Contradictions of the native plant movement

In a recent article in Bay Nature, the author tells the story of two California native trees and proposes to answer the question, “Now the question is, where do [these trees] belong?”  It’s a thorny question and one that illustrates one of the contradictions of the native plant movement.

The definition of “native” in California is based on the arrival of Europeans near the end of the 18th century, specifically 1769 in the San Francisco Bay Area.  Wherever plants or animals lived in 1769, they are considered native in the Bay Area.  If they were introduced or arrived after 1769, they are considered non-native, and in many cases designated as “invasive.”  Status as “invasive” makes them targets for eradication.

Monterey pine is a case in point. It has a small native range around Monterey, California, just 115 miles south of the East Bay. It was widely planted in the Bay Area by early settlers because the climate is similar to Monterey and much of the Bay Area was treeless. It is well adapted to conditions and thrives here.  Yet, it is being eradicated by most public land managers in the Bay Area because it is, strictly speaking, non-native according to the official classification of non-natives.

Bird’s eye view of San Francisco in 1868. US Library of Congress.  About half of San Francisco was treeless sand dunes before early settlers planted the non-native trees that now live there.

There is fossil evidence that Monterey pine has lived along the coast of California many times in the past, far outside its small present “native” range.  The evidence of its prehistoric past in the Bay Area has not saved it.  It wasn’t here in 1769, so it’s gotta go to suit the purist agenda of our public land managers.

But, wait!  Maybe our public land managers don’t have such a purist agenda. The same public land managers who are eradicating Monterey pine in the Bay Area are also planting Torrey pines and Catalina ironwood, which are native to the Channel Islands, off the coast of Southern California. These tree species weren’t here in 1769, nor were they here in the distant past.  In Monterey, where Monterey pines are native, Torrey pines are considered “invasive,” according to the interview with a tree advocate in Monterey who is quoted by the Bay Nature article.

Torrey pine, San Diego. Wikimedia commons
Catalina ironwood – Catalina Island. Wikimedia Commons

Where I have observed Torrey pines and Catalina ironwood being planted by East Bay Regional Park District and other public land managers, they seem to be doing well. Therefore, I have no objection to their being planted here, although they are not “native” as defined by local native plant advocates. I have one straight-forward criterion for “where trees belong:” they belong where they will survive.

The author of the Bay Nature article demurs, “Should we view Monterey pine as unwanted and possibly damaging invaders or welcome them as neo-natives? I couldn’t decide.” That seems to me an easy question to answer:  It makes no sense to destroy healthy Monterey pines that have lived in the Bay Area many times in the past, while simultaneously planting trees that have never lived here. If trees are adapted to present environmental conditions, they should be welcome here.

Nativist Dilemmas

The native plant movement is fraught with conundrums such as the vexing question of where Monterey pine and its close relative, Torrey pine, belong.

  • There are many unresolved debates about exactly where native ranges are. The rusty crayfish is “probably the most reviled crayfish species in North America,” yet there are as many opinions about its native range as there are publications about the species.  Pike is eradicated in Ireland, yet a recent genetic study revealed that there are actually two pike species, one of which is likely native and unique to Ireland.  (1)
  • There is little agreement among invasion biologists about the difference between non-native and “invasive.” Some consider all non-native species potentially invasive, some do not. (1) The California Invasive Plant Council recently designated nearly 100 plant species as invasive (in addition to 200 plants already on the list), despite the admission that they are not invasive in California.  Given that the behavior of plants depends to a great extent on local climate conditions, we cannot assume they will be invasive in California. When a plant is designated as “invasive” it becomes a target for eradication.  It is therefore a designation that should be used sparingly.
  • Public land managers in the Bay Area kill as many native plants as non-native plants because of their goal of recreating the landscape that existed here prior to the arrival of Europeans. That goal requires that early-succession native shrubs be killed when they naturally encroach on grassland if it is not burned periodically or grazed by animals.  Native coyote brush is eradicated as often as non-native broom when it “invades” grassland.  Are we trying to preserve native species or historical landscapes?  Those are different goals and they are often contradictory.

