesticated animals decades before then, most of the plants now designated as “invasive” in California were not widespread over 100 years ago.

Most of the globally extinct plant species had very small ranges and small populations. The smaller the population, the greater the chances of extinction. Most of the globally extinct plants were originally present in lowlands where most of the human population and habitat destruction are concentrated. Although there are many rare plants at higher altitudes, few are extinct. Plants limited to special habitats, like wetlands, seem to be more vulnerable to extinction. The primary drivers of plant extinction in California are agriculture, urbanization and development in general. Non-native plants are the innocent bystanders to disturbance.

“Invasive species” are mentioned only once in the inventory of extinct plants published by California Native Plant Society and only in combination with several other factors. However, the identity of this “invasive species” is not clear. Rejmanek suggests that the “invasive species” rating refers to animal “invasions” by predators and grazers. He says, “Indeed, one needs quite a bit of imagination to predict that any native plant species may be driven to extinction by invasive plants per se.” (Marcel Rejmanek, “Vascular plant extinctions in California: A critical assessment,” Diversity and Distributions, Journal of Conservation Biogeography, 2017)

TC: 90% of all insect species are specialists that have evolved in concert with only one or a few plant lineages. How can they cope with the loss of native plants?

DT: Native plants are adapting in evolutionary time. Specialization is a continuum. Few insects are confined to a single plant species, some are confined to one or two genera, and others are confined to one or two families of plants. But if you are looking at the number of plants available to them, only about 7% of plants they are adapted to are available to them. 93% of available plants are not viable hosts for insects. Everything is a specialist on one level of another.

CSN: That sounds like an argument for a diverse garden, with many plant species that offer more food sources for insects. That doesn’t seem a sound argument for eradicating non-native plants.

TC: I understand that some native plants are more useful to insects than others?

DT: These are the keystone species. Many native plants don’t support insects because plants are well-defended against them. Keystone species are making most of the food for the food web. Just 14% of native plants across the country are making 90% of food that drive the food web. 86% of the native plants are not driving the food web. Insect food comes from the big producers, like oaks, black cherries, hickories, and birches.

CSN: That is a mind-boggling admission!! Earlier Tallamy complained that non-native plants are hosting only 34% of butterflies in California. Now he says that only 14% of native plants are useful to insects. He asks home gardeners to plant only native plants as well as limit our plantings to a small subset of native plants.

Tallamy’s ideology is antithetical to the goal of biodiversity, which could be the salvation of ecosystems in a changing climate. Since we can’t predict the climate of the future, biodiversity provides more evolutionary options, which increases the chances that some species will survive. Tallamy asks us to put a few eggs in the huge basket of our ecosystems, reducing their ability to survive the challenges of our changing climate.

For example, in Oakland, California, where I live, there were approximately 10 species of native trees prior to settlement. In 1993, there were 350 tree species in Oakland. (David Nowak, “Historical vegetation change in Oakland and its implications for urban forest management,” Journal of Arboriculture, September 1993) The recently published draft o f Oakland’s Urban Forest Plan reports that there are now over 500 tree species in Oakland. I can’t fathom why Oakland would want to limit the planting of trees to only 10 native species.

I agree with Tallamy that many native plants are not useful to insects. I attend the annual conference of California Invasive Plant Council to give native plant advocates every opportunity to convince me of their viewpoint. At the most recent conference at the end of October, Corey Shake of Point Blue Conservation made a presentation about his project to “Evaluate native bee preference for common native and exotic plants.”

He designed 16 hedgerows around agricultural fields in Yolo County to determine if native bees have a preference for native plants or exotic plants, by controlling for availability of native plants compared to exotic plants. Here is his abstract:

“Farm edge restoration monitoring in Sacramento Valley highlights native bee use of some exotic plant floral resources. Corey Shake. Point Blue Conservation Science. cshake@pointblue.org

“Research of native bee preference for native versus exotic plant floral resources in California’s Sacramento Valley has shown mixed results. No studies have demonstrated a preference for exotic plants by native bees there, but some have highlighted the importance of exotic plant floral resources in plant-pollinator networks and expressed concern that rapid removal of exotic plants without restoring native plant populations could have negative impacts on native bees. We have been collecting native bee flower visitation, plant species, and floral abundance data on 16 farm edge restoration projects in Yolo County, California since 2019, which will allow us to assess bee preferences for some key native and exotic plants relative to their floral abundance. In our preliminary analysis, we see some important trends: (1) relative to their floral abundance in our plots, some native plant species are more frequently visited by native bees than other native plants that are infrequently or rarely visited, and (2) there is significant native bee visitation to some exotic plants relative to their floral abundance. We will further evaluate these data as well as our butterfly diversity and abundance data to provide plant-species specific insights to restoration practitioners and weed management specialists to help them reduce harmful impacts to native pollinators when executing restoration projects and managing weeds.”

In other words, not all species of native plants are useful to native bees and some species of non-native plant species are useful to native bees. Tallamy’s sweeping generalizations about the usefulness of native plants to insects are not supported by empirical or field studies. Although the characteristics of plants vary widely, the variation is unrelated to the national origins of plants.

From Micro to Macro Perspective

I recognize my voice in the questions Tom Christopher asked of Doug Tallamy, as well as Art Shapiro’s. Speaking for myself, not for Art, this interview misses the point of my criticism of native plant ideology. I like native plants as much as I like any plant and I encourage everyone to plant whatever they prefer. I only object to the pointless destruction of harmless non-native plants that thrive because they are best adapted to the conditions where they have naturalized. Non-native plants do particularly well in the wake of disturbance. Where they have replaced native plants, the natives were destroyed by disturbance, not by the hardy non-native plants that can tolerate disturbance. Non-native plants are a symptom of change, not the cause.

I object to destructive eradication projects because they poison the soil with herbicides, making it even less likely that non-native plants will be replaced by fragile native plants. I object to the loss of biodiversity which is a hedge against extinction in a rapidly changing climate. We don’t know which plants will be capable of surviving in the changed climate. We should not be taking cards out of the deck while we gamble with the future of the environment and everything that lives in it.

Unfortunately, native plant advocates take offense when anything positive is said about introduced plants. A positive statement about a non-native is routinely interpreted as a negative statement about native plants. It shouldn’t be. The emphasis on the negative assessment of introduced plants results in harmful land management decisions. The pros and cons of all plants should be considered before we condemn non-natives with a death sentence. Like our justice system for human society, all plants should be presumed innocent until proven guilty.

Thanks again for airing this debate on your podcast. I hope you will forward my email to Doug Tallamy

Webmaster, Conservation Sense and Nonsense

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Dana Milbank: “How I learned to love toxic chemicals”

Dana Milbank is a political commentator for the Washington Post. Like many city dwellers, Milbank moved his family from Washington DC to the Virginia countryside during the Covid pandemic.

His new home inspired him to become a native plant advocate with the usual corresponding hatred of non-native plants. He announced his new hobby of killing non-native plants in April 2023, as described in thi s response to his article by several defenders of the natural world as it exists, rather than as some might wish it to be.

In a more recent articl e, Milbank expressed his frustration at the failure of his early efforts to destroy non-native plants on his property without using herbicides: “When last I wrote about my battle of the brush, I was losing, badly, to the invasive vines and noxious weeds that had turned forest and field at my Virginia home into an impassable jungle. I’d cut them back, but they would return in even greater numbers.”

And he explained how he “learned to stop worrying and love chemicals.” He is now both a native plant advocate and a promoter of herbicides (specifically glyphosate) which is typical of most native plant advocates.

He justifies poisoning both his property and the Shenandoah National Park near his home by turning to advisors who tell him what he wants to hear, people who make their living using herbicides to eradicate non-native plants.

Of course, renowned native plant guru, Doug Tallamy, is one of his advisors. Although Tallamy advised residential gardeners against using herbicides in his book, Nature’s Best Hope, published in 2020, he has now changed his mind about herbicides. In Milbank’s article, Tallamy says that herbicides are an “essential tool:” “‘I think of it as chemotherapy,’ said Doug Tallamy, a University of Delaware entomologist and guru of the native-plant movement. ‘We have ecological tumors out there. If we don’t control them, we have ecological collapse. We have the collapse of the food web.’”

Poisoning the soil

Milbank admits that glyphosate (Roundup) is toxic and he wears protective gear when applying it, including a respirator (which is not required for glyphosate applications by California’s pesticides regulations). He describes his application technique: “My preferred technique is ‘hack and squirt.’ With my hatchet, I cut gouges around the circumference of the invading tree, then spray the poison inside. For smaller invaders, I can chop the whole thing down and apply the chemical as a ‘cut stump’ treatment.

I read most of the over one thousand comments on Milbank’s article to determine the public’s reaction. Although many commenters express reservations about the use of herbicides, the majority of commenters are supportive of the use of herbicides. The manufacturers of pesticides are definitely winning the public relations battle regarding chemical safety. When supporters reply to doubters of herbicide use, they defend Milbank’s application technique as “surgical.”

Cut stump and hack and squirt application methods are less likely to disperse chemicals in the air, but they increase soil contamination. These application methods work by applying herbicide shortly after the woody plant is cut, while the cambium layer (between the bark and the heart wood) is still functional. The cambium layer delivers the herbicide to the roots of the plant to kill the roots. The application may appear to be “surgical” from the standpoint of above-ground contamination, but the damage is being done in the soil, the plants growing in the soil, and the animals that eat those plants.

Source: https://www.acompletetreecare.com/blog/what-are-the-layers-of-a-tree-trunk/

There are many consequences of poisoning the soil:

Because the roots of plants are intertwined as well as connected to one another by fungal networks in the soil, non-target plants are harmed and often killed. It is not possible to poison one plant without poisoning others. HERE is an example of a forest of native trees that was damaged by spraying herbicide under the trees.
Herbicides kill beneficial microbes and fungi in the soil that contribute to plant health. (1) For example, fungal networks that are killed by herbicides transport moisture and nutrients from the soil to the plants. Whatever vegetation remains or is planted in the future is handicapped by the loss of this living support system.
Glyphosate binds minerals in the soil, preventing essential nutritional minerals such as iron and manganese in the soil from being taken up by plants. (2) Glyphosate is so widely used that it is found in the blood and urine of most of the population, including children. Could glyphosate be a factor in widespread iron-deficiency anemia in adolescent girls and young women? (3)
Glyphosate is a well-known anti-microbial agent. These effects raise concerns regarding glyphosate’s influence on human health and behavior through secondary means, such as our gastrointestinal microbiome, given what is now known regarding the gut microbiome and its influence on human health and disease. (4,5)

Source: https://symsoil.com/soil-food-web-soil-cities/

Who are the climate change deniers?

