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)

  1. Tallamy, Doug, “Flipping the Paradigm:  Landscapes that Welcome Wildlife,” chapter in Christopher, Thomas, The New American Landscape, Timber Press, 2011
  2. 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
  3. SD Graves and AM Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation, 110 (2003) 413-433
  4. 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.

Source: handsontheland.org

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 caterpillars.  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. 

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

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 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.

  1. Agosta, Salvatore J.; Jeffrey A. Klemens (2008). “Ecological fitting by phenotypically flexible genotypes: implications for species associations, community assembly and evolution”. Ecology Letters11 (11): 1123–1134. 
  2. “Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities” by Karin Burghardt, Doug Tallamy, et, al. (Ecosphere, November 2010).
  3. SD Graves and AM Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation, 110 (2003) 413-433

Highs and Lows of the 2018 Conference of the California Native Plant Society

I am pleased to publish the following report of one of our readers who attended the conference of the California Native Plant Society in Los Angeles at the beginning of February 2018. 

Million Trees


I attended the last conference of the California Native Plant Society in San Jose in January 2015.  It was interesting to note a few significant new themes in the recent conference in 2018.  Both fire and climate change were much more prominent themes in the recent conference.  While both are relevant to the future of native plants, neither seemed to have any effect on the “restoration” goals of the native plant movement.  For example, there were several presentations about massive die offs of native oak trees, resulting from higher temperatures, drought, and disease.  These presentations ended with urgent pleas to plant more oaks.  That seemed a fundamental contradiction and a denial of the reality of climate change.  When the climate changes, the landscape changes, but native plant advocates are not willing to acknowledge that.  In fact, the greater the threats to native plants, the greater the commitment to their preservation and “restoration.”

Beautiful pictures support nativist ideology

The conference began on a low point for me, but a high point for most attendees of the conference.  The keynote speaker was Doug Tallamy.  He was introduced as a “rock star” of the native plant movement, and indeed he is.  His presentation was very effective in delivering his message, which is that most insects are “specialists” with mutually exclusive relationships with native plants that evolved over “tens of thousands of years.”  If you believe that claim, you also believe that the absence of native plants will result in the absence of insects and ultimately the collapse of the entire food web.

Doug Tallamy’s closing photo, CNPS Conference 2018

Most native plant advocates believe that gloomy scenario, but few scientists still do, which creates a tension within this community of native plant advocates composed predominantly of amateur “botanists” and a smattering of academic ecologists.  For example, one of the first presentations after Tallamy’s keynote was an academic ecologist from UC Berkeley who advocated for accommodating the movement of plants outside of historical native ranges to accommodate climate change. (1) He said that restoring only with local natives is “maladaptive” and that a bioregional perspective is needed to create sustainable landscapes.  Allowing Monterey pines to grow in the San Francisco Bay Area, where they have grown in the past and are presently deemed “native” just 150 miles away, seems a good example of such a broader definition of “native.”  An amateur nativist, parroting Tallamy, asked this hostile question: “But if we move the plants how will wildlife survive?”  The academic delivered this tart dose of reality: “There are few mutually exclusive relationships in nature.  Wildlife will also move and will adapt to changes in vegetation.”

Science debunks a myth about eucalyptus

The high point of the conference for me was a presentation by Jennifer Yost, Assistant Professor at Cal Poly, San Luis Obispo.  She and her graduate student studied the claim that nothing grows under blue gum eucalyptus trees because of allelopathic chemicals emitted by eucalyptus that suppress the germination of other species of plants.  Two studies of this hypothesis were done in the 1960s, but the analytical methods used by those studies were misleading.

CNPS Conference 2018

Rigorous methods used by Yost’s team included planting seeds of 5 native plant species in the soil of eucalyptus forests and comparing germination rates of seeds planted in the soil of oak woodlands.  They also tested the effect of blue gum volatile leaf extracts, and water-soluble leaf extracts on germination and early seedling growth.

They concluded, “In these experiments, we found that germination and seedling growth of the species tested were not inhibited by chemical extracts of blue gum foliage, either at naturally-occurring or artificially concentrated levels.” (2)

CNPS Conference 2018

Yost observed that the lack of allelopathic effects of blue gum on the soil implies that blue gum forests theoretically can be successfully planted with native plants after removal of the trees.  However, she cautioned that those who destroy the blue gums should carefully consider what will replace them.  Will an aggressive non-native weed quickly colonize the bare ground?  If so, what is the benefit of destroying the blue gums? 