Invasion biology is an ideology, not a science

These—and many other—contradictions are symptoms of a pseudoscience, struggling to make sense, but failing to do so.  Invasion biology made the initial mistake of picking a specific point in time to define the “ideal” landscape.  Because nature is dynamic, in response to a constantly changing environment, the arbitrary selection of a static landscape will always be a source of contradictions.  The environment has changed radically since 1769 and even more radically since the 400-year baseline used on the East Coast.  We cannot expect to recreate that historical landscape because we cannot recreate the environment in which it existed. 

  1. C. Guiasu, C.W. Tindale, “Logical fallacies and invasion biology,” Biological Philosophy, Sept 2018, 33(5)

 

 

Nativism in the Natural World

Invasion biology is the scientific discipline that spawned the native plant movement.   Charles Elton published a book in 1958 that is considered the origin of the modern version of invasion biology, although there are precursors centuries earlier.  These are the basic tenets of modern invasion biology:

  • Plants and animals that are “native” to a specific location are considered members of an ideal ecosystem that have co-evolved over thousands of years so that members of the community are dependent upon one another.
  • Plants and animals introduced to an ecosystem by humans are assumed to disrupt the equilibrium balance of the community and threaten its existence because introduced plants and animals do not have predators that would control their spread.  All introduced plants and animals are therefore considered potentially invasive.
  • Animals are believed to be dependent upon the plants with which they evolved—and only these plants–and these mutually exclusive relationships are disturbed by the introduction of new plants and animals. 
  • Adaptation and evolution of introduced plants and animals is believed to be too slow for introduced plants and animals to successfully enter the food web.
  • Native members of the ecosystem are presumed to be inherently superior to introduced plants and animals.  Invasion biology does not acknowledge that introduced plants and animals are often functional members of the ecological community.
  • Native ecosystems are said to be in “balance” and introduced species are presumed to cause “imbalance.”  Introduced species must be eradicated to restore balance to the ecosystem, presumed to be the ideal for a particular location.

Hundreds of empirical studies have been conducted since the 1960s to test these assumptions.  Little scientific evidence has been found to support them. Current knowledge of ecology explains why the assumptions of invasion biology are mistaken. 

What is native?

The native plant movement defines native as the plant species that lived in a specific location prior to the arrival of Europeans.    In the San Francisco Bay Area, “native” is defined by native plant advocates as the plants and animals that lived here prior to 1769 when Europeans first laid eyes on San Francisco Bay.  When Europeans arrived, the San Francisco Bay Area was already occupied by indigenous people who had arrived approximately 10,000 years earlier. 

The arbitrary selection of the pre-European settlement period to define the ideal landscape was based on the mistaken assumption that the indigenous human population had not radically altered the land. Anthropological and paleontological research informs us that the landscape was essentially gardened by the indigenous population to provide food and cultural implements. 

Pomo gathering seeds, 1924. Smithsonian photo archive

The landscape found by Europeans at the end of the 18th century was not “natural.”  It was altered by humans to serve humans who lived as hunters and gatherers.  Since modern society no longer hunts and gathers for its food and shelter, the landscape that served that lifestyle cannot be maintained without mimicking the land management practices of native people such as frequent burning of the landscape and grazing by animals.  Indigenous people in California did not have domesticated animals (except dogs), but the grassland was grazed by wild deer, elk, and antelope. 

Plants and animals have migrated around the world without the assistance of humans since life began.  The seeds of plants are carried in the stomachs of migrating birds and on the winds of storms.  Animals, including humans, move to wherever they can find what they need to survive.  Migration is natural and often necessary for survival.  Making a distinction between species moved by humans and those moved by natural forces is pointless and usually impossible to distinguish. 