Milbank repeats his accusation that those who believe the threat of non-native plants is exaggerated, are climate change deniers. He turns to the Executive Director of the federal Invasive Species Council for confirmation, who calls the threats of non-native plants “settled science.” Science is, by definition, never settled. Science is a process, not a conclusion. Every scientific hypothesis is constantly tested and usually refined or overturned as new knowledge and methods are available. Many scientists are testing the hypotheses of invasion biology and questioning their validity in a changing climate.

The only issue about invasion biology that is “settled” is that it has created a multi-billion dollar “restoration” industry that relies on and benefits the manufacturers of pesticides, as well as creating vested interests that perpetuate the industry.

Milbank also quotes one of his advisors who claims that native plants are better adapted to the changed climate than non-native plants: “The natives have the best ability to adapt — they’ve been adapting for tens of thousands of years in these areas — so they’ve got the ability to change as the climates and the landscapes have been changing.” This statement seems comical, given that the topic is the extreme difficulty of eradicating non-native plants and the fact that they are out-competing native plants. There is zero evidence that native plants are better adapted to the changed climate than the non-native plants that have replaced them. 500 million years of geologic history on Earth has informed us that when the climate changes—as it has many times–the vegetation changes.

All plants, whether native or non-native, convert carbon dioxide to oxygen and store carbon. Destroying them contributes to greenhouse gases causing climate change by releasing their stored carbon into the atmosphere and reducing the capacity of the landscape to absorb more carbon in the future. To deny that fact, is to be a climate change denier.

Reality trumps unrealistic hopes

Milbank describes the landscape he hopes to achieve with the help of herbicides. It is the landscape that existed in the distant past, in a different climate, before the environment was altered by the activities of humans. I am reminded of one of the presentations at the most recen t conference of the California Native Plant Society, an event where the audience hopes and the speakers douse the audience’s hope with the reality of their unsuccessful efforts. The presenter described a 20-year effort at the Santa Rosa Plateau Ecological Reserve to convert non-native annual grassland to native grassland, using annual (sometimes bi-annual) prescribed burns. Many different methods were used, varying timing, intensity, etc. The abstract for this presentation reports failure of the 20-year effort: “Non-native grass cover significantly decreased after prescribed fire but recovered to pre-fire cover or higher one year after fire. Native grass cover decreased after prescribed fire then recovered to pre-burn levels within five years, but never increased over time. The response of native grass to fire (wild and prescribed) was different across time and within management units, but overall native grass declined.” The audience was audibly unhappy with this presentation. One person asked if the speaker was aware of other places where non-native grass was successfully converted to native grass. The speaker chuckled and emphatically said, “NO. I am not aware of any place where native grasses were successfully reintroduced.”

(1) “Glyphosate kills microorganisms beneficial to plants, animals, and humans,” Beyond Pesticides, October 2021.
(2) “Glyphosate, a chelating agent—relevant for ecological risk assessment?” Environmental Science and Pollution Research International, 2018
(3) “Prevalence of Iron Deficiency and Iron-Deficiency Anemia in US Females Aged 12-21 Years, 2003-2020,” Journal of American Medical Association, 2023
(4) “Is the Use of Glyphosate in Modern Agriculture Resulting in Increased Neuropsychiatric Conditions Through Modulation of the Gut-brain-microbiome Axis?” Frontiers in Nutrition, 2022
(5) “Toxic Effects of Glyphosate on the Nervous System: A Systematic Review” International Journal Molecular Science, 2022

Talking back to nativism

Dana Milbank is a political columnist for the Washington Post. He broke out of his political mold on April 7, 2023 to write an article about gardening published by the Post, which repeats every myth of the nativist ideology.

A team of dismayed critics of invasion biology has responded to excerpts of Milbank’s column:

Marlene A. Condon is a garden writer based in Virginia and the author of The Nature Friendly Garden. She has a degree in physics. Her entire critique of Milbank’s column is available on her website. Her comments address the reader.
Carol Reese is a retired Extension Horticulture Specialist who conducted her 27 year career from the University of Tennessee’s West Tennessee AgResearch and Education Center in Jackson, where a large and diverse display garden gave her the opportunity to observe biodiversity in action on an enormous range of plant species from other parts of the world. She describes herself as a farm raised country girl tomboy who has looked at the natural world in hundreds of settings and landscapes, natural and manmade, and read countless books and articles. She has written for several magazines, newspapers, articles for Garden Rant as well as university publications. Her speaking engagements around the country have allowed her to engage with many other green industry professionals. Dana Milbank’s column prompted her to email him directly with her concerns, directly addressing some of his assertions. I publish some excerpts here from her emails sent directly to Milbank.
Conservation Sense and Nonsense is the webmaster of this website. I have studied invasion biology and the native plant movement it spawned for over 25 years. I’ve watched forests of healthy, non-native trees in California be destroyed and replaced by weedy grassland. I have used what I have learned to advocate for a less destructive approach to restoration, a word I am reluctant to use to describe projects that use herbicides to eradicate harmless plants and trees. My comments are addressed to the reader.

What follows are excerpts from Dana Milbank’s column with responses from Marlene Condon, Carol Reese, and Conservation Sense and Nonsense, just three of many skeptics of invasion biology. To summarize the point of our criticism:

Insects are not dependent on native plants. They are just as likely to use related non-native plants in the same genus or even plant family with similar chemical properties and nutritional value.
While some non-native plants have potential to be harmful, many are beneficial. There are pros and cons to both native and non-native plants and that judgment varies from one animal species to another, including humans. For example, we don’t like mosquitoes, but they are important food for bats and birds.
All plants, whether native or non-native convert carbon dioxide to oxygen and store carbon. Destroying them contributes to greenhouse gases causing climate change.
When the climate changes, vegetation must also change. Many non-native plants are better adapted to current climate and environmental conditions in disturbed ecosystems.

Conservation Sense and Nonsense

“I’m no genius about genuses, but your garden is killing the Earth”
By Dana Milbank
Washington Post, April 7, 2023

Milbank: I did almost everything wrong.

Reese: I’m so sorry you thought this!

Milbank: For 20 years, I found the latest, greatest horticultural marvels at garden centers and planted them in my yard: sunny knock-out roses, encore azaleas, merlot redbud, summer snowflake viburnum, genie magnolia, firepower nandina

In between them flowed my lush, deep-green lawn. I hauled sod directly from the farm and rolled it out in neat rows. I core-aerated, I conditioned, I thatched, I overseeded, I fertilized. I weeded by hand, protecting each prized blade of tall fescue from crabgrass and clover.

In this season, a symphony of color performs in my yard. The fading daffodils, cherry blossoms, saucer magnolias, hyacinths and camellias meet the arriving tulips, lilacs, creeping phlox and azaleas, with the promise of rhododendrons, peonies, hydrangeas, day lilies and roses to debut in the coming weeks.

But this year, the bloom is off the rose. And the hydrangea. And the rhododendron. And all the rest. It turns out I’ve been filling my yard with a mix of ecological junk food and horticultural terrorists.

Condon: When Mr. Milbank posits that he’s “been filling his yard with a mix of ecological junk food and horticultural terrorists,” he’s channeling the kind of words Bringing Nature Home author Doug Tallamy loves to employ: Biased expressions that implant negative images in the reader’s mind so he will become yet another minion of this scientist. Nowadays you can’t read a garden or environmental column without being accosted with the same words or variations thereof, as if everyone has become a mouthpiece for Doug Tallamy, which I’ve never seen done more obviously than in this column by Dana Milbank.

Conservation Sense and Nonsense: Milbank’s lengthy list of “bad” plants in his garden paints with too broad a brush. For example, instead of identifying a particular species of hydrangea and rhododendron, Milbank condemns an entire genus. Both hydrangea and rhododendron genera have several native species within the genus. Most (all?) species of phlox are also native to North America.

Milbank: When it comes to the world’s biodiversity crisis — as many as 1 million plant and animal species face near-term extinction because of habitat loss ― I am part of the problem. I’m sorry to say that if you have a typical urban or suburban landscape, your lawn and garden are also dooming the Earth.

Reese: YIKES! This is pretty extreme, and dare I say inaccurate? No, home gardeners are part of the solution, no matter the plants in their garden. Doom will come from lack of diverse green space. Doom will come from climate warming as a result, as well as from pollution, tillage, factory farming and development.

Milbank: I came to understand the magnitude of my offenses after enlisting in nature boot camp this spring. I’m in “basic training” with the state-sponsored Virginia Master Naturalist program. While others sleep in on rainy weekend mornings, my unit, the Arlington Regional Master Naturalists, has us plebes out in the wetlands distinguishing a yellow-bellied sap sucker from a pileated woodpecker.

I’m no genius with genuses, but I know a quercus from a kalmia, and because of my gardening experience, I began the program with confidence. Instead, I’ve discovered that all the backbreaking work I’ve done in my yard over the years has produced virtually nothing of ecological value — and some things that do actual harm.

A few of the shrubs I planted were invasive and known to escape into the wild. They crowd out native plants and threaten the entire ecosystem. Our local insects, which evolved to eat native plants, starve because they can’t eat the invasive plants or don’t recognize the invaders as food.

Anise swallowtail on non-native fennel. Courtesy urbanwildness.org “Papilio zelicaon, the anise swallowtail, typically has one to two generations in the mountains and foothills of California where it feeds on native apiaceous hosts. However, along the coast, in the San Francisco Bay Area and the urbanized south coastal plains and in the Central Valley, P. zelicaon feeds on introduced sweet fennel, Foeniculum vulgare, and produces four to six or more generations each year… the use of exotics has greatly extended the range of P. zelicaon in lowland California.” SD Graves and A Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation, 2003.

