Doug Tallamy – Conservation Sense and Nonsense

Nativism turns a blind eye to climate change

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

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

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

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

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

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

Where did Ms. Renkl learn these myths?

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

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

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

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

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

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

Treat the cause, not the symptom

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

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

Photo credit for featured photo: Garden Rant, Marianne Willburn

Doug Tallamy’s Blame Game

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

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

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

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

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

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

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

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

Why are insect populations declining?

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

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

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

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

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

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

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

Why are native plant populations declining?

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

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

My New Year’s Wish

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

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

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

Re-upping on Reality

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

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

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

Prefatory Comments

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

References:

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

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

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

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

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

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

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

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

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

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

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

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

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

In conclusion

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

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

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

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

Cedar Waxwings in crab apple tree. Wikimedia Commons

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

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

Generalizations unsupported by evidence

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

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

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

In conclusion

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Soapberry bug on balloon vine. Scott Carroll, UC Davis

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

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

Monachs in eucalyptus, Pacific Grove Museum

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

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

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

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

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

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

_______________________________________________

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

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

Study design determines study findings

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

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

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

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

Ecological Laboratory Science

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

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

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

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

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

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

Limitations of laboratory studies

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

The study results are not relevant to all insects. The findings apply only to the larvae stages of moths and 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.

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.

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

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.

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)

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.

“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
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 5^th, 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?

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.

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

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 20^th 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

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 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.

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