Climate change renders the concept of “native plants” meaningless because when the climate changes, the vegetation changes.  The plants that live in tropical climates will not survive in arctic cold and vice versa.  Introduced plants are often better adapted to current climate conditions than their native predecessors because the climate has changed and it will continue to change. 

Mistaken assumptions about evolution

Animals rarely depend upon a single plant species for survival.  Such mutually exclusive relationships rarely exist in nature because they are evolutionary dead-ends. Animals can, and often do, adapt quickly to changes in the environment.  Transitions from native to introduced plants are routinely made by animals, including humans.  Indigenous hunter/gatherers quickly incorporated plants introduced by European settlers into their diets.  Plants in the same family and genus are often chemically similar, making the transition more likely. 

Native plant advocates assume that evolution only occurs slowly, over thousands of years, but evolution can be faster than they assume.  Rapid environmental change accelerates the speed of evolution because extreme weather events caused by climate change increase the speed of natural selection, the primary tool of evolution.  When cataclysmic events such as hurricanes, droughts, floods, extreme temperatures kill many members of a species population, these are selection events in which the fittest members survive to breed and the next generation inherits the genetic traits that helped their parents survive.  The classic example of this principle is the finches in the Galapagos Islands who died if they didn’t have big enough beaks to eat the seeds of the only plant that survived extreme drought.  The next generation of finches had bigger beaks. 

Darwin’s finches are an example of rapid evolution

Evolution occurs when genetic changes enable future generations to inherit the genetic change.  Adaptation occurs when animals respond to environmental challenges by changing behaviors that aren’t necessarily inherited by the next generation.  Adaptation to changed environmental conditions is even more rapid than evolution and equally effective to ensure survival. Genetic changes are not required for an insect to make the transition from a native host plant to a chemically similar introduced plant.   Extreme temperatures require that plants and animals move to more temperate climates.  “Native” ranges must change to survive changes in the environment.  A plant or animal that cannot survive extreme heat will migrate (if it can) into regions where temperatures are not as warm.  They should not be prevented from doing so. 

Adaptation to Climate Change. IPCC

Plant and animal species with large populations and short lives, such as insects, evolve more quickly.  This more rapid pace of evolution enables a more rapid transition from native host plants to closely related introduced plants.

soapberry bug made transition from native to non-native balloon vine in 20-50 years. Scott Carroll, UC Davis

Nativism and the native plant movement

The native plant movement is based on the belief that native plants are superior to introduced plants, that native plants are somehow “better” than immigrant plants.  That assumption of superiority is the definition of nativism.  It is as specious an assumption in the natural world as it is in human society and it is equally dangerous. 

There are pros and cons to everything living in the natural world and there is no right answer to the question of which species is “best.” When evaluating introduced plants, nativists consider only the negative aspects. They refuse to acknowledge that there are also advantages and a death verdict should take both into consideration.  For example, native plant advocates want all eucalyptus trees in California cut down because they were planted here after European settlement.  This negative judgment of eucalyptus does not take into consideration that 75% of monarch butterflies who spend the winter in California use eucalyptus trees for their safe haven. Also, eucalyptus blooms in California from November to May, providing nectar to butterflies, hummingbirds, and bees at a time of year when native plants are not blooming.  Eucalyptus trees are also nesting homes of owls and other raptors.  Cutting down eucalyptus trees simply because they are not native in California ignores the many benefits they provide to wildlife. 

Monarch butterflies over-winter in California’s eucalyptus groves

Confusing cause and effect

The native plant movement mistakenly assumes that the mere existence of introduced plants threatens the existence of native plants.  They believe that native plants will magically emerge if introduced plants are eradicated.  They have spent 25 years eradicating non-native plants and do not seem to have noticed that native plants have not returned.  They make this mistake because they do not acknowledge the changes in the environment that make non-native species better adapted to current environmental conditions. 