Reese: It sounds so logical, but is sooo inaccurate. Ask any entomologist that has spent their careers “fighting pests” on valued crop or ornamental plants. Remember Pange a [when all continents were fused into one]? More recently, have you thought about the exchange of plants and animals across Berengia when we were still connected to Asia? We can trace those relationships/kinships of our plants to Asian/Eurasian plants now through DNA. They eventually differentiated into species (a continuum of change caused by climate and geologic pressures until we [Man] declare it as a different species, though biologically it is still basically the same nutritional makeup)

Condon also dissects Milbank’s statement:

“They crowd out native plants and threaten the entire ecosystem.” Read virtually any description of where you find so-called invasive plant species and you will find the word “disturbed.” This tells you the soil profile has been negatively impacted by people, animals, or weather, and usually means the topsoil is gone. Only very tough plants—known as colonizers—can grow in disturbed areas because the soil is nutrient-poor and is typically compacted. Consequently, these areas may fill with a mix of native and nonnative plants, or mainly one or the other—but every single plant is a colonizer that is working to rehabilitate the land for the benefit of the native plants that require topsoil in which to grow. “Invasiveness” is nothing more than a derogatory word used by people with contempt for alien-plant colonization. Conclusions: Alien plants can’t “crowd out” native plants because once the soil is disturbed and thus degraded, most of our native plants can’t grow there and thus are not there to be crowded out. As for “threatening the entire ecosystem,” to the contrary, alien colonizers are helping to restore it.
“Our local insects, which evolved to eat native plants, starve because they can’t eat the invasive plants or don’t recognize the invaders as food.” This oft-repeated distorted premise comes straight out of Bringing Nature Home, in which Doug Tallamy deceptively writes about “an excellent demonstration of how restricted a specialist’s [an insect with particular food preference] diet is.” Tallamy tells the story of Eastern Tent caterpillars on a cherry tree denuded of its own leaves but hosting a Japanese Honeysuckle vine. He writes that the caterpillars didn’t recognize the honeysuckle as food (sound familiar?) But, of course, they didn’t because this species of insect can only eat plants in the Rose Family, which does not include honeysuckle. What Doug Tallamy doesn’t tell the reader is that the tent caterpillars could certainly have eaten the so-called invasive Multiflora Rose, which I’ve documented in the photo below. Conclusion: Native insects did not evolve to eat only local (native) plants, but rather can typically feed upon dozens, if not hundreds or thousands, of plants related to each other by family classification, even though they grow in other countries.

Tent caterpillar on multiflora rose. Photo by Marlene Condon.

Milbank: This in turn threatens our birds, amphibians, reptiles, rodents and others all the way up the food chain. Incredibly, nurseries still sell these nasties — without so much as a warning label.

Reese: As I read, I also watch the many birds on my lawn, the fence lizards on my decks, the insects humming among the flowers in my diverse collection of native cultivars and introduced plants.

Hummingbird in eucalyptus flower. Eucalyptus blooms from November to May. It is one of the few sources of nectar and pollen for birds and bees during the winter months when little else is blooming. Courtesy Melanie Hoffman

Eucalyptus leaf litter makes excellent camouflage for this garter snake. Courtesy Urban Wildness

Milbank: Most of my other plants, including my beloved lawn, are ecological junk food.

Reese: Now, now! Many (most) natives do not supply useful forage either. All plants supply some benefit. They provide shelter, create, improve and anchor soil, cleanse air and water, make oxygen and cool the planet. The plant must be judged on benefits versus detriments in each situation. If a nonnative plant is the only thing that will flourish in bombed out rubble, or contaminated soil, if it is providing many benefits, shall we rip it out because caterpillars won’t eat it? If we let it get established, will it ready the site for other species with more benefits to become established? Shall we get out of the way and let nature do what she does, which is heal herself?

Milbank: The trees, shrubs and perennials are mostly “naturalized” plants from Asia or Europe or “cultivars,” human-made varieties of native plants bred to be extra showy or disease resistant but lacking genetic diversity or value to animals. I, like other gardeners I know, planted them after mistaking them for their native cousins. They’re not doing harm, but neither are they doing anything to arrest the spiral toward mass extinction.

Reese: Please know that the most influential native plant botanical garden in the country (Mt. Cuba Center) has trialed the cultivars of native plants for their ecological benefits and found as should be expected, that each cultivar must be judged on its own merits. Some are better than the straight native as in the coneflowers where ‘Fragrant Angel’ scored tops for pollinators and many others were very close to being as good as straight species. These cultivars were even better than the other species of Echinacea tested. BTW, I grow E. purpurea, pallida, paradoxa, tennesseensis and laevigata as well as many cultivars. Remember that cultivars should also be judged on not just nutritional value, but other factors that increase benefits, such as length of bloom period, numbers of blooms, drought resistance, heat tolerance, hardiness, ease of production (cost) and durability. Please ask to speak to Sam Hoadley there as he leads the research on beneficial cultivars and has completed and undertaken several studies of different native species. Great guy and great speaker.

Please be aware that many cultivars originated as naturally occurring deviations in seedling populations, and as we know this actually diversifies the genetic pool, allowing Mother Nature to select the better form. We sometimes agree with her, and other times we may move along that diversifying form by crossing it with others that are demonstrating genetic variance. Logically, this actually furthers the cause of a broader genetic pool that can help in today’s crisis in showing which can cope and flourish.

Milbank: To get a sense of my missteps, I asked Matt Bright, who runs the nonprofit Earth Sangha, a native-plant nursery in Fairfax County (and a lecturer on botany for my nature boot camp) to walk through my yard with me.

He took aim at my day lilies: “I would remove them all. Those have also become badly invasive.”

He spied my creeping jenny on a slope: “Another nasty invasive.”

He condemned to death my rose of Sharon shrubs (natural areas “have really been torn up by these guys”) and my innocuously named summer snowflake viburnum.

Worst was my row of nandinas — “heavenly bamboo” — along the foundation. “You definitely want to remove it,” he advised. Its cyanide-laced berries poison birds.

Condon: This tactic is typical of the followers of Tallamy who want folks to perceive supposedly invasive plants as “bad” even though no evidence exists to support their accusations, especially in this instance. Mr. Milbank and Mr. Bright, who obviously supplied this information, have misspoken here. A study out of the University of North Carolina, Chapel Hill, published in 2022, explains that Cedar Waxwings are the only birds that might be poisoned, and that’s only going to happen if someone grows so many nandinas that these birds consume large numbers of fruits in a single feeding bout. If you grow just one or even a few plants, you are not going to poison waxwings.

Conservation Sense and Nonsense: Here in California, most berry-producing, non-native plants are considered “invasive” based on the assumption that birds eat the berries and spread the plants. Nandina was briefly on the list of invasive plants in California until knowledgeable people informed the California Invasive Plant Council that birds don’t eat the toxic berries. Nandina was removed from the invasive plant inventory long ago.

Bumblebee on Cotoneaster, Albany, CA. Cotoneaster is one of many berry-producing non-native plants on the list of invasive plants in California. Himalayan blackberries are another target for eradication in California. They are frequently sprayed with herbicide in public parks where children and other park visitors eat the blackberries.

I also have personal experience with nandina and cedar waxwings. Flocks of waxwings visited my holly trees in San Francisco every year. They did not touch my three nandina plants.

California buckeye (Aesculus californica) is an example of a native tree that is toxic. Its flowers are toxic to honeybees and its big brown seeds for which it is named were used by Indigenous people to stun fish to make them easier to catch. The bark, leaves, and fruits contain neurotoxic glycoside aesculin. Every negative characteristic attributed to some non-native plant species is equally true of some native plant species. No one mentions buckeye’s toxic characteristics because it’s a beautiful native tree. Photo Sacramento Tree Foundation

Condon: I’ve had a Rose of Sharon (Hibiscus syriacus) growing in my yard since I moved to my home in Virginia almost 40 years ago. In all this time, only one seedling from the plant I brought here has ever “volunteered” to become a second yard denizen. During the past 37 years, pollinators have fed at the original plant and then also at its offspring. What I’ve found by experience in my yard is that few plants can successfully move into a space that’s already filled with other plants. (Proving what physics tells us–that no two physical objects can occupy the same space). I’ve brought home numerous so-called invasive plants, only to have them disappear or simply stay put where I planted them. That’s because hundreds, if not thousands, of plants fill my yard.

Conservation Sense and Nonsense: Virginia is one of only four states in which rose of Sharon is considered invasive. Condon’s experience with rose of Sharon in Virginia suggests that lists of “invasive plants” are either inaccurate or are serving another purpose (perhaps both). The longer the list of “invasive plants” the more work is created for the “restoration” (AKA eradication) industry.

Rose of Sharon is not considered invasive in California. This is a reminder that the behavior of plants varies because of the wide range of climate and environmental conditions. Nearly one third of the plants on California’s list of invasive plants are not considered invasive in California. They are on the list because they are considered invasive in Hawaii, a state with a warmer, wetter climate than California. In naming rose of Sharon as a dangerous invasive, a media resource with a national readership has made a generalization that red-lines more plants than necessary. They become targets for eradication with herbicide and they deprive us of the biodiversity that is particularly important in a changing climate in which biodiversity ensures resiliency.

Milbank: Bright did praise two “good” species I have that contribute to biodiversity: a sycamore and a catalpa as well as a “great” American elm and a “phenomenal” dogwood. (I couldn’t take much pride in them, though, because all four were here long before I arrived.) And Bright assured me I wasn’t a particularly egregious offender; my one-sixth acre lot in town is typical of the urban/suburban landscape.

● ● ●

Lawns, and those useless, ubiquitous cultivars of trees, shrubs and perennials sold by the major garden centers, are squelching the genetic variety nature needs to adapt to climate change.

Reese: It’s actually the opposite. We need more plants in the mix. We need “the tumult of nature” to decide. We aren’t the jury, and we continue to interfere with our well-intended assumptions that we know best.

Lawns are full of wildlife when management is minimal. Mow. That’s all. Mow judiciously when “lawn weeds” are blooming. Watch birds feed on the many insects in the lawn including lepidopteran larvae. Realize that many moths pupate underground. Think of your lawn as haven for them and for the grubs birds relish as millions of acres across our country are being tilled for factory farms. Remember that the best habitat is mixed. Open areas bordered by wooded areas and most species love the borders. Our suburban landscapes are ideal if we just stop killing things.

This is a lawn that serves pollinators. Homestead Stencil Company

Milbank: The resulting loss of native plants in our fragmented urban and suburban landscapes deprives both plants and wildlife of the contiguous habitats they need to breed and, over time, to migrate in response to climate change.

The deck is stacked against nature in this fight.

● ● ●

If possible, you should remove the nastiest of the invasive plants if you have them: burning bush, Japanese barberry, Asian bush honeysuckle, English ivy, callery (Bradford) pear and a few others.

But leave the rest of your plants alone, for now. Tallamy ultimately wants to cut lawn acreage in half, but “there is room for compromise,” he said. Think of your noninvasive plants and cultivars as “decorations.”

Janet Davis, who runs Hill House Farm & Nursery in Castleton, Va., has a similar message for the purists who make you feel bad about your blue hydrangea. “Don’t give me crap about something that’s not native but not invasive,” she said. “I’m never going to tell you you can’t have your grandmother’s peony.”

Thus absolved, I shed my guilt about my yard full of ecological empty calories. I kept my hydrangeas, azaleas and roses but pulled out the truly bad stuff. I dug up the nandinas and replaced them with native winterberry holly, red chokeberry and maple-leaf viburnum. I removed the rose of Sharon and substituted American hazelnut and witch hazel. I uprooted the invasive viburnum and planted a native arrowwood viburnum in its place.