I had a conversation with one of the most influential nativists in the San Francisco Bay Area after Yost’s presentation.  This new scientific information does not alter his commitment to destroying blue gum eucalyptus in the Bay Area.  After all, there are many more negative claims that remain unchallenged by scientific studies.  For example, there are no studies that prove that blue gums use more water than native trees, as nativists claim.  Nor are there any studies that prove that eucalyptus leaves contain less moisture than the leaves of native oak or bay laurel trees, which theoretically makes eucalyptus more flammable, as nativists claim.  The lack of scientific evidence enables the persistence of speculation justifying irrational fear of blue gum eucalyptus.

Nativism dies hard because of lack of scientific studies

There appeared to be three distinct groups of people in the crowd of about 900 conferees.  There was a large contingent of grey-haired volunteers who are the backbone of every native plant “restoration.”  They are the dedicated weed pullers.  There is an equally large contingent of young people who are making their living writing the “restoration” plans and directing the activities of the volunteers.  The smallest contingent is a few academic scientists who study the underlying issues in their ivory tower.  The goals and conclusions of these three groups are increasingly divergent as scientific studies disprove the assumptions of the citizen “scientists.”

The tension between science and the citizenry is as evident within the native plant movement as it is in American politics at the present time. The general public rejects scientific evidence at its peril.  The rejection of science will not end well.  In the case of uninformed nativism in the natural world, the result will be a barren, poisoned landscape.


  1. “Climate change and open space conservation: Lessons from TBC3’s researcher-land manager partnerships in the San Francisco Bay Area,” David Ackerly1, Naia Morueta-Holme5, Sam Veloz3, Lisa Micheli2, Nicole Heller4 1University of California, Berkeley, Berkeley, CA, USA, 2Pepperwood, Santa Rosa, CA, USA, 3Point Blue Conservation Science, Petaluma, CA, USA, 4Peninsula Open Space Trust, Palo Alto, CA, USA, 5University of Copenhagen, Copenhagen, Denmark
  2. Abstracts of CNPS conference presentations are available here:  CNPS Conference abstracts

Butterfly Bush: An example of the escalating war on non-native plants

The war on so-called “invasive species” continues to escalate.  One of the indicators of this escalation is the recently revised California Invasive Plant Council’s (Cal-IPC) inventory of “invasive” plants.  Nearly 100 plant species were added, a 50% increase in the inventory. 

Scabiosa is one of 87 plants recently added to the inventory of “invasive” plants in California, despite the fact that is isn’t invasive in California. Scabiosa is very useful to bees because it blooms prolifically for much of the year.

More alarming is that most of the additions to the list are not considered “invasive” in California.  Rather, a new category of “potentially” invasive plants was created, based on their behavior elsewhere.  Many of the plants in the new category are considered invasive in Hawaii, a place with a distinctly different climate than California.  Hawaii is a tropical climate, hotter than much of California and wetter and more humid than everywhere in California.

The big increase in the number of plant species now designated as “invasive” in California is a concern partly because of the herbicides that are usually used to eradicate them.  Not only do we lose that plant species in our landscape when it is added to the hit list, we can also expect to see an increase in the use of the herbicides that are used to kill it.

Increased use of herbicides

Native plant advocates are aggressively defending the use of herbicides. Policies and practices are being developed to accommodate increased use of herbicides on our public lands.

East Bay Municipal Utilities District (EBMUD) is evaluating its Integrated Pest Management Program (IPM), including practices and policies regarding pesticide use.  The first draft of EBMUD’s revised IPM program was made available to the public in July 2017.  The draft adds several new goals to the IPM program:  “habitat protection and restoration,” reducing populations of “invasive plant species,” and “use of alternative vegetation such as native plants.”  EBMUD is the supplier of our drinking water in the East Bay and the quality of the water they supply should be the top—if not the only—priority.  If destroying non-native plants requires greater use of herbicides, that goal contradicts EBMUD’s obligation to providing safe drinking water.

Garlon sprayed on the trail in a San Francisco park. San Francisco Forest Alliance

San Francisco’s IPM program has also changed some policies to accommodate use of herbicides in parks on plants the Natural Resource Division of the parks department considers “invasive.”  The parks department restricts all park access to the established trails in the 33 “natural areas” where non-native plants are eradicated and replaced by native plants.  The new IPM policy permits the spraying of herbicides without posting pesticide application notices in places that are “publicly inaccessible.”  In other words, pesticide application notices are no longer required in the “natural areas” unless herbicides are sprayed on the trails.  One way to reduce the public’s opposition to pesticides is to hide their use and this policy seems designed to do that.