Many of the changes in the environment that are inhospitable to native species are caused by structural changes made to accommodate human activities, not by introduced species.  For example, all the major rivers in California have been dammed to prevent floods and store water for use during the dry season.  These dams have fundamentally altered the ecology of our rivers.  There are no longer cleansing spring floods that clear rivers of accumulated mud and vegetation.  Channeled rivers are deeper and warmer.  Salmon can no longer get to their spawning grounds past the dams.  The altered structural conditions are more hospitable to bass than to trout.  Aquatic plants from tropical regions become invasive in warmer water.  None of these conditions are reversed by spraying aquatic plants with herbicide or killing introduced bass.

Butterfly bush (buddleia) is now being eradicated by nativists.. butterflybush.com

Wherever “invasions” are observed, no thought is given to why.  Instead, a convenient plant or animal scapegoat is found and poisoned.  That death sentence doesn’t reverse the underlying reason for the invasion.  Therefore, the invasion persists.  Society is unwilling to make the sacrifices, even inconveniences, needed to address the underlying cause of the “invasion.”  We have done little to address the causes of climate change.  We are unwilling to destroy the dams and the system of supplying water to serve agriculture needs.  Invasions are the symptom, not the cause of the changes in nature.

Conservation Sense and Nonsense

You are receiving this announcement of our changed focus and new name because you are a subscriber to our original Million Trees blog.  This is our revised mission for the Conservation Sense and Nonsense blog:

Conservation Sense and Nonsense began in 2010 as the Million Trees blog to defend urban forests in the San Francisco Bay Area that were being destroyed because they are predominantly non-native.  In renaming the Million Trees blog to Conservation Sense and Nonsense, we shift the focus away from specific projects toward the science that informed our opposition to those projects. 

Many ecological studies have been published in the past 20 years, but most are not readily available to the public and scientists are often talking to one another, not to the general public.  We hope to help you navigate the scientific jargon so that scientific information is more accessible to you.  If this information enables you to evaluate proposed “restoration” projects to decide if you can or cannot support them, so much the better.

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

Since 2010, we have learned more about the ideology of invasion biology that spawned the native plant movement and the “restoration” industry that attempts to eradicate non-native plants and trees, usually using herbicides.  We have read scores of books and studies that find little scientific evidence in support of the hypotheses of invasion biology.  We have studied the dangers of pesticides and the growing body of evidence of the damage they do to the environment and all life. 

Meanwhile, climate change has taken center stage as the environmental issue of our time.  Climate change renders the concept of “native plants” meaningless because when the climate changes, vegetation changes.  The ranges of plants and animals have changed and will continue to change to adapt to the changing climate.  Attempting to freeze the landscape to an arbitrary historical standard is unrealistic because nature is dynamic.  Evolution cannot be stopped, nor should it be.

Destroying healthy trees contributes to climate change by releasing stored carbon into the atmosphere.  Both native and non-native trees store carbon and are therefore equally valuable to combat climate change.  Native vegetation is not inherently less flammable than non-native vegetation.  There are advantages and disadvantages to both native and non-native vegetation. 

The forests of the Earth are storing much of the carbon that is the primary source of greenhouse gases causing climate change.  Deforestation is therefore contributing to climate change.  By destroying healthy trees, the native plant movement is damaging the environment and its inhabitants.

Housekeeping

All of the articles on the Million Trees blog are still available in the archive on the home page.  The search box on the home page will take you to specific subjects of interest.  Visit the pages listed in the sidebar of the new home page for discussion of each of the main topics by clicking on the links above.  Readers who subscribed to the Million Trees blog will receive new articles posted to Conservation Sense and Nonsense unless they unsubscribe.  Thank you for your readership.  Your comments are welcome and will be posted unless they are abusive or repetitive. 