I also took a small step in the painful task of killing my beloved lawn. I used landscape fabric to smother about 400 square feet of turf. In its place, I planted a smattering of canopy trees (two white and two northern red oaks), understory trees (ironwood, eastern redbud), shrubs (wild hydrangea, black haw viburnum) and various perennials and grasses (Virginia wild rye, blue-stemmed goldenrod, American alumroot, woodrush, spreading sedge).

My 38 plants cost $439 at Earth Sangha. But these natives, adapted to our soil and conditions, don’t require fertilizer, soil amendments or, eventually, much watering. Over time, I’ll save money on mulch and mowing.

Reese: This one is so oft repeated and so very wrong. It depends on the plant, and it depends on the site. Plants in the wild require no input to succeed whether native or not because we have not messed up the soil and we have let the natural cycles of plant debris/decay improve the soil as it was meant to, creating a live, moist, interaction of microorganisms that work symbiotically to support the plant, which, btw has also been selected by nature for that site. It has absolutely nothing to do with origins. In fact, why would nonnative plants become “invasive” if they did not adapt as well or better than the native plants? I want to snort with laughter!

Milbank: Right now, my seedlings look pretty sad. Where once there were healthy lawn and vibrant shrubs, there is now mud and scrawny sprigs poking from the ground every few feet. I put up chicken wire to keep the kids (and me) from trampling them. The carcasses of my invasive plants lie in a heap on the gravel.

Condon: This statement supports my contention that ridding your yard (and, in the case of government, natural areas and parks) of “invasive” plants destroys habitat, leaving our wildlife high and dry. Follow the advice of Doug Tallamy, via Dana Milbank (and many others) and you make the environment far less hospitable to our wildlife by removing plants that supplied habitat NOW when our critters need it to survive.

Conservation Sense and Nonsense: This description of Milbank’s ravaged garden is consistent with my 25 years of observing native plant “restorations” on public land. They all begin with destruction, usually accomplished with herbicides. The first stage of these projects is often described as “scorched earth.” Years later, there is rarely habitat comparable to what was destroyed. Colored flags usually outnumber plants.

This is what a native plant garden on Sunset Blvd in San Francisco looked like after two years of effort: more colored flags than plants. The sign claims it is “pollinator habitat.” Since when do pollinators eat flags?

Milbank: But in a couple of seasons, if all goes well, my yard will be full of pollinators, birds and other visitors in need of an urban oasis. Years from now, those tender oak seedlings, now 6-inch twigs, will stretch as high as 100 feet, feeding and sheltering generations of wild animals struggling to survive climate change and habitat loss.

Conservation Sense and Nonsense: Destroying harmless vegetation contributes to climate change by releasing carbon stored in the living vegetation and reducing the capacity to sequester more carbon. Above-ground carbon storage is proportional to the biomass of the living vegetation. Destroying large, mature plants and trees releases more greenhouse gases in the atmosphere than the young plants and trees can sequester. Meanwhile, the climate continues to change and the native plants that Milbank prefers are less and less likely to be adapted to conditions. Native plant ideology is a form of climate-change denial.

A small forest of non-native trees was destroyed in a San Francisco park to create a native plant garden. Nine months later, this is what the project looked like: a tree graveyard.

Milbank: I won’t be alive to see it. Yet even now, my infant oaks give me something the most stunning cherry blossom never could: a sense of hope.

Conservation Sense and Nonsense: I feel bad for Dana Milbank. He has been successfully guilt-tripped into believing he has damaged the environment. He hasn’t, but destroying his harmless garden WILL damage the environment.

We hope he will find his way back to a less gloomy outlook on nature, which will outlast us all in the end. Altered perhaps, but always knowing best what it takes to survive. The way back from the cliff he is standing on is through a study of evolutionary change through deep time to appreciate the dynamic resilience of nature, which may or may not include humans in the distant future. Our message is “Embrace the change because change will enable survival.”

Suggested reading for those standing on the steep cliff created by nativism in the natural world:

Nativism turns a blind eye to climate change

“Reflexive demonization of alien species ignores the beautiful but complex truth that nature fights to find a way—and for a planet navigating the pressures of climate change and overpopulation, that just might be our saving grace.” – Marianne Willburn, Garden Rant

Margaret Renkl writes an opinion column for the New York Times that I usually enjoy because she frequently writes about nature, often based on observations of wildlife in her own garden. She lives in her childhood home in Nashville, Tennessee. Much of her garden was planted with non-native plants and trees decades ago by her deceased mother. Yet, in a recent column, Ms. Renkl blames non-native plants for a variety of crimes against nature.

She suggests that non-native trees are blooming earlier than native trees, which she says has “skewed our experience of spring.” She is apparently unaware that spring does indeed arrive earlier than it has in the past because of climate change. Warmer weather arrives earlier, triggering the blooms of spring, not vice versa. Both native and non-native plants are blooming earlier than they did in the past.
She suggests that gardens planted with non-native plants are “blooming wastelands where the flowers feed nobody at all,” yet her columns are usually filled with the wildlife that lives in her own garden, with introduced plant species.
Although she does not use pesticides in her own garden, she believes that her neighbors’ non-native gardens require them to use pesticides that kill wildlife. She says, “The typical suburban yard is actually worse than a wasteland. It’s a death trap.” She does not seem to know that most herbicide is used to kill non-native plants, not native plants nor does she seem to realize the contradiction in her indictment of gardening with non-native plants. If there are more insects living in native gardens than non-native gardens, why would more pesticide be needed in non-native gardens? If people could learn to love the clover, dandelions, and English daisies in their lawns as much as I do, they would use less “weed killers” on their lawns.

Ms. Renkl’s misperceptions about non-native plants seem to be based on a mistaken belief in their origins. She says, “Ambulatory and omnivorous, human beings are a migratory species. That’s not true for the vast majority of plants.” In fact, plants are just as mobile as animals, including humans. Plants are carried by birds, animals, wind, ocean currents, etc. They come and go as the climate changes, as it has many times in the past 500 million years that plants have existed on Earth. Plants now considered non-native existed here in the distant past, in a different climate. Here are a few examples of such dispersals; most occurred before humans even existed:

Although Dawn Redwoods are now considered non-native in California, there is fossil evidence that they lived here 40 million years ago.
Pollen remains of rhododendron now being eradicated as an “invasive” alien in the United Kingdom, indicate that they lived there over 30,000 years ago.
The oldest known fossils of eucalyptus are 52 million years old. They were found in Argentina, where eucalyptus no longer grows naturally. Eucalypts were planted in Argentina by Europeans as agricultural crop protection and more recently for pulp, as they were in many countries all over the world.
This is a map from a study, showing pre-historic oceanic dispersals of plants from one continent to another: (1)

The ability to migrate is essential to the survival of plant and animal species. As the climate changes, this survival strategy is quickly becoming even more important. When we demand that plants be restricted to their historical “native” ranges, we doom them to extinction because when the climate changes, the vegetation must change.

Where did Ms. Renkl learn these myths?

Ms. Renkl’s cites Doug Tallamy’s Nature’s Best Hope as one of the sources of her mistaken beliefs. Tallamy considers the existence of non-native plants the root of all evil in nature. He calls them “ecological tumors.” He blames non-native plants for declining populations of both native plants and insects, and by extension to declining populations of birds that eat insects.

In Nature’s Best Hope, Tallamy says, “…we must not use climate change as an excuse to do nothing. Most species of plants and animals are far more resilient to climate variability than we give them credit for. Besides, increasing the number and biomass of the plantings in our yards and public spaces is one of our most accessible and convenient tools to fight climate change.” The problem with Tallamy’s dogma is that it inspires the public and land managers to eradicate established landscapes that are not native based on Tallamy’s claims that non-natives are “crowding out” native species and depriving wildlife of food. All native plant “restorations” begin by eradicating non-native plants, usually with herbicides that retard new growth. In other words, the native plant ideology is causing the loss of vegetation and therefore the loss of stored carbon and the reduced capacity for carbon sequestration in the future. The native plant ideology is not increasing biodiversity, nor is it “fighting climate change.” It is more destructive than constructive.

I’m not looking for “an excuse to do nothing.” On the contrary, I believe every effort must be made to stop or at least slow down the inexorable advance of climate change. The most basic effort we can make is to stop destroying functional vegetation, especially trees. Then, there is a lengthy list of what we should be doing to reduce greenhouse gas emissions, which is another, equally important topic.

Native plant advocates consider climate change irrelevant because they believe the existence of non-native plants is the sole culprit of all problems in the environment. They see every environmental issue through the narrow lens of their dogma. This comment on an article about the value of non-native plants by Marlene Condon published in [Chesapeake] Bay Journal is an example of such a misinterpretation of an environmental issue:

“English ivy is an evergreen, non-native, invasive groundcover that has demolished undisturbed natural areas…In salmon country that’s the difference between clean, cold streams and warmer streams filled with sediment.”

Eradicating ivy on stream banks is likely to produce more sediment because it will take some time for replacement vegetation to cover the ground, especially if herbicides are used to eradicate the ivy. Water is warmer in streams because of climate change and because there is less water due to water diversion and droughts. There are many other reasons for declining populations of salmon, particularly dams that prevent salmon from reaching their spawning grounds upstream.

Treat the cause, not the symptom

The native plant ideology ignores the underlying causes of changing ecosystems. Most changes are caused by the activities of humans, such as agriculture, development, water diversion, and pesticides. Climate change is the underlying cause of some changes in nature and it will steadily become a more important factor. Eradicating non-native plants will not reverse any of those changes nor will it prevent changes in the climate.

Alan de Queiroz, “The resurrection of oceanic dispersal in historical biogeography,” Trends in Ecology and Evolution, 20 No. 2, February 2005

Photo credit for featured photo: Garden Rant, Marianne Willburn

Doug Tallamy’s Blame Game

The fact that insect populations are declining in many places around the world is well known, but the reasons for the decline are not well known. Where there is uncertainty, there is speculation and where there is speculation, there is debate.

Doug Tallamy recently stepped into that debate by publishing a review article about insects and their use of plants. The article is a mind-numbing list of studies that find both positive and negative relationships between insects and non-native plants.

Tallamy contends those studies add up to support for his belief that non-native plants are bad for insects and native plants are good for insects. He suggests that declining populations of native plants should be considered one of the reasons for declining populations of insects, but then he goes one step further. Tallamy suggests that non-native plants are responsible for declining populations of native plants. It follows that Tallamy blames non-native plants for the disappearance of insects.

My interpretation of the studies in Tallamy’s review is different. The studies tell me that there is too much variation in insect-plant relationships to generalize about the relative value of native vs. non-native plants to insects. A more accurate conclusion would be that sometimes insects make a successful transition from a native to a non-native plant—especially in the absence of a native in the same lineage—and sometimes they don’t…or at least they haven’t yet.