Update:   The San Francisco Forest Alliance (SFFA) has informed me that Chris Geiger, head of San Francisco’s IPM program, has given assurances that the IPM program will no longer offer City departments a blanket exemption to apply herbicides without posting in areas the department considers “publicly inaccessible”.  Previous to this, each land manager was empowered to make their own decisions as to which areas they considered “publicly inaccessible”.  The IPM group did not provide oversight of the decisions or keep records of which areas were exempted.  Now specific exemptions will be issued and recorded on the IPM exemptions webpage.  Chris Geiger reports RPD will not be requesting any posting exemptions.   SFFA is still waiting for formal written documentation of this change.

San Francisco’s IPM Program is also demonstrating its commitment to native plants and the eradication of non-native plants by sponsoring a webinar on October 5th, 2017, featuring Doug Tallamy:  The Plant-Pollinator Connection: Why Pollinators Need Native Plants.”  Tallamy is the academic entomologist who has devoted his career to the promotion of native plants based on his claim that insects at the base of the food web are dependent upon native plants.  He has said in many publications that non-native plants will cause the collapse of our ecosystems.  Many of the statements he makes in support of his dire theory are not accurate.

This post will focus on the intersection of these symptoms of the escalating war on “invasive” plants:  the expansion of California’s inventory of “invasive” plants and the closely associated claim that non-native plants must be eradicated because they compete with the native plants required by wildlife.  We use buddleia, commonly known as butterfly bush, as an example.

Butterfly Bush (Buddleia):  friend or enemy of butterflies?

Monarch nectaring on butterfly bush. butterflybush.com

Buddleia is one of 87 plant species recently added to Cal-IPC’s inventory of “invasive” plant because it is considered invasive outside of California.  Buddleia is called butterfly bush because it produces large quantities of nectar that attract swarms of butterflies.  Since buddleia is very appealing to butterflies, it is popular with gardeners who like to see butterflies in their gardens.

Since buddleia is obviously useful to butterflies and Doug Tallamy claims to be concerned about the welfare of our pollinators, why is he telling gardeners to quit planting buddleia?  His advice is based on the fact that buddleia is considered invasive in some places and his belief that it will eventually be invasive everywhere.  In fact, that’s his belief about all non-native plants:  they may not be invasive now, but he predicts that eventually they all will be invasive.

Secondly, Tallamy argues that although buddleia provides food for butterflies, it is not a host plant for butterflies.  The host plant is where butterflies lay their eggs and where the caterpillar feeds when the eggs hatch.  The choice of host plant species is much smaller than the number of food plant species available to butterflies, but it is not as small as Tallamy thinks it is.  Tallamy does not seem to realize that many plants are chemically similar, which enables butterflies to make a transition from a native plant to a chemically similar non-native plant.  Here in California, many butterfly species have made that transition and a few butterfly species are dependent upon abundant non-native plants that are available year-around because their original native host plant is dormant much of the year.

Buddleia “starves” butterflies?

This is Tallamy’s apocalyptic prediction about the fate of butterflies if gardeners continue to plant buddleia:

“It’s no exaggeration to say that when you choose which plants to include in your garden, even the beautiful, seemingly harmless butterfly bush, you’re deciding if members of your community’s local food web will be nourished or unintentionally starved.  And to get to that mind frame, which is a way of thinking that truly benefits nature, including its butterflies, you’re going to have to come to a harsh realization: You need to stop planting the butterfly bush—forever.” (1)

Ironically, this harsh verdict on buddleia was published by a blog entitled, “Organic Life.”  Is Organic Life unaware of the fact that the most widely used method of eradicating non-native plants is spraying herbicides?  The consequence of adding more plant species to the long list of “bad plants,” is more pesticide use.  That’s not very “organic.”

What amoral, selfish gardener would plant buddleia in their garden after such a severe scolding?  First, let’s stop and think about the logic of the claim that buddleia will disrupt the “food web” and starve butterflies.  Since buddleia is an excellent source of nectar and swarms of butterflies are observed nectaring on buddleia, how could we be “starving” them?  Professor Art Shapiro (UC Davis), our local butterfly expert, said when asked about this article, “The ‘disrupting food webs’ argument is ludicrous. It’s equivalent to saying that if you eat popcorn rather than apples, you’re contributing to unemployment in the apple-picking industry.”

Is buddleia a host plant for butterflies?

Now let’s consider the argument that we should not plant buddleia in our gardens because although it feeds butterflies, it isn’t their host plant where they lay their eggs.  The problem with that argument is that it isn’t true!!

Checkerspot laying eggs on buddleia, near Santa Barbara. Photo by Marc Kummel

In 1940, Charles M. Dammers reported that the Variable Checkerspot (Euphydryas chalcedona) “can use” buddleia as a substitute for its usual native host in southern California desert-mountain areas, based on a laboratory study of the larval stages of its caterpillar on buddleia.   In 2001, chemical analysis of buddleia found that it is chemically similar to the native host of the checkerspot, which confirmed the potential for such a substitution.