Another innocent introduced plant with a bad rap

Professor Mark Davis, known as “a friend to aliens,” is one of a growing number of academic scientists who are critical of invasion biology.  His research has exonerated some of the introduced plants that are accused of harming native plants. Garlic mustard is an example.  Professor Davis has briefly summarized his research of garlic mustard for publication on Million Trees.

The Nature Conservancy says, “…garlic mustard [is] one of the ten most destructive invasive species in Indiana today… It…displaces native or other desired plants in a relatively short period of time.” Although the Nature Conservancy description of the lifecycle of garlic mustard is identical to Professor Davis’s description, it reaches the opposite conclusion about its effect on native plants.  The Nature Conservancy describes laborious methods of eradication and concludes that herbicides and prescribed burns may be necessary. 

It is precisely that accusation that non-native plants “displace” native plants that is at the heart of the debate about so-called “invasive species.”  Nativists do not believe that native and non-native plants can co-exist.  In fact, when empirical studies test that hypothesis, they rarely find evidence of such displacement.  The fact is, the environmental conditions that support native plants also support non-native plants and the result is usually more biodiversity, rather than less. 

The first sighting of the garlic mustard herb in the U.S.A. was in 1868 in Long Island, New York. Garlic mustard was intentionally introduced into the northeastern United States for food, erosion control, and medicine. Pretty isn’t it? It’s tasty as well.

If native plants are not doing well, compared to non-natives, it is usually for other reasons, such as changes in the climate or available water or the arrival of a new insect or pathogen.  It is rarely because of the arrival of new plants.  Killing non-native plants with herbicide is simultaneously killing native plants and damaging the soil they live in.

There is room for everyone in the human world and in the natural world.  Nativism in the human realm is as unnecessary as it is in the natural world.  Fear and anger about human immigrants to America is strongest in places with few immigrants.  I am fortunate to live in a place with many immigrants.  I can see first-hand how they enrich my community.  I also see first-hand how introduced plants are not doing any harm, but the futile efforts to kill them with herbicides IS doing a great deal of harm to every living thing in the environment.

Million Trees


Garlic mustard, Alliaria petiolata (Brassicaceae), is a European biennial plant (completing its life cycle over 2 years) now common in many Midwestern and eastern North American forests. It is self-pollinating, has abundant seeds that are viable in the soil for long periods of time, and can grow in a variety of forest environments.  These characteristics allow garlic mustard to spread easily. Many consider it to be an invasive species that displaces native herbs and inhibits tree seedling growth and survival.

For the past six years I have worked with colleagues and students to determine to what extent garlic mustard is negatively affecting native herbs and tree seedlings in an oak forest in east-central Minnesota.  Based on our monitoring of garlic mustard and the other plant species, garlic mustard appears to be acting similarly to other species.

Specifically, garlic mustard and the other common native herbs seem to be changing in abundance largely independent of one another. In other words, changes in abundance of one species is unrelated to changes in other.  In six years of study, we have not been able to document any substantial effects by garlic mustard on other plant species, positive or negative.

In fact, the best predictor of garlic mustard presence is high diversity of native plants.  The most likely explanation for this fact is that all the species, garlic mustard included, are simply establishing in microsites favorable to plants in general.  The same conditions that benefit native plant species also probably benefit non-native plant species.

Overall, our findings are not consistent with the common claim that garlic mustard is a noxious invasive species responsible for the decline of many North American native forest herbs. Rather, our findings are more consistent with other recent studies and reviews that have concluded that garlic mustard is primarily responding to ecological changes in North American forests.

In other words, at many sites garlic mustard is not a significant driver of change but rather is a passenger of change.  What might be the drivers of change, the real causes of declines of many native wildflowers?  Evidence points to both earthworms and white-tailed deer.

Earthworms, which have been introduced from Europe and Asia, have drastically reduced the abundance of litter in these forests.  The near absence of litter has been shown to negatively impact many of the native species.  However, eradicating earthworms would be impossible and attempting to do so would cause more damage to the environment.