Anise swallowtail butterfly is one of many insects that have made a successful transition from a disappearing native plant to an introduced non-native plant in the same lineage. Prior to that transition, swallowtails were able to lay eggs only once a year, when the native was available. The introduced non-native is available year around, which enables the swallowtail to lay its eggs year around. Courtesy urbanwildlife.org

Since evolution is a process and not a historical event, these insect/plant relationships will continue to change. There are many studies that document such transitions and Tallamy cites some of them in his review. Tallamy assumes insects will be forever handicapped, if not killed, by whatever deficiencies there are in the non-native substitute. I assume the insect is more likely to adapt and eventually evolve to cope with those deficiencies. Both our assumptions are just guesses. Tallamy considers nature immutable, while I consider it dynamic. Where Tallamy sees doom and gloom, I see opportunity.

Professor Art Shapiro’s (Distinguished Professor of Evolution and Ecology, UC Davis) assessment of Tallamy’s review article is less equivocal than mine. Keep in mind when reading his assessment that he is far more knowledgeable than I am:

“There is little evidence known to me of alien plants (‘invasives’) competitively displacing natives in ‘communities’ except in highly disturbed environments, except in the case of ‘ecological engineer’ species like Japanese honeysuckle, Himalayan Blackberry, climbing fern in Florida, Purple Loosestrife, etc. — things that drastically alter the ground rules for structuring the vegetation by smothering or prompting fire.
“The use of natives and non-natives by insects has a long and venerable history, going back to T.R.E. Southwood and his comparisons of insect faunas on British trees to Godwin’s history of the British flora, Azevedo’s student study at SF State, etc. — demonstrating overall that enemies accumulate in time on naturalized aliens, but it may be a very slow process if there is no phylogenetic or chemical bridge to their colonization. Experiments using haphazardly-selected species to examine acceptability are basically silly, and very easy to ‘stack’ if one knows one’s phytochemistry.
“As I have repeatedly pointed out, ‘weed’ eradication would lead rapidly to the extirpation of nearly all of the non-tree-feeding urban and suburban butterfly fauna in lowland California (and many other places).”

Why are insect populations declining?

A 2017 study revealed a shocking 76 percent decline in the biomass of flying insects over 27 years in protected areas in Germany. The German study does not offer specific explanations for the significant decline in insects, but it speculates about probable cause: “Agricultural intensification (e.g. pesticide usage, year-round tillage, increased use of fertilizers and frequency of agronomic measures) that we could not incorporate in our analyses, may form a plausible cause. The reserves in which the traps were placed are of limited size in this typical fragmented West-European landscape, and almost all locations (94%) are enclosed by agricultural fields. Part of the explanation could therefore be that the protected areas (serving as insect sources) are affected and drained by the agricultural fields in the broader surroundings (serving as sinks or even as ecological traps). Increased agricultural intensification may have aggravated this reduction in insect abundance in the protected areas over the last few decades.” Presumably “protected areas” in Germany are not landscaped with non-native plants, rendering the use of this study to corroborate Tallamy’s hypothesis irrelevant.

A comprehensive review of 73 reports of declining insect populations around the globe was published in 2019. These studies report the reasons for declining populations: “The main drivers of species declines appear to be in order of importance: i) habitat loss and conversion to intensive agriculture and urbanization; ii) pollution, mainly that by synthetic pesticides and fertilisers; iii) biological factors, including pathogens and introduced species; and iv) climate change. The latter factor is particularly important in tropical regions, but only affects a minority of species in colder climes and mountain settings of temperate zones.” The “introduced species” are usually insects rather than plants.

In a Yale e360 article about Tallamy’s review, one commenter offers his opinion that the over-population of deer and their preference for eating native vegetation is likely a greater threat to native plants than the existence of non-native plants that provide an alternative source of food for deer, thereby reducing predation of native plants. Tallamy seems to agree that deer are a problem for native plants, while rejecting deer as a greater threat to native plants than the existence of non-native plants.

The list of reasons for declining insect populations is long and will probably get longer as more research is done. If the existence of non-native plants is on that list, it is unlikely to be higher on a prioritized list than the pesticides that are being used to eradicate non-native plants. The more herbicide that is used to eradicate non-native plants, the more harm is done to insects.

EPA Biological Evaluation of glyphosate is a black eye for native plant “restorations” that use herbicide

The Environmental Protection Agency has finally published its Biological Evaluation (BE) of the impact of glyphosate products (all registered formulations of glyphosate products were studied) on endangered animals (mammals, birds, amphibians, reptiles, fish, invertebrates) and plants. The BE reports that 1,676 endangered species are “likely adversely affected” by glyphosate products. That is 93% of the total of 1,795 endangered species evaluated by the study. Of the total of 792 critical habitats of endangered species, 759 (96%) were “likely adversely affected” by glyphosate products. Most of those critical habitats probably contain predominantly native plants that are clearly not benefiting from herbicides used to kill their competitors.

Both agricultural and non-agricultural uses of glyphosate products were evaluated by the BE. Although only endangered plants and animals were evaluated by the BE, we should assume that all other plants and animals are likewise harmed by glyphosate because the botanical and physiological functions of plants and animals are the same, whether or not they are endangered. Herbicides, specifically glyphosate products, are used by the majority of projects that attempt to eradicate non-native plants. As a result, the crusade against non-native plants is undoubtedly a far more important factor in the decline of insect populations than their mere existence.

Why are native plant populations declining?

There are many reasons why native plant populations are declining, but there is little evidence that non-native plants are the cause of declining populations of native plants. Many of the causes of declining insect populations are also causes of declining populations of native plants. A recent study reports that 65 taxa of native plants in the US and Canada are thought to be extinct. The study did not report a single case in which the extinction was caused by the existence of non-native plants. Sixty-four percent of extinct plants were single-site endemics. The same drivers cited by recent insect studies appear on the list of causes of plant extinctions. Nearly half of the extinctions occurred more than 100 years ago, long before introduced plants were considered an issue.

Butterfly bush is a host plant of Variable checkerspot butterflies. It is also an important source of nectar for butterflies and bees. It is being eradicated on public land because it is not a native plant. butterflybush.com

My New Year’s Wish

Nature is too complex to be reduced to a single cause for changes in the environment. Human knowledge is insufficient to identify all of the causes. That’s why we make many mistakes when trying to fix a perceived problem in nature. Our own priorities influence our evaluation of changes in the environment. We should not automatically assume that a change is a problem or that it must be reversed.

The existence of novel ecosystems is a case in point. They can as easily be seen as positive as negative. If a native plant or animal is no longer adapted to changes in the environment, such as climate change, we should be grateful that a non-native substitute is capable of tolerating the change. Where some see enemies, others see friends.

I wish you all a very happy New Year in 2021. I can’t wish 2020 a fond farewell. I can only say good riddance! I am hopeful for a more peaceful year, one in which we befriend our enemies and work together for a better world for nature and for humanity. I am grateful for your readership.

Re-upping on Reality

A book review **by Marlene A. Condon©of Wild Urban Plants of the Northeast by Peter Del Tredici**

Marlene A. Condon is the author/photographer of The Nature-friendly Garden: Creating a Backyard Haven for Plants, Wildlife, and People (Stackpole Books 2006; information at www.marlenecondon.com). Please visit her blog, In Defense of Nature. You can reach her at marlenecondon@aol.com

To the farmer’s eye, Eastern Redcedar trees “invade” his cow fields where he would prefer only grass to grow. To the ecologist’s eye, the trees signify the need for soil remediation. Photo credit Marlene A. Condon

Prefatory Comments

When I was a student in the mid-1970s at Virginia Tech, small farms surrounded the town of Blacksburg. I spent time at many of the cow farms, where I constantly heard complaints by agriculturalists about the Eastern Redcedar (Juniperous virginiana) perpetually invading their fields.

After getting my degree in physics, I moved north to Charlottesville, a 140-mile highway drive through rural areas. In the ensuing decades, numerous small farms were abandoned as it became more difficult for farmers to make a living from them.

On frequent trips back to Blacksburg, I watched as the forsaken cow fields began to fill with cedar trees. Then, as time went on, Autumn Olive (Eleagnus umbellata) shrubs began to show up as well. It took decades for those fields to become a forest of cedars, olives, or a mix of both; succession was a slow process because the soils had been emptied of their nutrients, and they were compacted by the generations of half-ton animals that had trod upon them.

What the farmers didn’t understand in the 70s, and what most people still don’t understand today, is that Mother Nature tries constantly to replenish degraded areas by sending in colonizers—plants capable of growing in and enriching exhausted soil. Because very few kinds of plants can perform this natural restorative work, their presence in an area is a sure sign of impoverished land.

Virginia Cedar, Virginia Pine (Pinus virginiana), and Broomsedge (Andropogon virgincus) comprise the most-common native species that move into old Virginia cow fields, sometimes accompanied by Black Locust (Robinia pseudoacacia) that is somewhat beyond its original range. But Autumn Olive, from Asia, is a far superior restorer. It not only enriches the soil with nitrogen, an essential nutrient for plant growth, but also provides for wildlife far better than these other plants. I can’t think of another species that feeds such an abundance of pollinators in the spring with its fragrant blooms, and birds and mammals in mid-to-late summer with fruits and again in late winter by way of its buds.

Yet Autumn Olive is one of the most despised plants of people going after so-called invasive-plant species, the presence of which in our environment they don’t understand and have misinterpreted. For example, University of Delaware entomology professor Doug Tallamy starts Bringing Nature Home (published in 2007) with an explanation of how he came to write his book: He and his wife had moved seven years earlier to 10 acres in southeastern Pennsylvania where he found “at least 35 percent of the vegetation on our property (yes, I measured it) consisted of aggressive plant species from other continents that were replacing what native plants we did have.”

Despite his knowledge that the area “had been farmed for centuries before being subdivided and sold to people like [him and his wife]”, this entomologist clearly had no clue about the full story of the landscape he had bought. The presence of Multiflora Rose (Rosa multiflora), Autumn Olive, and other much denigrated alien species that occupied about a third of his property revealed a prior history that Dr. Tallamy and other invasion proponents ignore.

The farmer’s land had obviously stood idle for some years, giving the variety of plants mentioned plenty of time to move in to rehabilitate the soil. These alien species didn’t suddenly appear and grow to full size overnight; we know the plants had been growing for a long time because the author tells us: “In places on [his] land, bittersweet…was supported at the base by vines with six-inch diameters.”

They weren’t “taking over the land” by “push[ing] out any existing natives,” as Dr. Tallamy erroneously asserts. Ecological succession is defined as “a gradual and orderly process of change brought about by the progressive replacement of one community [herbaceous plants to woody shrubs] by another until a stable climax [forest] is established.” (1) If Professor Tallamy truly understood how the natural world works, he would realize he can now grow his preferred climax community of native trees only because the alien “invaders” prepared the site for him to do so.