The first actual observation of checkerspot butterflies breeding spontaneously and successfully on buddleia was in Mariposa County, California in the Sierra Nevada foothills.  “Mariposa” is Spanish for butterfly.  Mariposa County was named by an early Spanish explorer who saw many butterflies near Chowchilla.

Checkerspot bred 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.” (2)

Variable checkerspot. Photo by Roger Hall

More recently, a gardener in Mendocino County 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.’” (3)

Bad rap for non-native plants

When the native plant movement began some 30 years ago, native plant advocates promoted their agenda with a straight-forward claim that they are superior to non-native plants.  The public was initially resistant to that argument because non-native plants have been around for a long time and people have become fond of them.

Native plant advocates began to fabricate stories about the evils of non-native plants to convince the public that eradicating them was necessary because they are harmful to wildlife and they damage the environment.  The Million Trees blog was created to address those claims.

But Doug Tallamy’s active participation in the crusade against non-native plants is a special case because he is an academic entomologist, credentials that make him more influential with the public.  For that reason, Million Trees has critiqued several of his publications.  We publish this critique of Tallamy’s opinion of buddleia for several reasons:

  • Buddleia is very useful to butterflies. The loss of buddleia in our gardens would be a loss to butterflies.
  • San Francisco’s IPM program is using Doug Tallamy’s mistaken theories to promote the use of herbicides to eradicate non-native plants in San Francisco.
  • Buddleia is one of 87 plants that have been classified as “invasive” by the California Invasive Plant Council despite the fact that it is NOT invasive in California. The expansion of the list of “invasive” plants in California to include plants that are NOT invasive in California, will increase the use of herbicides and will eliminate plants that are performing valuable ecological functions.

  1. https://www.rodalesorganiclife.com/garden/never-plant-butterfly-bush  (N.B.  The butterfly in the photo in this article is European Small Tortoiseshell, found in Britain and in Europe.  The caterpillar in the photo is the monarch caterpillar on its host plant, milkweed.  Buddleia is food for both of these butterfly species.)
  2. Arthur M. Shapiro and Katie Hertfelder, “Use of Buddleja as Host Plant by Euphydryas chalcedona in the Sierra Nevada foothills, California,” News of the Lepidopterists’ Society, Spring 2009
  3. http://plantwhateverbringsyoujoy.com/never-pull-up-and-discard-what-you-cannot-identify/

Do insects prefer native plants?

We follow Doug Tallamy’s publications closely because he is the academic scientist most often quoted by native plant advocates to support their belief that insects require native plants and that the absence of the native plants will result in the collapse of entire ecosystems:   “…our wholesale replacement of native plant communities with disparate collections of plants from other parts of the world is pushing our local animals to the brink of extinction—and the ecosystems that sustain human societies to the edge of collapse.” (1)

Main fountains of Longwood Gardens.  Creative Commons - Share Alike
Main fountains of Longwood Gardens. Creative Commons – Share Alike

Tallamy co-authored his most recent publication, The Living Landscape:  Designing for Beauty and Diversity in the Home Garden, with Rick Darke, curator of plants at Longwood Gardens for 20 years.  Longwood is a formal garden outside of Philadelphia, which seems at odds with the exclusively native gardens for which Tallamy advocates.  And so we were intrigued by this unlikely team.  Darke’s introduction to the book implies a departure from Tallamy’s usual mantra:

“Is this a book only about gardening with native plants?  No.  It’s a book about how native plants can play essential roles in gardens designed for multiple purposes, with a focus on proven functionality.  For better and worse, the native plant movement in North America has evolved in the last decade…One of the most important functionalities is durability:  the capacity to thrive over a long time without dependence on resource-consuming maintenance regimes.  Claims that natives are always better than exotics fail to take into account radically altered environmental conditions in many suburban landscapes…In most cases and most places, the design of broadly functional ecologically sound, resource-conserving residential gardens requires a carefully balanced mix of native and non-native plants.  It’s time to stop worrying about where plants come from and instead focus on how they function in today’s ecology.  After all, it’s the only one we have.”  (2)

Tallamy writes his own introduction to The Living Landscape, which suggests a softening of his hard-line insistence upon gardening exclusively with native plants:

“What is native in any given place today wasn’t native if we look back far enough in time, and it is certain that what will be native in that same place in the future will be different from what is native now.  Functional ecological relationships take a long time to evolve—often thousands of years—but they do evolve.  Humanity’s challenge is to reduce its introduction of rapid environmental changes that are currently causing extinctions to occur faster than the evolution of new species.”  (2)   

Has Tallamy’s viewpoint evolved?

