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
  • Doug Tallamy’s Nature’s Best Hope perpetuates the myth that berry-producing non-native plants must be eradicated because they are less nutritious than the berries of native plants.  Available HERE
  • Doug Tallamy believes we must prevent hybridization.  Hybridization is a natural process that increases biodiversity and enables plants and animals to adapt to changes in the environment.  Available HERE.
  • There is NO evidence to support Doug Tallamy’s claim that insect populations are declining because of the existence of non-native plants.  Available 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

California Academy of Sciences: “Evolution in the Park”

In 2003, when the great debate with native plant advocates about the future of San Francisco’s public parks reached a fever pitch, the California Academy of Sciences stepped into the fray by publishing this article in their quarterly publication, California Wild.  This article was written by Gordy Slack, free lance science writer and former editor of California Wild

Golden Gate Park in 1880. The trees are about 10 years old. In the distance, looking south, we see the sand dunes of the Sunset District. That’s what most of Golden Gate Park looked like before the trees were planted.

As you will see, “Evolution in the Park”  (1) urges the public to consider that the parks of San Francisco have been transformed over the past 150 years from predominantly barren sand dunes to green oases of non-native trees and plants.  Using Golden Gate Park as an example, Mr. Slack reminds us that the non-native trees provided the windbreak needed to protect the entire landscape which we admire today.  The park has changed and it will continue to change, because nature is dynamic.  The forces of evolution are stronger than human desires to freeze-frame our world.

At the time, we were deeply grateful to Mr. Slack and to the Academy of Sciences for taking a position on the controversy.  We remember thinking that the opinion of this prestigious institution would surely settle the controversy.  But we were mistaken, because native plant advocates would not even read this article, let alone heed its message.

As the Environmental Impact Report for the Natural Areas Program undergoes revision and the controversy heats up again, we reprint “Evolution in the Park.”  We can only hope that someday reason will prevail.

Tree ferns from New Zealand are one of many species of non-native trees that make Golden Gate Park the beautiful place it is today. Creative Commons

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“When San Francisco officials asked the great nineteenth-century landscape architect Frederick Law Olmstead how to turn the wind-scoured dunes of the western half of the city into a green, rambling park, he was happy to offer advice: Don’t bother, he said, it’ll never work.

They went ahead anyhow, establishing three-mile-long, 1,107-acre Golden Gate Park on April 4, 1870. The decades that followed saw an almost unbelievable transformation under the strong hand of the park’s first superintendent, William Hammond Hall. He shaped glades and grew forests, dug lakes and planted lawns, until people nearly forgot that under the acres of grass and trees and shrubs lay mountains of sand.

The invention of Golden Gate Park was an amazing engineering and horticultural accomplishment, but it was not an environmental one—at least not in the sense of conserving native natural resources. If the California Native Plant Society (CNPS) had existed then, it would never have allowed Hall to spread tons of exotic barley seed over the dunes as part of his plan to “reclaim” them. The barley achieved what Hall wanted—to create favorable conditions for the thousands of alien trees and shrubs he would soon plant. And yet the CNPS now meets in Strybing Arboretum and botanists love the park. Everyone does. It would seem as silly to criticize the park’s blue gum trees for being out-of-towners as it would be to criticize most of us for being exotics. The park is as much an urban invention as the parking lot or the shopping mall, only much better. There is nothing wild about it…except what goes on there.

Nancy de Stefanis is the Director of San Francisco Nature Education, a group that leads nature studies in Golden Gate Park. She is perhaps best known for her discovery in 1993 that great blue herons were nesting in the park’s Stow Lake, and for her efforts to protect those nests from raccoons and other threats (California Wild, Summer 2002). A few days ago, on an early morning walk in the park, she saw a great blue plucking endangered red-legged frogs out of a pond. She saw feral cats, gray squirrels, and a three-foot-long box turtle. All this, and she had intended to look for birds! She saw those, too: an albino robin, varied thrushes, ravens, white- and gold-crowned sparrows, and a courting pair of red-tail hawks doing loopty-loops and dives. She saw a bevy of seven California quail running through Strybing Arboretum, the only population of quail left in the park. “It was incredible,” she said. “We saw 25 bird species easy.”