It’s unfortunate that Doug Tallamy’s false version of nature has been given much credence and publicity. Thanks to conservationists and governments at all levels rallying around his contrived version of reality, huge areas of well functioning habitat have been, and continue to be, destroyed throughout the United States. Adding insult to injury, the “mission” to get rid of supposedly invasive plants has usually been accomplished with the use of herbicides deadly to wildlife.

Book review of Wild Urban Plants of the Northeast by Peter Del Tredici

The natural world would currently be in far better shape if years ago the press had instead taken note of urban ecologist and Harvard botanist Peter Del Tredici’s book, Wild Urban Plants of the Northeast (first published by Cornell Press in 2010, with an expanded version out this year). Unlike Dr. Tallamy, Dr. Del Tredici recognizes the substantial modifications to our environment wrought by development and climate change, such as soil degradation that goes hand in hand with construction, and drought that is more severe and more frequent due to climate warming.

Anyone knowledgeable about plants should recognize that these changes are quite consequential for these organisms. Perhaps Professor Tallamy doesn’t “get it” because he’s focused only on insects and knows very little about animal/plant relationships. For example, he erroneously writes (2) that the Tulip Poplar tree (Liriodendron tulipifera) “is one of the least productive forest species in terms of its ability to support wildlife—insects and vertebrates alike.” He doesn’t know Tulip Poplar blooms feed a myriad of insects along with hummingbirds, and its seeds are taken by the Eastern Gray Squirrel and other rodents, as well as birds like the Carolina Chickadee, the mascot for his cause célѐbre.

It’s a shame that Wild Urban Plants of the Northeast is referred to as a field guide on its cover and in advertisements. People are bound to think this book is mainly for identification of plants growing in urban areas, but it is so much more. Conservationists and gardeners throughout the entire country—and certainly students learning about plants—would do well to read the 29-page “Introduction”.

The true value of this work lies in the author’s explanatory text about why the 268 covered species show up in the cracks and crevices of city sidewalks and deserted parking lots, as well as from the walls of decrepit buildings. It’s an ecology lesson that is far more illustrative than the dry text you might read in a book devoted to the subject for the classroom.

An urban Krakatoa. This sea of urban blacktop is like a volcanic lava flow, and the plants that grow here, including mullein (Verbascum thapsus) , chicory (Cichorium intybus), New England hawkweed (Hiercium saubadum), and white heath aster (Symphyotrichum pilosum), can tolerate extreme heat and drought. Courtesy Peter Del Tredici

For example, in Wild Urban Plants, the reader views a photo of an abandoned building with its fissured parking lot in which a variety of wildflowers grow. The caption likens the “sea of urban blacktop” to “a volcanic lava flow” where plants must be able to tolerate extreme heat and drought. What a superb metaphor! It conveys the environmental conditions to which these plants are subjected while also making very clear to the reader why only certain plants germinate and survive well in such places.

Princess Tree (Paulownia tomentosa) colonizing an abandoned building in New London, Connecticut. From the plants’ perspective, a decaying brick wall is just a limestone cliff. Courtesy Peter Tredici.

In Wild Urban Plants, Princess Tree (Paulownia tomentosa) is seen growing out the side of a neglected painted-brick building in New London, Connecticut. The caption informs us that, “From the plants’ perspective, a decaying brick wall is just a limestone cliff.” How marvelously enlightening!

The urban glacier leaves a trail of compacted glacial till in its wake. Courtesy Peter Del Tredici

Perhaps the most unique metaphor of all can be found in the picture of a backhoe sitting atop a hill of dirt. The author tells us “The urban glacier [referring to the backhoe] leaves a trail of compacted glacial till in its wake.” A conglomerate of unsorted broken rocks, till does not provide amenable growing conditions for very many species of plants.

The author doesn’t go into this subject, but moss is often the first colonizing organism to move in. It secretes organic acids that break down the rocks into soil, paving the way for plants with the ability to fix nitrogen to come in, and over time, as plants die, the soil is enriched via their nitrogen, allowing other kinds of plants to live here. An understanding of this process is sorely lacking among those conservationists who insist that “invasive” plant species serve no useful purpose in the environment. In fact, it’s a darned good thing they are here, given their ability to flourish under present environmental conditions. This is the explanation, after all, for their apparent invasiveness.

Dr. Tredici’s “Introduction” should be required reading for everyone involved in conservation. With a better comprehension of how the natural world works, people should be able to realize that the United States is wasting many millions of taxpayer dollars every year to remove alien plants. And annually putting millions of pounds of herbicides into our environment (according to a 2012 Environmental Protection Agency report (3)) manifests a horrendous crime against nature.

This counterproductive war on nonnative plants must be stopped quickly; far too much damage has already been done. Spread the word about this book to everyone you know.

References:

https://www.thefreedictionary.com/Plant+succession
Bringing Nature Home, Doug Tallamy
https://www.epa.gov/sites/production/files/2017-01/documents/pesticides-industry-sales-usage-2016_0.pdf

Doug Tallamy’s Nature’s Best Hope denies the value of hybridization

In Nature’s Best Hope, Doug Tallamy concedes that there is no evidence of extinctions of native plants being caused by the introduction of non-native plants in the Continental US. However, he accuses non-native plants of something more nefarious: “There is one biological phenomenon associated with some plant invasions that is so pernicious, even continental scales are not protecting natives from invasive species. I speak of…introgressive hybridization, where the invasive species hybridizes with a closely related native, and then through repeated backcrosses and directional gene flow, the gene pool moves closer and closer to that of the invader.”

Jake Sigg calls this phenomenon, genetic pollution. Both Tallamy and Sigg consider such hybridization a loss of the native species and, indeed, it can be the end of localized variants of a species. However, hybridization is often instrumental in the creation of a new species, one that is often superior to its ancestors because it is better adapted to present environmental conditions.

In a recently published study of the evolution of oaks, scientists traced the 56 million year evolutionary history of roughly 435 species of oak across 5 continents where they are found today. Oaks are wind-pollinated, leaving pollen fossil records of their presence where they may no longer live. Using DNA analysis of fossil pollen, scientists tell us when and where oaks have lived. Their presence or absence was determined by changes in climate that created or eliminated land bridges between continents enabling movement of plants and animals, as well as providing the climate conditions in which oaks can survive.

Hybridization was instrumental in the formation of oak species and the ability of oaks to survive in different climate conditions. The article in Scientific American about the genetic study of oak species concludes: “A firm grasp of when, where and how oaks came to be so diverse is crucial to understanding how oaks will resist and adapt to rapidly changing environments. Oaks migrated rapidly as continental glaciers receded starting around 20,000 years ago, and hybridization between species appears to have been key to their rapid response. The insights we can gain from elucidating the adaptive benefits of gene flow are critical to predicting how resilient oaks may be as climate change exposes them to fungal and insect diseases with which they did not evolve.”

In fact, a recent study suggests that assisted species migration and intentional hybridization are necessary to prevent the extinction of plants in Arctic regions, where the climate is warming the fastest. Intentionally planting species from warmer regions into colder regions in anticipation of climate warming is called assisted migration. It is not a new concept. The study acknowledges that intentional hybridization is a radical suggestion that contradicts conventional wisdom: “Traditionally, hybridization is viewed as negative and leading to a loss of biodiversity, even though hybridization has increased biodiversity over geological times.” This study acknowledges the role that hybridization plays in increasing biodiversity.

In the Bay Area, we are surrounded by examples of hybridization, some intentional and tolerated and some natural, but not tolerated:

Sycamores are the most common street tree in the United States and we have many here in the Bay Area. They are a hybrid of London Plane Trees and our native Sycamore. The California native was intentionally bred with the London Plane Tree to increase its drought tolerance. Sycamore street trees are one of the most popular because they are extremely hardy and tolerant of challenging conditions in urban settings. They are also the host trees of one of our native butterflies, Western Tiger Swallowtail. The Tiger Swallowtail probably used our native Sycamore in the past, but made a seamless transition to the hybrid.

Update: I learned about the hybrid origins of our local Sycamore street tree in an urban forestry class at UC Berkeley. Peter Del Tredici has sent me this correction: “The london plane tree, Platanus x acerifolia is generally considered to be a hybrid between the european species, P. orientalis and the eastern species, P. occidentalis. the west coast species, P. racemosa is not part of the mix.”

Spartina alterniflora is a marsh grass that is native on the East Coast. It grows taller and denser than our native marsh grass, Spartina foliosa that also dies back in winter, unlike the East Coast native that does not. In other words, non-native spartina is superior protection from winter storm surges compared to native spartina. Yet, non-native spartina is being eradicated using herbicides along the entire West Coast of the country because it hybridizes with the native spartina species. The herbicide used for that purpose has been sprayed for about 15 years, which is probably why attempts to plant native spartina as a replacement have been unsuccessful. The result of the eradication project has been bare mud that provides no protection from erosion caused by rising sea levels and more intense winter storms. In other words, if non-native spartina were permitted to hybridize with native spartina on the West Coast, the result would be a new species that is better adapted to face the threats of rising sea levels and intense storm surges.

Doug Tallamy’s closing photo of his keynote speech to the California Native Plant Society Conference, 2018

Fear of hybridization is akin to fear of mongrelization–the mixing of races–by racists and xenophobes. It is closely related to the fear of non-native plant and animal species, a short-step away from the fear of human immigrants. Concern about racial purity is not far from fear of “genetic pollution.” State laws in the US prohibiting interracial marriage were not repealed until 1967, when the US Supreme Court ruled in Loving v. Virginia that such laws were unconstitutional in the 16 states in which these laws still remained. These are cultural fears, not grounded in biological science.

In conclusion

Doug Tallamy’s intended audience is home gardeners. Although he urges his readers to remove invasive species, he does not endorse the use of herbicides. Unfortunately, his work is used by public land managers to justify their eradication projects that usually use herbicides. If Tallamy’s work stayed in its home gardening lane, it would do less damage to the environment.

Baseless generalizations in Doug Tallamy’s Nature’s Best Hope

Doug Tallamy’s latest book, Nature’s Best Hope, continues his crusade against non-native plants. He now calls invasive plants “ecological tumors.” You might be tempted to respond that invasive plants are a small subset of non-native plants until you realize that Tallamy calls 3,300 plant species in North America “invasive.” There are approximately 6,500 species of native plants in California, which reminds us that introduced plants are often a significant portion of our urban landscapes. The title of Tallamy’s book is a misnomer. Nature is not confined to native plants, as Tallamy wishes it to be.