When we reported on Tallamy’s previous publication in 2012, we quoted him as saying that a graduate student under his direction could not find any evidence that native plants were eaten by insects more frequently than non-native plants:

“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 landscapes 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)

May we conclude that Tallamy no longer believes that native plants are required by insects?  No, we may not.  In Living Landscape he takes a different approach to this question.  He collaborated in three studies which found more insects in native gardens than in non-native gardens:

  • Significantly more caterpillars of butterflies and moths were found in suburban gardens of predominantly native plants compared to gardens of predominantly non-native plants. This study also quantified the number of birds found in these gardens and concluded that “…the negative relationship between non-native plant abundance and bird community integrity is apparent in managed ecosystems as well, regardless of whether the non-native species are invasive.”  This seemed a leap of faith, given that the inventory of insects was done in a six-week period in August and September and the inventory of birds was done in a six-week period in June and July, rendering a cause-and-effect relationship dubious.
  • Two other studies were conducted in a constructed garden in which native and non-native plants were paired for comparisons. Some of the pairs were in the same genus.  Again, significantly more caterpillars and other plant-eating arthropods were found on native plants, although the differences were much smaller when the plants were in the same genus, which are often—but not always–chemically similar.

Reconciling apparent contradictions

So, how are we to reconcile these studies which find more insects on native plants with other studies which report otherwise?

  • Here in the San Francisco Bay Area, we rely on the research of Professor Arthur Shapiro to inform us of which plants are useful to our butterflies. He tells us:  “Most California natives in cultivation are of no more butterfly interest than nonnatives, and most of the best butterfly flowers in our area are exotic.” (3) The difference between Professor Shapiro’s studies and those cited by Professor Tallamy is that Professor Shapiro has been studying butterflies in “natural areas” rather than cultivated gardens.  Most of the plants that he finds butterflies using are considered weeds, such as non-native fennel and star thistle, which we wouldn’t find in suburban gardens.  We speculate that this difference accounts for some of the difference in findings. 
  • Furthermore, the studies reported by Professor Tallamy only seem contradictory. In fact, if we look at them closely we find that one reports no difference in what caterpillars eat, but considerable difference in where they are found.  And this strange difference is consistent with the scientific literature.  A meta-analysis of hundreds of studies of insect-plant interactions published by Annual Review of Entomology reports these findings:  “Herbivore densities are lower on invasive plants than on native plants, but there is no evidence that invasive plants overall suffer from less damage inflicted by native herbivores.” (4)

Go figure!  More herbivores are found on native plants, but they don’t eat more native plants than they do non-native plants.

A parting shot

Professor Tallamy urges suburban gardeners to take insects into account when making their gardening choices and, of course, we agree.  However, he closes his pitch for gardening with natives in The Living Landscape with a story which seems superficially compelling but doesn’t hold up to close scrutiny.

Eumaeus atala butterfly laying eggs on coontie.  Creative Commons - Share Alike
Eumaeus atala butterfly laying eggs on coontie. Creative Commons – Share Alike

There is a beautiful butterfly (Eumaeus atala) in Florida that was historically dependent upon a particular native plant, coontie, which is a species of cycad.  Coontie was popular with early settlers as a food flavoring and was nearly wiped out early in the 20th century, along with the atala butterfly which was dependent upon it as its host plant.  Tallamy claims that the atala made a comeback when coontie became a popular plant for suburban gardens.  This makes a powerful case for how suburban gardeners can participate in efforts to conserve our native butterfly fauna.

Coontie.  Photo by Dan Culbert, University of Florida
Coontie. Photo by Dan Culbert, University of Florida

But is it true?  Wikipedia says it’s not:  “The atala is now common locally in southeast Florida rebounding to some extent as it has begun to use ornamental cycads planted in suburban areas.”    This is an example of how chemically similar plants can be useful to native insects, whether they are native plants or introduced, non-native, ornamental plants.

Sago cycad palm
Sago cycad palm is an example of an ornamental cycad

We apologize for being repetitive, but for the record we will close with the reminder that Million Trees urges everyone to plant whatever they want in their own gardens.  In public open spaces, which belong to everyone, we ask only that land managers quit destroying trees and using pesticides for the sole purpose of attempting to eradicate non-native plants.  The audience for Professor Tallamy’s publications is private gardeners, so we don’t really have a beef with him.  We critique his rationale for his preference for native plants only because it is often cited by those who demand the eradication of non-native plants and trees in our public open spaces.