Due to the abundance of habitat, secondary forests, and lack of predators, white-tailed deer populations have exploded throughout many of the Midwestern and eastern forests.  And, native plants are a common food of the deer.  The deer tend to avoid garlic mustard due to its garlicy oils.

Mark Davis, Macalester College


Professor Davis’s studies of garlic mustard:

Davis MA, Anderson MD, Bock-Brownstein L, Staudenmaier A, Suliteanu M, Wareham A, and Dosch JJ.  2015.  Little evidence of native and non-native species influencing one another’s abundance and distribution in the herb layer of an oak woodland.  Journal of Vegetation Science 26:105-112.  PDF

Davis MA, MacMillen C, LeFevre-Lefy M, Dallavalle C, Kriegel N, Tyndel S, Martinez Y, Anderson MD, and Dosch JJ.  2014. Population and plant community dynamics involving garlic
mustard (Alliaria petiolata) in a Minnesota Oak Woodland: a four year study. Journal of the Torrey Botanical Society 141: 205–216.  PDF

Davis MA, Colehour A, Daney J, Foster E, MacMillen C, Merrill E, O’Neil J, Pearson M, Whitney M, Anderson MD, and Dosch JJ. 2012.  The population dynamics and ecological effects of garlic mustard, Alliaria petiolata, in a Minnesota oak woodland. American Midland Naturalist 168: 364-374.  PDF

A Milestone for Million Trees

As the Million Trees blog approaches the anniversary of its eighth year, we are celebrating a milestone. Yesterday, Million Trees reached a total of 250,000 individual views of posts on Million Trees.  We now have over 300 subscribers and we are averaging about 150 views per day.  About 25% of our readers are outside the United States.  Since nativism in the natural world is an international fad, we are gratified that Million Trees is being read by people in other countries.  Million Trees is also proud and grateful for the participation of several academic scientists who have written informative guest posts for Million Trees in the past year.  Thank you, Dr. Matt Chew, Professors Mark Davis and Art Shapiro, and Dr. Jacques Tassin for your help!

Our most popular posts have each been visited by over 10,000 readers.  They are, in the order of their popularity:

  • “Darwin’s Finches: An opportunity to observe evolution in action.”  This article about the speed with which adaptation and evolution occur in a rapidly changing environment is the bedrock of the Million Trees blog.  Nativists mistakenly believe that evolution is much slower than it is.  Therefore, nativists believe plant and animal species are nearly immutable and that they are locked into mutually exclusive relationships, which are, in fact, extremely rare in nature.
  • “Nearly a HALF MILLION trees will be destroyed in the East Bay if these projects are approved.” The Million Trees blog was created to inform the public that nativism is destroying our urban forest in the San Francisco Bay Area.  Our urban forest is composed of predominantly non-native trees.  If they are destroyed, we will not have an urban forest because native trees will not survive in our changed and rapidly changing environment.  Non-native trees were planted here because people wanted trees and native trees existed only in riparian corridors where they were sheltered from the wind and there was sufficient water.
  • “Falling from Grace: The history of eucalyptus in California.”  Because people wanted trees, they planted non-native trees that were capable of surviving in the San Francisco Bay Area.  Non-native trees were valued for nearly one hundred years until nativism got a death grip on our public lands. This article on Million Trees tells the history of why eucalypts were planted and why they “fell from grace.”

In the past year, one of the most popular posts on Million Trees was “Krakatoa:  A case study for species dispersal.”  This post has been viewed by over 7,000 readers.  Understanding how plants and animals were dispersed around the world by natural means–such as by birds, wind, and ocean currents—is another way to realize that the concept of “native vs. non-native” is an artificial construct with little practical meaning.  Plants and animals have always moved and they will continue to move.  In fact, as the climate changes, they MUST move if they are to find the environmental conditions in which they can survive.