Tallamy makes no meaningful distinction between “invasive” and “non-native.” The classification of berry-producing non-native plants as “invasive” is a case in point. Although Himalayan blackberries are invasive, most other berry-producing non-natives in California are not. Cotoneaster, pyracantha, and holly are a few examples of berry-producing plants being eradicated in the Bay Area that are not inherently “invasive.” They spread because birds eat the berries and deposit the seeds elsewhere.

Cedar Waxwings in crab apple tree. Wikimedia Commons

Eradicating berry-producing plants deprives birds of an important source of food. If herbicides are used to kill the plant, the birds are also exposed to harmful chemicals, known to reduce reproductive success and cause other sub-lethal health issues in wildlife. In the case of Himalayan blackberries, they are frequently eaten by children and adults, who are then exposed to the herbicides used to kill the shrubs that are often widespread in our parks and open spaces. San Francisco’s Recreation and Park Department sprayed blackberries in San Francisco’s parks and open spaces 23 times in 2019.

Tallamy and his nativist allies claim that native plants are beneficial to wildlife, especially birds. How can they claim that eradicating berry-producing plants benefits birds? They do so by claiming that native berries are more nutritious than non-native berries. In particular, they claim that native berries contain more fat than sugar and that migrating birds require berries with high fat content. Tallamy cites one study in support of that claim, a study that compared fat and sugar levels in the berries of 9 species of plants in the Northeast, 5 native species and 4 introduced species. They found that the native species they analyzed had more fat content than the introduced species they analyzed. (1)

Generalizations unsupported by evidence

From that single study of nine plant species, Tallamy generalizes that berries of plants that are considered native in Asia are less nutritious for migrating birds than the berries of native plants in North America are. (None of the nine plant species studied occurs in California.) Does that generalization make sense?

Tallamy does not provide any evidence that there are fewer migratory birds in Asia, or that the nutritional needs of migratory birds in Asia are different than those in North America. In fact, looking at the migratory patterns of birds confirms that migratory routes of birds span several continents. The intercontinental flights of birds sometimes span both Asia and North America. There is no logical or evidentiary explanation for berries of native plants in Asia being uniformly less nutritious than native plants in North America.
However, Tallamy offers evidence of the similarity between plants in Asia and closely related plants in North America. Wooly adelgids quickly made a transition to native hemlocks when they arrived in North America from Asia because its native host in Asia is closely related to the American native. The adelgid has “all but eliminated hemlocks” in America, according to Tallamy. The emerald ash borer has killed millions of ash trees in America when it arrived from Asia, where its native host was closely related. On one hand, Tallamy claims that native plants in America are unique, completely different from plants in Asia, yet he recognizes that insects from Asia rapidly adapt to closely related host plants in America.
Asian species are not so foreign to America as Tallamy wishes us to believe. There are relicts of vegetation that extended completely around the Northern Hemisphere about 50 million years ago that were broken up by a combination of mountain-building and climate change. Tree of Heaven, Gingko, and Dawn Redwood, now considered introduced trees from Asia, occurred here naturally during that geologic period. Tallamy says we must confine our choices to plants that “share an evolutionary history.” In fact, many plants now considered non-native shared an evolutionary history with plants now considered native. Trees are time travelers, marching to the beat of the Earth’s geologic and climate drum. Now they must be on the move to survive our changing climate. We should not stand in their way.

Such generalizations unsupported by evidence are typical of Tallamy’s work. In “Native plants improve breeding and foraging habitat for an insectivorous bird,” Tallamy and his collaborators conclude, “We demonstrate that residential yards dominated by nonnative plants have lower arthropod abundance…that function as population sinks for insectivorous birds.” The data provided do not support such a broad generalization. They studied one species of bird, in one geographic location, in a short period of time. They inventoried arthropods for two years in a single month time-frame. They quantify only one variable (plant foliage biomass) in addition to the nativity of plants, the abundance of insects, and the breeding success of one bird species. They have not taken into consideration intervening variables such as variations in temperature, rainfall, pesticide use, etc. The bird species studied is abundant within its range. Its conservation status is “Least Concern.” The abundance of this bird species does not justify the dire predictions of Tallamy’s study.

In conclusion

I have focused on just one of the many controversies discussed in Doug Tallamy’s new book. I haven’t touched on the two most fundamental errors in Tallamy’s work:

Tallamy underestimates the speed of adaptation and evolution. There is ample evidence of rapid adaptation to non-native vegetation, including Tallamy’s examples of wooly adelgid and emerald ash borer making a quick transition to North American native trees after arriving from Asia.
He exaggerates the degree of specialization among insects. For example, he claims that 30% of native bees are “host-plant specialists,” yet Bees of the World (Michener, Johns Hopkins University) estimates a global average of 9% of bee species use plants within the same genus and it is “exceedingly rare” for bee species to be confined to only one plant species.

We have explored those issues in Tallamy’s work in previous articles:

Doug Tallamy claims that insects eat only native plants, yet his own study proves otherwise: HERE
Doug Tallamy claims that non-native plants are “ecological traps for birds.” HERE is an article that disputes that theory.
Doug Tallamy claims that native and non-native plants in the same genus are not equally useful to wildlife, but he is wrong about that. Story is HERE.
Doug Tallamy advocates for the eradication of butterfly bush (Buddleia) because it is not native. He claims it is not useful to butterflies, but he is wrong about that. Story is HERE.
Doug Tallamy publishes a laboratory study that he believes contradicts field studies, but he is wrong about that. Story is HERE.
Doug Tallamy speaks to Smithsonian Magazine, Art Shapiro responds, Million Trees fills in the gaps: HERE

(1) B. Smith, et. al., “The value of native and invasive fruit-bearing shrubs for migrating birds,” Northeastern Naturalist, 2013, 20(1): 171-84.

Doug Tallamy speaks…Art Shapiro responds…Million Trees fills in the gaps

Smithsonian Magazine published an interview with Professor Doug Tallamy, the entomologist who is committed to the eradication of non-native plants and most influential with native plant advocates in the United States. The Smithsonian article gives Professor Art Shapiro an inadequate opportunity to respond to Tallamy’s assertions about the superiority of native plants. Million Trees steps up to fill in the gaps in response to Tallamy.

The Smithsonian article says, “As a scientist, Tallamy realized his initial obligation was to prove his insight empirically. He began with the essential first step of any scientific undertaking, by applying for research grants, the first of which took until 2005 to materialize. Then followed five years of work by relays of students.”

The first study that Tallamy conducted is not mentioned in this article because it disproved his hypothesis: “Erin [Reed] compared the amount of damage sucking and chewing insects made on the ornamental plants at six suburban properties landscaped primarily with species native to the area and six properties landscaped traditionally. After two years of measurements Erin found that only a tiny percentage of leaves were damaged on either set of properties at the end of the season….Erin’s most important result, however, was that there was no statistical difference in the amount of damage on either landscape type.” (1)

The Smithsonian article says, “… insects tend to be specialists, feeding on and pollinating a narrow spectrum of plant life, sometimes just a single species. ‘Ninety percent of the insects that eat plants can develop and reproduce only on the plants with which they share an evolutionary history’…:”

Anise Swallowtail butterfly in non-native fennel. Courtesy urbanwildness.org

A “specialist” insect is rarely confined to using a single plant species. Mutually exclusive relationships in nature are very rare because they are usually evolutionary dead-ends. The study in which this claim about “specialization” originated, actually concluded: “More than 90 percent of all insects sampled associate with just one or two plant families.”* There are over 600 plant families and thousands of plant species within those families. Most plant families include both native and non-native plant species. An insect that uses one or two plant families, is therefore capable of using both native and non-native plant species. For example, there are 20,000 plant members of the Asteraceae family, including native sagebrush (Artemisia) and non-native African daisy. In other words, the insect that confines its diet to one family of plants is not very specialized.

The Smithsonian article says, “But he [Tallamy] thinks this [transition of insects to non-native plants] is likely to take thousands of generations to have an impact on the food web. Shapiro maintains he has seen it occur within his own lifetime.”

There are many empirical studies that document the transition that insects make from native to non-native plants within generations. Professor Tallamy provides a few examples of such rapid transitions in his first book, Bringing Nature Home: wooly adelgids from Asia have had a devastating effect on native hemlock forests in the eastern United States; Japanese beetles introduced to the United States are eating the foliage of over 400 plant species (according to Professor Tallamy), some of which are native (according to the USDA invasive species website).

Soapberry bug on balloon vine. Scott Carroll, UC Davis

The soapberry bug made a transition from a native plant in the soapberry family in less than 100 generations over a period of 20 to 50 years. The soapberry bug-balloon vine story is especially instructive because it entailed very rapid morphological as well as behavioral change; the beak length was quickly (a few years) selected for the dimensions of the fruit of the new host. (2)

Doug Tallamy claims that Art Shapiro’s findings are “anecdotal.” They are not. Art Shapiro’s published study is based on nearly 40 years of data. (3)

Monachs in eucalyptus, Pacific Grove Museum

In a recent NY Times article about declining populations of monarch butterflies on the West Coast, an academic scientist explains how he used Professor Shapiro’s data set to study the decline: “The monarch’s decline is part of a larger trend among dozens of butterfly species in the West, including creatures with names like field crescents, large marbles and Nevada skippers,” said Matt Forister, an insect ecologist at the University of Nevada, Reno, whose conclusions are based on a nearly 50-year set of data compiled by Art Shapiro, a researcher at the University of California, Davis. “The monarch is very clearly part of a larger decline of butterflies in the West.” Clearly, other academic entomologists do not consider Professor Shapiro’s data “anecdotal.”

The Burghardt/Tallamy study (4) does not contradict the findings of Professor Art Shapiro because Professor Shapiro is studying butterflies (not moths) in “natural areas” that have not been artificially created by choosing a limited number of plant species, as Tallamy’s study did. In other words, the adult and larvae stages of butterflies that Professor Shapiro studies have more options, and when they do they are as likely to choose a non-native plant as a native plant for both host plant and food plant. You might say, Professor Shapiro’s study occurs in the “real world” and the Burghardt/Tallamy study occurs in an artificially created world.

Dismissing observations as anecdotal is a well-worn rhetorical device. Creationists often claim that evolution cannot be proven because the theory is based on millions of observations, rather than empirically tested by experiments. Yet, virtually all scientists are firm believers in the validity of evolutionary principles.

Tallamy dismisses climate change as a factor in plant and animal extinctions, preferring to place the blame solely on the mere existence of non-native plants.

This claim is contradicted by a multitude of studies, such as a collection of studies recently reported by Yale E360 that concludes: “A growing number of studies show that warming temperatures are increasing mortality in creatures ranging from birds in the Mojave Desert, to mammals in Australia, to bumblebees in North America. Researchers warn that heat stress could become a major factor in future extinctions.”

Climate change is the environmental issue of our time. When the climate changes, the vegetation changes. When the vegetation changes, wildlife adapts or dies. Non-native plants are one of the consequences, not the cause of climate change or plant and animal extinctions.