The Living Landscape is a beautiful book, which we recommend to our readers for its lovely photos of naturalistic landscapes.

 


 

  1. Doug Tallamy, “Flipping the Paradigm:  Landscapes that Welcome Wildlife,” chapter in Christopher, Thomas,The New American Landscape, Timber Press, 2011
  2. Rick Darke and Doug Tallamy, The Living Landscape: Designing for Beauty and Diversity in the Home Garden, Timber Press, 2014
  3. Arthur Shapiro, Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions, University of California Press, 2007
  4. Martijn Bezemer, et. al., “Response to Native Insect Communities to Invasive Plants,” Annual Review of Entomology, January 2014.

Are non-native plants “ecological traps” for birds?

One of the reasons why native plant advocates want the managers of our public lands to destroy non-native plants and replace them with native plants is that they believe native plants provide superior habitat for birds.  However, empirical studies do not support this belief, as we have explained in earlier posts.  Today we will examine an article recently published in an advocacy magazine, making the claim that non-native plants are “ecological traps” for birds:  “Nonnative Plants: Ecological Traps Offering alluring habitat for songbirds, exotic plants may actually decrease the animals’ long-term survival and fitness” (1)

Japanese honeysuckle.  Attribution William Rafti
Japanese honeysuckle. Attribution William Rafti

The article begins auspiciously with the good news that populations of some bird species have increased significantly in recent decades because of the spread of non-native plant species which are valuable sources of food:  “…a 2011 paper, published in the journal Diversity and Distributions, concluding that the number of fruit-eating birds such as cardinals, robins and catbirds tripled during the past three decades in parts of central Pennsylvania due to the spread of nonnative honeysuckles.”  (1)  And then the article attempts to contradict this good news by turning to the usual nativist caveats.

Generalists vs. Specialists

Nativists claim that the animal kingdom is divided into generalists and specialists.  The generalists are theoretically omnivores—they have a varied diet—and so depriving them of native plants will not prevent their survival.  Specialists, on the other hand, are dependent upon a narrow range of plant or animal species for survival.  We are expected to believe that specialists far outnumber generalists and that we doom them to extinction when one particular species of native plant or animal is unavailable to them.

Monarch butterfly caterpillar - Creative Commons - Share Alike
Monarch butterfly caterpillar – Creative Commons – Share Alike

Doug Tallamy is the purveyor of the generalist vs. specialist overstatement.  We have critiqued his assumptions in an earlier post.  In a nutshell, there are few mutually exclusive relationships in nature because they are a risky evolutionary strategy.  The plant or animal that is dependent upon one other species is significantly less likely to survive in the long term than an animal with more dietary options.  The perception that there are immutable relationships between insects and plants also underestimates the speed of adaptation and evolution, particularly of insects with large populations and short lifespans.

For example, a bird that eats insects usually eats all manner of insects as well as spiders.  They are not dependent solely upon caterpillars as Mr. Tallamy seems to believe:  “…warblers and chickadees rely on caterpillars for 90 percent of their diet during the breeding season, eating hundreds per day. ‘That’s a lot of insects,’ Tallamy says. ‘If you don’t have those insects, you don’t have the birds.’” (1)

According to Cornell Ornithology Lab–America’s most prestigious research institution for birds–warblers and chickadees have a much more varied diet than Mr. Tallamy believes.  (We chose specific species with ranges and abundant populations in Delaware where Mr. Tallamy lives.  However, the diet of all species of chickadees and warblers are similar.)

  • Black-capped Chickadee:  “In winter Black-capped Chickadees eat about half seeds, berries, and other plant matter, and half animal food (insects, spiders, suet, and sometimes fat and bits of meat from frozen carcasses). In spring, summer, and fall, insects, spiders, and other animal food make up 80-90 percent of their diet. At feeders they take mostly sunflower seeds, peanuts, suet, peanut butter, and mealworms.” (2)
  • Orange-crowned Warbler:  “insects and spiders.” (2) Most insects are not caterpillars and many are not herbivores.