Anyone who’s spent much time in Golden Gate Park has wild stories to tell. My own favorite took place a few years ago, after I’d pulled an all-nighter at the magazine and was tired enough to sleep dangling off of Half Dome. Half Dome was too far away, so I walked a few hundred yards east on Middle Drive and up a tiny path back to Lily Pond. I walked the perimeter looking for a place to sleep. The pond had steep vegetated banks all around except for a small, reasonably sloped patch of dirt on the east side. I kicked away some guano, put a newspaper under my head, and fell asleep.

I woke up half an hour later; something soft was tickling my arm. I raised my head slowly to find myself surrounded by mallards. There must have been 20 and they filled every inch of the dirt patch around me. One nestled comfortably between my outstretched arm and my torso.

Each duck had its head swiveled and tucked into the feathers on its back. When I lifted my own head, the birds next to me raised and turned theirs as well, and a couple of them stirred, causing a chain reaction of awakenings in the ultimate morning-after surprise. No one lost his or her cool, though. I tiptoed out of their realm and headed back to work, downy feathers clinging to my sweater and my hair. That was how I became the Man Who Sleeps with Ducks.

Raymond Bandar, a field associate in the Academy’s Department of Ornithology and Mammalogy, grew up in Golden Gate Park and has a thousand wild stories to tell. He says that in the 1940s, when he was a boy, it was a “biologist’s paradise.” He spent long summer days hunting for garter snakes, alligator and fence lizards, bush rabbits, Pacific pond turtles, weasels, and red-legged frogs. Peacocks roamed free in the park, and there Bandar courted his wife, Alkmene. They took long, moonlit walks from the beach to the park’s entrance on Stanyan, stopping to spoon in the Valley of the Moon.

Most old-timers like Bandar long for the good old days, when the park was “less manicured.” It’s hard to tell if this is because the park used to be wilder, or because the old-timers were. But there’s no doubt that the park refuses to cooperate by holding still. As Heracleitus said, “You can’t step into the same river twice.” (Or as Cratylus, Heracleitus’s follower, trumped, ‘You can’t step into the same river once.’)

The park’s “ecosystems” are a moving target, changing with park administrations and larger cycles of growth and death. In recent years, the Parks Department has cleared away much of the undergrowth that had been protecting ground-nesting birds—and homeless humans. Other forces originate outside the park but have an influence by increasing, diminishing, or eliminating the animals that live within. If there are no peregrines anywhere else in California, there aren’t going to be any in Golden Gate Park either.

Late Academy ornithologist Luis Baptista used to talk about the 1980s in the park. California quail were common then, running here and there on urgent business. Native bush rabbits lived here, too. The rabbits are now gone and the quail nearly so. I’ve heard speculation that the rabbit population may have collapsed partly under predation by humans, too. But both are most likely victims of the park’s shifting food web.

Raptors returned when their populations rebounded from the DDT poisoning that largely ended four decades ago. Recently, red-tailed, red-shouldered, and Cooper’s hawks have moved in, according to Douglas Bell, a biology professor at California State University in Sacramento. The park is now “probably a sink for white-crowns” he says. “It draws them in, but because of the intense predation, their survivorship is very low.” But even bigger players in the songbird and quail equation are the park’s resident feral cats. According to Baptista and Bell, white-crowned sparrow deaths in the park are probably due mostly to cats.

In addition to feral cats and other predators, floral changes affect park wildlife as well. Many of the Park’s trees are reaching climax now, says Peter Dallman, who is writing a natural history guide to Strybing Arboretum. The big trees are falling or are being cut down in anticipation of their natural collapse. The pygmy nuthatch, a bird that nests in the park’s climax Monterey pine forest, will likely flee the park when those trees come down.

Today, raccoons are plentiful. So are ravens, though Bell remembers that not long ago no ravens nested here. Exotic cowbirds have arrived, too; they lay their eggs in the nests of other birds, which then raise ravenous cowbird chicks, often at the expense of their own young. Squirrels are multiplying out of control, says Dallman.

Cat populations are strong, but not as strong as their political lobby. The bison herd, introduced to the park in 1894, remains stable at eleven. But the reintroduced grizzly bears (Bandar remembers when there were at least two sad grizzlies in cages in the park’s southeast corner) are long gone. The last Golden Gate grizzly, Monarch, is now stuffed and on exhibit in the Academy’s Wild California Hall.

These changes and conflicts raise some uncomfortable questions about the park and its mission. By what standard can the costs or benefits of these changes be measured? Should we be trying to restore Golden Gate Park systems and populations that are at bottom artificial? Should we simply maintain the species we prefer, and get rid of, or let slip away, the unpopular ones? Should “maximum diversity” be the goal, and mandate mediations of conflicts that arise between incompatible species, such as cat and quail?