*Professor Shapiro has provided a caveat to this definition of specialization of insects in a private communication, published with his permission: “A couple of observations: Hardly any insects feed on entire plant families. Rather, they feed on specific lineages within those families, typically defined by secondary chemistry (which is the necessary releaser for oviposition and/or feeding behavior). The relationship was summed up symbolically by A.J.Thorsteinson half a century ago: feeding=presence of nutrients+presence of required secondary chemicals-deterrents-antifeedants-toxins. Thus the Anise Swallowtail species-group feeds on the carrot family, Apiaceae, but NOT on Apiaceae lacking the proper chemistry.But they DO feed on some Rutaceae (including Citrus) that, though unrelated, are chemically similar. That was worked out by Vincent Dethier in the 1940s and further developed by John Thompson at UC Santa Cruz. A whole slew of things require iridoid glycosides as oviposition and feeding stimulants. Most plants containing these were in the family Scrophulariaceae before DNA systematics led to its dismemberment, but one whole branch of Scrophs is chemically unsuitable. Milkweed bugs eat milkweed, but they also eat the Brassicaceous genera Erysimum and Cheiranthus, which are chemically similar to milkweeds but not to other Brassicaceae…and so on. Native vs. non-native has nothing to do with it.” (emphasis added)

Tallamy, Doug, “Flipping the Paradigm: Landscapes that Welcome Wildlife,” chapter in Christopher, Thomas, The New American Landscape, Timber Press, 2011
Carroll, Scott P., et. al., “Genetic architecture of adaptive differentiation in evolving host races of the soapberry bug, Jadera haematoloma,” Genetica, 112-113: 257-272, 2001
SD Graves and AM Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation, 110 (2003) 413-433
Karin Burghardt, Doug Tallamy, et. al., “Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities,” Ecosphere,November 2010

Study design determines study findings

Million Trees can never resist a response to misinformation we find in Jake Sigg’s Nature News. (In this case, the statement originates with one of Jake’s readers, not Jake himself.)

“This study takes some of the life out of Art Shapiro’s ecological fitting theory: Non‐native plants supported significantly fewer caterpillars of significantly fewer specialist and generalist species even when the non‐natives were close relatives of native host plants.” “Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities” by Karin Burghardt, Doug Tallamy, et, al. (Ecosphere, November 2010).

We’ll get to the study later, but first let’s address the statement about ecological fitting. Ecological fitting is more accurately described as an observation, rather than a theory or hypothesis and it does not originate with Art Shapiro. The first observation of ecological fitting was recorded by Dan Janzen in 1980 and described by other ecologists as “the process whereby organisms colonize and persist in novel environments, use novel resources or form novel associations with other species as a result of the suites of traits that they carry at the time they encounter the novel condition.” (1) Ecological fitting is an alternative to the view that relationships between plants and insects and parasites and hosts are the result of co-evolution. It is consistent with the observation that adaptation to new arrivals in an ecosystem often occurs without evolutionary change and can occur more rapidly than co-evolution would require.

The Colorado potato beetle readily devours an introduced relative of its *Solanum* hosts as a result of ecological fitting. (Hsiao, T. H. (1978). “Host plant adaptations among geographic populations of the Colorado potato beetle”. *Entomologia Experimentalis et Applicata*. 24 (3)) USDA photo

Ecological Laboratory Science

The Burghardt/Tallamy study is a laboratory experiment in the sense that it creates an artificial environment by planting a garden in which it chooses the plant species and then inventories the insect visitors to the garden. In one garden, native plant species were paired with a closely related species of non-native plant in the same genus (called congeners). In another, distant garden, native plant species were paired with unrelated species of non-native plants. The insect visitors that were counted are specifically the larvae stages (caterpillars) of lepidoptera (moths and butterflies). The adult stage of the caterpillars (moths and butterflies) were not inventoried, nor were members of the other 28 insect orders.

The study considers caterpillars “specialists” if they feed on three or fewer plant families. The authors make this determination based on scientific literature and on observations of their artificially created garden. Using scientific literature, 30% of visiting caterpillar species to the experimental garden were specialists. Using actual visits to their experimental garden, 64% of visiting caterpillars were specialists. The difference is as we should expect because the scientific literature is based on the behavior of caterpillars in the field, but the study confines the choices of the caterpillars to a few specific plant species chosen by the authors of the study. In other words, caterpillars in the experimental garden had fewer choices of plant species.

The inventory of caterpillars was conducted over two summer months in 2008 and three summer months in 2009. Findings were very different in the two years of the study: “We found no difference between the total Lepidoptera larvae supported by native plants and their non-native congeners in 2008, but found over three fold more larvae on natives in 2009. In 2008 there was no difference in the abundance of generalists on native and non-native congeners, but natives supported more than twice as many generalists as non-natives in 2009.” (2) Similar results were reported for species richness (number of different larvae species). When paired with unrelated non-native plants, caterpillars showed a significant preference for native plant species, as we should expect because the plants were not chemically similar.

Caterpillar of Anise swallowtail butterfly on its host plant, non-native fennel. Berkeley, California

Although on average, native species attracted more caterpillars than the non-native congener with which they were paired, the strength of that difference varied significantly. One matched pair attracted eight times as many caterpillars to the native plant compared to the non-native plant. Another matched pair attracted slightly more caterpillars to the non-native plant compared to the native plant.

The study authors interpret the significant differences between findings in the first and second years as an indication that caterpillars accumulated more rapidly on native plants than on non-native plants. They speculate that a longer study would have found even greater preferences for native plants compared to non-native congeners. Given that adaptation to introduced species occurs over time that is a counter-intuitive prediction. In fact, many studies find that insects have made a successful transition from native to non-native hosts within a few generations.

Limitations of laboratory studies

The Burghardt/Tallamy study is often cited by native plant advocates in support of their belief that insects require native plants for survival. This generalization is not supported by the results of the Burghardt/Tallamy study because:

The study results are not relevant to all insects. The findings apply only to the larvae stages of moths and butterflies. The adult stages of moths and butterflies also require nectar and pollen from a much broader range of plants than their host plant, where the adult lays its eggs and caterpillars feed before becoming flying adults. At the adult stage of their lives, they become pollinators. Studies of the preferences of pollinators consistently find that a diverse garden that prolongs the blooming period is most useful to them.
The study does not support the claim that caterpillars consistently choose native plants in preference to closely related non-native plants over time. In fact, other studies find such preferences fade over longer periods of time.
Statements made by native plant advocates about the degree to which caterpillars are “specialized” are often exaggerated. When a diverse landscape is available to caterpillars, scientific literature reports that specialization to a few plant families is found in only 30% of the 72 caterpillar species identified by this study.
The Burghardt/Tallamy study was conducted on the East Coast where the climate is different than California. It snows in the winter and it rains during the summer, unlike most of California. Our native plants are therefore different from natives on the East Coast. The Burghardt/Tallamy study was conducted in the summer months from June to August. Native plants in California are no longer blooming and many are dormant during summer months unless they are irrigated. The findings of the Burghardt/Tallamy study are therefore not applicable to California unless they can be replicated here.

This is the Serpentine Prairie in Oakland. It is one of the native plant “restorations” done by East Bay Regional Park District. About 500 trees (including native oaks) were destroyed to return the prairie to native grassland. This is what it looks like in June.

Comparison of laboratory with field studies

The Burghardt/Tallamy study does not contradict the findings of Professor Art Shapiro because Professor Shapiro is studying butterflies (not moths) in “natural areas” that have not been artificially created by choosing a limited number of plant species. In other words, the adult and larvae stages of butterflies that Professor Shapiro studies have more options, and when they do they are as likely to choose a non-native plant as a native plant for both host plant and food plant. You might say, Professor Shapiro’s study occurs in the “real world” and the Burghardt/Tallamy study occurs in an artificially created world.

The credibility and relevance of Professor Shapiro’s studies are also based on 47 years of visiting his research plots at least 250 days per year, that is, year around. During that period of time, he recorded his observations and they were statistically analyzed for the study he published in 2003. (3) His study is of particular interest as the climate changes rapidly because the length of the study also enables us to observe the impact of climate change on our butterfly population in the Bay Area. In contrast the Burghardt/Tallamy study was conducted in a total of 5 months over a total of two years. Population trends cannot be determined from such a short study.

Burghardt/Tallamy study is consistent with mission of Million Trees

The Burghardt/Tallamy study does not contradict anything Million Trees advocates for. Decisions to plant a particular species and the decision to eradicate a particular species are entirely different. Gardeners should plant whatever they prefer, in my opinion. When planting decisions are made for public land, I prefer that plants be capable of surviving current local and climate conditions. When my tax dollars are being spent, I prefer that they not be wasted. Besides, I hate watching plants and trees die in the parks I visit.

This study is consistent with my view that non-native plants don’t threaten the survival of insects unless they replace native plants that insects prefer. The Burghardt/Tallamy study quite rightly does not say that they do. Local experience in the Bay Area informs me that they rarely do. To the extent that they have replaced native plants, they are better adapted to current conditions in a specific location. Eradicating them rarely results in native plants successfully replacing them. As the climate continues to rapidly change, the failure of native plant “restorations” is inevitable because vegetation changes when the climate changes.

Site 29 on Claremont Blvd in Oakland is one of the places where UC Berkeley destroyed about 19,000 trees about 14 years ago. Non-native weeds thrive in the sun where trees were destroyed. Poison hemlock and thistle (both non-native) are 8 feet tall where not sprayed with herbicide. Site 29, May 2016.

The Burghardt/Tallamy study does not justify eradication of non-native plants because it does not take into account the damage done by the methods used to eradicate non-native plants. Since most eradication projects use herbicides, we speculate that more harm is done to insects by herbicides than by the existence of non-native plants.

The decision to eradicate non-native plants must also take into consideration whatever benefit the plants may provide, such as food for wildlife. For example, even if a plant species isn’t a host plant, it might be a food plant. Butterfly bush (Buddleia) is an example of a plant that is very useful to pollinators, including butterflies, but native plant activists advocate for its eradication.

Monarch nectaring on butterfly bush. butterflybush.com

Many thanks to Jake Sigg for creating this opportunity for dialogue with native plant advocates. I am grateful for the window into the community of native plant advocates that Jake’s Nature News provides.

Agosta, Salvatore J.; Jeffrey A. Klemens (2008). “Ecological fitting by phenotypically flexible genotypes: implications for species associations, community assembly and evolution”. Ecology Letters. 11 (11): 1123–1134.
“Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities” by Karin Burghardt, Doug Tallamy, et, al. (Ecosphere, November 2010).
SD Graves and AM Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation, 110 (2003) 413-433