Black-capped Chickadee - Creative Commons - Share Alike
Black-capped Chickadee – Creative Commons – Share Alike

No evidence that insects require native plants

Mr. Tallamy is focused on caterpillars because they are herbivores, that is, they eat plants.  Just as he believes that the birds need native plants, he also believes that plant-eating insects need native plants.  However, Mr. Tallamy disproved his own theory about an immutable relationship between native plants and insects when he supervised a graduate student whose thesis concluded: 

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.” (3)

This empirical study, supervised by Mr. Tallamy, was unable to find evidence that there are more plant-eating insects in a native garden than in a landscaped garden of non-native cultivars.  Yet, Mr. Tallamy continues to claim that insects require native plants and birds require those insects for their survival:  “Tallamy’s research shows that birds also may be harmed indirectly because nonnative plants affect insects. He has found that the number and diversity of plant-eating insects, especially caterpillars, drops dramatically when exotic plants invade…[Tallamy said,] ‘My prediction is that birds that specialize on insect herbivores will take a bigger hit than those that eat other insects,’” (1)

The study by Mr. Tallamy’s student about the relationship between native plants and insects is not the only empirical evidence that his assumption is incorrect.  We have published several articles about local studies that have found no such relationship:

Native plant advocates have also offered “evidence” of insect populations in the local eucalyptus forest.  UCSF produced a video to promote their original plan to destroy most of the eucalypts on Mount Sutro (now on hold indefinitely).  An arborist shows us eucalyptus leaves that have been chewed by insects.  He claims that a drastically thinned forest will be healthier because it will have fewer insect predators.  So, there are insects in the eucalyptus forest when it suits native plant advocates’ purposes and there are no insects in the eucalyptus forest when it does not.  They want more insects when they are advocating on behalf of birds and they want fewer insects when they are demanding that trees be destroyed.  It’s rather confusing.

Insects ARE important to birds

We agree with Mr. Tallamy that insects are very important to birds because they are a major source of food, especially during the nesting season when their high-protein content is vital to nestlings.  Therefore, we believe that Mr. Tallamy should join us in making climate change our highest environmental priority.  Because insects are cold-blooded, they are particularly vulnerable to the extreme weather conditions associated with climate change.  They cannot adjust their body temperature as warm-blooded animals can in response to such fluctuations in temperature.  A recent study predicts devastating consequences for insect populations in coming decades:  “Our predictions are that some species [of insects] would disappear entirely in the next few decades, even when they have a fairly wide distribution that currently covers hundreds of kilometers.” (4)

We believe that a single-minded focus on native plants is misguided because in a rapidly changing climate the entire concept of “native” becomes meaningless.  Just as insects are unlikely to survive radical changes in temperature, the ranges of native plants must change if species are to survive.

Stay tuned for Part II

In our next post, we will continue our critique of the article that theorizes that non-native plants are “ecological traps.”  We will tell our readers about the published research that contradicts statements in the article about predation of cardinal nests in non-native honeysuckle.  The author of one of the studies is quoted in this article, saying something completely different than her own published study.  It’s an intriguing contradiction.

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(1)    John Carey, “Nonnative Plants: Ecological Traps Offering alluring habitat for songbirds, exotic plants may actually decrease the animals’ long-term survival and fitness,” National Wildlife Federation, January 14, 2013

(2)    Cornell Ornithology Laboratory, Guide to Birds

(3)     Tallamy, Doug, “Flipping the Paradigm:  Landscapes that Welcome Wildlife,” chapter in Christopher, Thomas, The New American Landscape, Timber Press, 2011

(4)    “Extreme weather caused by climate change decides distribution of insects, study shows,”  Science Digest, February 20, 2014

Doug Tallamy refutes his own theory without changing his ideology

In our debates with native plant advocates, the scientist who is most often quoted to support their beliefs is Doug Tallamy who wrote an influential book, Bringing Nature Home:  How Native Plants Sustain Wildlife in our Gardens.    Professor Tallamy is an entomologist at the University of Delaware.

Professor Tallamy’s hypothesis is that native insects require native plants because they have evolved together “over thousands of generations.”  Because insects are an essential ingredient in the food web, he speculates that the absence of native plants would ultimately result in “ecological collapse” as other animals in the food web are starved by the loss of insects. (1)

Professor Tallamy freely admits that his theory is based on his anecdotal observations in his own garden, not on scientific evidence:  “How do we know the actual extent to which our native insect generalists are eating alien plants?  We don’t until we go into the field and see exactly what is eating what.  Unfortunately, this important but simple task has been all but ignored so far.”  (1)

This research has now been done to Professor Tallamy’s satisfaction by a Master’s Degree student under his direction.  The report of that study does not substantiate Professor Tallamy’s belief that insects eat only native plants.  In his own words, Professor Tallamy now tells us:

“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.” (2)

Corroborating Evidence

This finding that insects are equally likely to eat native and non-native plants may be new to Professor Tallamy, but it isn’t new to the readers of Million Trees.  We have reported many studies which are consistent with this finding.

Anise Swallowtail butterfly in non-native fennel

The English garden, where plants from all over the world are welcome

Specialists vs. Generalists

When debating with native plant advocates, one quickly learns that the debate isn’t ended by putting facts such as these on the table.  In this case, the comeback is, “The insects using non-native plants are generalists.  Insects that are specialists will not make that transition.”  Generalists are insects that eat a wide variety of plants, while specialists are limited to only one plant or plants in the same family which are chemically similar.