Million Trees Commitment

Million Trees will continue to advocate for the preservation of our urban forest in the San Francisco Bay Area.  Our strategy is to inform the public of the many projects that are destroying our forests and to describe the damage that is being done by those projects.  We are particularly concerned about the use of pesticides to eradicate non-native plants and trees.  We are equally committed to providing our readers the latest scientific discoveries that relegate invasion biology to a scientific back-water.  We are hopeful that the gap between public policy and the scientific knowledge discrediting invasion biology will eventually be bridged and bring an end to this destructive fad.

Stevie Nicks, Naturalized Species, and the future of the biosphere

Professor Arthur M. Shapiro, at work, UC Davis

Art Shapiro is no stranger to the long-time readers of Million Trees.  Professor Shapiro is Distinguished Professor of Ecology and Evolution at UC Davis, and a renowned expert on the butterflies of California.  He is the author of a seminal, frequently cited study of California butterflies that reported the results of 30 years of observing butterflies in his research transects. (1)  He summarized this study in his Field Guide to the Butterflies of the San Francisco and Sacramento Valley Regions:

“California butterflies, for better or worse are heavily invested in the anthropic landscape [altered by humans].  About a third of all California butterfly species have been recorded either ovipositing [laying eggs] or feeding on nonnative plants.  Roughly half of the Central Valley and inland Bay Area fauna is now using nonnative host plants heavily or even exclusively.  Our urban and suburban multivoltine [multiple generations in one year] butterfly fauna is basically dependent on ‘weeds.’  We have one species, the Gulf Fritillary that can exist here only on introduced hosts.  Perhaps the commonest urban butterfly in San Francisco and the East Bay, the Red Admiral is overwhelmingly dependent on an exotic host, pellitory. And that’s the way it is.”

 Professor Shapiro has given us permission to reprint his Amazon review of the most recently published critique of invasion biology, Inheritors of the Earth, by Professor Chris Thomas (University of York, United Kingdom).  We recommend Professor Thomas’s book to our readers.  Although it is learned, it is accessible to the general public.  This book is another step forward in the long march to acceptance of the reality of existing landscapes that are adapted to present climate conditions.

 Million Trees


2011 Chris Thomas published a paper in the journal “Trends in Ecology and Evolution” entitled “Translocation of species, climate change, and the end of trying to recreate past ecological communities.” I immediately e-mailed him (April 11, 2011): “I have been delivering the same message in my advanced courses in Community Ecology and Biogeography for years, and have found the students by-and-large highly receptive, especially when they have internalized the overwhelming evidence for wild fluctuations in climate and vegetation since the end of the Ice Age 10-20,000 years ago. But over and over I have been told ‘but of course that is not the Party line…restoration ecology,’ blah, blah….Thank you for giving me a respectable citation, since merely citing one’s self can never do.” He e-mailed back: “…the conservation community in Britain seems mainly to be treating me with bewildered patience! I think that it will take time for everyone to become re-programmed to accept change as a reality.”

But of course change is not only a reality, it is the norm in ecology. Belief in equilibrium states and a “balance of nature” has been a dogma without a rationale beyond sentimentalism for many decades. There are coevolved segments of communities that are intimately synchronized and interdependent (say, figs and fig wasps or yuccas and their moth pollinators), but a great deal of any community is the product not of coevolution but of what Dan Janzen calls “ecological fitting,” whereby things haphazardly thrown together by the vicissitudes of geology, climate or commerce just happen to click. We are surrounded all over the globe by functioning communities and ecosystems with little to no history in geologic time. For about 40 years I have asked my students on their final exam how one might go about telling the difference between coevolved communities and “communities” assembled by chance. It is an exceedingly difficult question.