To maintain quail in the park for the long term, for instance, would require “intensive and sustained human intervention,” says Bell. “We’d have to rely on the full range of wildlife management techniques.” Predation would have to be monitored and protective habitat cultivated. New quail stocks would have to be introduced, and electrically charged wire cages (through which the quail could fit, but not cats or ravens or raptors) could be built around nesting areas. But without heroic and constant human support, the quails’ days in the park are numbered.

Like its creation, the park’s future will be shaped by human invention, its progression determined by our priorities.”

Golden Gate Park, aerial view. Gnu Free Documentation

(1) Gordy Slack, “Evolution in the Park,” California Wild, Spring 2003 [reprinted with permission of author]

Nature is resilient, animals can adapt to change

We are always puzzled by the widespread belief amongst native plant advocates that native animals are dependent upon native plants and the corollary argument that non-native plants are invasive because they have no predators.  We suspect that one of many reasons for this assumption is a lack of understanding about evolution.  That is, if you believe that animals are unable to adapt to new plant species, then you probably assume that the new plant species are not useful nor are they prey to native animals.  

The Gallup Poll tracks the opinions of Americans regarding evolution.  In 2010 a surprisingly small percentage of Americans (16%) believed in the evolution of man unguided by God.  Even amongst those who believe in evolution, it is often seen as an historical process that moves too slowly to be perceived.  Science has only recently found living examples of on-going evolution:

“A growing appreciation that organic evolution, like mountain building, is an ongoing rather than simply historical process has stimulated an infusion of evolutionary thinking into mainstream ecology.”(1)

The Soapberry Bug

Soapberry bug on balloon vine. Scott Carroll, UC Davis

The soapberry bug (Jadera haematoloma) is an example of a native insect that has changed genetically in less than 100 generations over a period of 20 to 50 years in response to a new non-native plant host. 

The soapberry bug is named for the plant upon which it depends for both food and reproduction, the Sapindaceae family (‘Soapberry’ family).  In southern Florida, the native host plant of the soapberry bug was the balloon vine (Cardiospermum corindum).  As its name suggests, its seed is large and round.  The soapberry bug that feeds on that seed has a large jaw–up to about 70% of its body length–that enables it to get the seed into its mouth. 

In the 1950s a new member of the Sapindaceae family of plants was introduced to southern Florida, the Chinese flametree (Koelrueleria elegans) as an ornamental.  Its seed is much smaller than the seed of the balloon vine.  The soapberry bug quickly made a transition to its new host and over time it evolved several adaptations to it.  The jaw of the soapberry bug that feeds on the flametree is much smaller, as little as 50% of its body length. 

The life cycle of the soapberry bug has also changed and is better suited to the brief, simultaneous availability of seeds of its new host, the flametree:  “The flametree-specialized race [of soapberry bug] has a briefer development time (and thus an earlier age at first reproduction), greater fecundity, and exhibits greater expenditure of effort towards reproduction than the balloon vine race of J. haematoloma from which they originated.”(2)

In south Florida, the soapberry bug now has two genetically distinct races that are suited to their specific hosts–one native, one not.  The original race has not changed where its host is the native balloon vine.  The soapberry bug is not very mobile, so these two populations are physically separated.  This is an example of increased genetic biodiversity in response to an introduced plant. 

There are 400 genera and 1,500 species of plants in the Sapindaceae family all over the world(3), so we should not be surprised to find many other examples in the scientific literature of insect hosts that are adapted to them, whether they are native or introduced plants, as well as differences in those insects that are suited to the specific plants and/or their locations.  The soapberry bug isn’t an isolated example of an insect that has rapidly evolved to adapt to new hosts.  On the other hand, science cautions us against generalizing to all insects. 

We offer our readers three sources of information, depending upon their scientific knowledge.  The National Public Radio story about soapberry bug evolution is addressed to the layman.  At the opposite extreme, the citation in our footnotes is addressed to scientists with expertise in genetics.  The middle ground, from which we drew most heavily, is a website about soapberry bugs

Cheerful conclusion

As we often do, we conclude cheerfully that nature is remarkably resilient.  Although nature is less fragile than native plant advocates believe it to be, we don’t take that as an invitation to abuse it.  We treat nature with respect, and that includes taking care of what is here, whether it is native or non-native.  