Professor Tallamy offers in support of this contention that only “…about 10 percent of the insect herbivores in a given ecosystem [are not specialists],” implying that few insects are capable of making a transition to another host plant.

However, categorizing insects as specialists or generalists is not a dichotomy.  At one extreme, there are some insects that choose a single species of plant as its host or its meal.  At the other extreme, there are insects that feed on more than three different plant families.  It is only that extreme category which has been estimated at only 10% of all phytophagous (plant-eating) insects.  The majority of insects are in the middle of the continuum.  They are generally confined to a single plant family in which the plants are chemically similar.

Putting that definition of “specialist” as confined to one plant family into perspective, let us consider the size of plant families.  For example, there are 20,000 plant members of the Asteraceae family, including the native sagebrush (Artemisia) and the non-native African daisy.  In other words, the insect that confines its diet to one family of plants is not very specialized. 

Soapberry bug on balloon vine. Scott Carroll. UC Davis

Professor Tallamy offers his readers an explanation for why specialist insects cannot make the transition from native to non-native plants.  He claims that many non-native plants are chemically unique and therefore insects are unable to adapt to them.  He offers examples of non-native plants and trees which “are not related to any lineage of plants in North America.”  One of his examples is the goldenrain tree (Koelreuteria paniculata).  This is the member of the soapberry (Sapindaceae) family to which the soapberry bug has made a transition from a native plant in the soapberry family in less than 100 generations over a period of 20 to 50 years.  Professor Tallamy’s other examples of unique non-native plant species are also members of large plant families which probably contain native members.  Professor Tallamy is apparently mistaken in his assumption that most or all non-native plants are unique, with no native relatives. 

The pace of evolution

Even if insects are “specialists” we should not assume that their dependence on a native plant is incapable of changing over time.  Professor Tallamy’s hypothesis about the mutually exclusive relationships between native animals and native plants is based on an outdated notion of the slow pace of evolution.  The assumption amongst native plant advocates is that these relationships are nearly immutable.

In fact, evolution continues today and is sometimes even visible within the lifetime of observers.  Professor Tallamy provides his readers with examples of non-native insects that made quick transitions to native plants:

  • The hemlock wooly adelgids from Asia have had a devastating effect on native hemlock forests in the eastern United States.
  • The Japanese beetle introduced to the United States is now eating the foliage of over 400 plants (according to Professor Tallamy), some of which are native (according to the USDA invasive species website).

These insects apparently made transitions to chemically similar native plants without evolutionary adaptation. If non-native insects quickly adapt to new hosts, doesn’t it seem likely that native insects are capable of doing the same?  That is both logical and consistent with our experience.    For example, the native soapberry bug mentioned above has undergone rapid evolution of its beak length to adapt to a new host.

Although Professor Tallamy tells us that the relationship between insects and plants evolved over “thousands of generations,” he acknowledges much faster changes in plants when he explains why non-native plants become invasive decades after their arrival:  “Japanese honeysuckle, for example, was planted as an ornamental for 80 years before it escaped cultivation.  No one is sure why this lag time occurs.  Perhaps during the lag period, the plant is changing genetically through natural selection to better fit its new environment.”  Does it make sense that the evolution of plants would be much more rapid than the evolution of insects?  Since the lifetime of most insects is not substantially longer than the lifetime of most plants, we don’t see the logic in this assumption.

Beliefs die hard

Although Professor Tallamy now concedes that there is no evidence that insects are dependent upon native plants, he continues to believe that the absence of native plants will cause “ecological collapse.”  In the same book in which he reports the study of his graduate student, Professor Tallamy repeats his mantra:  “…our wholesale replacement of native plant communities with disparate collections of plants from other parts of the world is pushing our local animals to the brink of extinction—and the ecosystems that sustain human societies to the edge of collapse.”

This alarmist conclusion is offered without providing examples of any animals being “pushed to the brink of extinction.”  In fact, available scientific evidence contradicts this alarmist conclusion. (3)

Here are more articles about the mistaken theories of Doug Tallamy:

  • 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

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(1)    Tallamy, Doug, Bringing Nature Home, Timber Press, 2007

(2)    Tallamy, Doug, “Flipping the Paradigm:  Landscapes that Welcome Wildlife,” chapter in

Christopher, Thomas, The New American Landscape, Timber Press, 2011

(3)    Erle C. Ellis, et. al., “All Is Not Loss:  Plant Biodiversity in the Anthropocene,” http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0030535