(1) 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

Evolution didn’t stop in 1492

One of the most appealing of the many arguments used by native plant advocates in support of their ideology is the evolutionary concept of “co-evolution.”  Co-evolution is defined by Forgotten Pollinators(1) as “The idea in evolutionary ecology that certain mutualistic organisms have directed or redirected each other’s evolutionary trajectory.”  The implication of this theory is that plants and animals that have evolved together are interdependent and that loss of a particular plant will result in the loss of the animals with which it evolved.  Native plant advocates sometimes describe these relationships as “a lock and key,” implying that native plants and animals fit together in a mutually beneficial relationship which is exclusive. 

Those who believe this theory are obviously deeply committed to saving all native plants because they believe the loss of any single plant would inevitably lead to the loss of the animals that are dependent upon it.  Likewise, non-native animals are often exterminated based on the assumption that they compete with native animals and that loss of native animals will lead to the loss of native plants.

There are three problems with this theory. First, while there are some examples of truly exclusive co-evolved relationships in which both species cannot survive without the presence of the other, the number of such relationships is quite small.  Second, even these relationships are not immutable because evolution has not stopped, and therefore other species may develop mutualistic relationships with the prior exclusively mutualistic species.  And third, organisms are opportunistic and are quick to take advantage of any new opportunities, meaning that many interactions observed between species in the wild are not co-evolved at all.  For example, the honeybee pollinates hundreds of species of North American plants and it didn’t evolve with any of them (since honeybees were introduced into North America from Europe, which had introduced them from Africa).

Why is “co-evolution” rare in nature?

When defining “co-evolution” Forgotten Pollinators adds this caveat, “Good examples of truly reciprocal coevolution are difficult to find.”  Although the concept of “co-evolution” has a certain logical appeal, the explanation for why it is rare in nature is even more logical:  it is a risky survival strategy in a world that is constantly changing.  If, for example, the specific plant upon which a specific animal depends doesn’t bloom or doesn’t return from its dormant phase because of a sudden, even temporary, change in the climate, the animal that is dependent upon that plant is out of luck.  Since such fluctuations of environmental conditions are common, natural selection does not favor the animal that is restricted to a single plant for which there is no substitute.  Such exclusive relationships therefore do not persist in nature.

Nature provides “back-ups” that will enable plants and animals to respond to fluctuating environmental conditions.  For example, few plants have a single pollinator.  Most have several, usually of several different types.  One bee may be a particularly effective pollinator of a particular plant, but that plant is probably also visited by a fly, a butterfly, a bird, a beetle, etc.  As humans do, plants and animals don’t just give up when conditions change.  We all look for and usually find other alternatives. 

Native bumblebee gathering nectar and/or pollen from non-native cotoneaster. Albany Bulb, Albany, California

“Evolution right under our nose”

The Science Section of yesterday’s New York Times features an article about evolution of animals in New York City In the most densely populated city in the country, founded nearly 400 years ago, 74% of the native plant species that existed when the city was founded in 1624, still exist there.(2)  San Francisco has an even lower rate of extirpation of its native plants since it was founded in 1850.  Ninety-seven percent of the 714 plant species known to exist in San Francisco in 1850 are still found in San Francisco

Midtown Manhattan as seen from the Empire State Building. Creative Commons Attribution Share Alike

The fascinating article in the New York Times reports that the ability of animals to evolve in response to changing environmental conditions has enabled their survival in the urban environment. 

The white-footed mouse is an example of a native animal that is thriving in New York City.  The urban environment creates isolated urban islands, such as parks.  Scientists find that virtually every park in New York City has a population of genetically unique white-footed mice.  In fact, “The amount of [genetic] differences you see among populations of mice in the same borough is similar to what you’d see across the whole southeastern United States,” according to the scientist studying this mouse in New York City.

It’s difficult to imagine a more altered, artificial environment than the road medians on Broadway on the Upper West Side of Manhattan, which are composed of landfill used to cover the subway tunnel.  However, scientists have found 13 species of ants living in some of these medians.  Nine of the thirteen species are native. 

Nature is opportunistic and resilient.  It isn’t necessary to eradicate non-native plants and animals to ensure the survival of native plants and animals.  What greater laboratory to illustrate the resilience of nature than New York City? 


(1) Buchmann and Nabhan, The Forgotten Pollinators, Island Press, 1996

(2) Duncan et al, “Plant traits and extinction in urban areas:  a meta-analysis of 11 cities,” Global Ecology and Biogeography, July 2011