So this book is an expansion of the TREE [Trends in Ecology and Evolution] paper, and its message is vital. Resources for conservation are limited, and one must prioritize. The vast majority of naturalized alien species are harmless and many may be potentially beneficial. The ones that are genuinely harmful should be fought tooth and nail, but of course we do that anyway–we call it “pest management” and “public health.” The blanket indictment of “invasive species” makes no more sense than the blanket condemnation of human immigrants. Of course, when we say this, Thomas and I and Fred Pearce and “that Marris woman!” are immediately called out as shills for the extractive industries or the nursery industry or the Bilderbergers or the Zelosophists (conspiracy theory villains!!) or some despicable cartel of nature-haters. Pure poppycock. Truth-tellers attract trolls. That’s just the way it is.

Quite a few years ago a group of us took a prominent visiting British ecologist (not Thomas) on a field trip to the Sierra Nevada. We had half a dozen grad students and a few faculty crammed in a van. On the way up, one of the students sort-of apologized for the predominance of naturalized alien plant species in the lower foothill landscape. Our guest demurred forcefully: “Why must you consider this some kind of tragedy? Why don’t you see it as an opportunity for all kinds of evolutionary novelty to arise?” Indeed.
Thomas asks (p. 104): “How long will it be before the environmental police force of ecologists and conservationists is prepared to step back and decriminalize introduced species that have had the temerity to be successful?” An excellent question.

Stevie Nicks got over her fear of change: “Time makes you bolder…children get older…I’m getting older too.” Maybe conservationists can mature after all.

Arthur Shapiro


Professor Thomas’s book is very much in the mainstream.  The Economist magazine included it in their list of important books published in 2017.  It is one of only a few books in the category of “Science and technology” and it is at the top of the list.  The Economist says of the book, “Humans have consigned species to extinction at an alarming rate.  But hybridization and speciation is happening quickly too.  An ecologist at the University of York shows how humans are bringing about a great new age of biological diversity.  Extinctions ain’t what they used to be.”

The New York Times published a review of “Inheritors…” on New Year’s Eve.  The reviewer summarizes Thomas’s main argument: “He argues that new species are arriving and evolving faster than old species are dying out globally…Instead of the sixth extinction, it’s a sixth genesis.”  The reviewer faults Thomas for not portraying the “wonder of nature” and for giving oceans short shrift.  But, the reviewer concludes with this observation about the unhelpful role that humans often play in conservation efforts: “It is human concerns that determine everything here on Earth now.  An animal that arrived in a particular location hundreds or thousands of years ago is fine with us, while a more recent immigrant, like garlic mustard, is cause for alarm and extensive campaigns to extirpate the interloper.  Nostalgia is deadly, as people kill to preserve or restore some ill-remembered but more natural past, and we disdain new species as weeds.”  That observation about human attempts to control nature says it all.  Plants and animals are not to blame for the damage we are doing to satisfy our ideological commitment to the distant past. They are symptoms of change, not the cause of change.

Happy New Year!

Million Trees

Update:  Professor Thomas gave a presentation to the Long Now Foundation in San Francisco on June 19, 2018.  HERE is a video of his presentation.  If you haven’t read his book, his presentation is a good summary of the issues he covers in his book.  MT

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


  1. Arthur M. Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation,110, 413-433, 2003

Name-calling as a substitute for scientific evidence

In the 20-plus years we have advocated for the preservation of our urban forest, my collaborators and I have been accused of many nefarious motivations and deeds.  Here is a small sample of what we have been accused of:

Discrediting one’s opponents is a standard debating tactic and we are neither surprised nor dissuaded by such name-calling.  So, why are we raising the issue today?  Because this name-calling has migrated into the realm of academic science.  We find that shocking because academia is a place where we expect reason to prevail and debates to be based on evidence, rather than ad hominem attacks.

Academic scientists in New Zealand resort to name-calling

We have published several articles about the projects that are dumping rodenticides on islands all over the world to kill animals believed to be the predators of birds.  The most aggressive projects are found in New Zealand where rodenticides have been aerial bombed on small islands for over 60 years.  Recently New Zealand has made a commitment to expand that program to the mainland of New Zealand to kill all wild mammals that have been introduced by humans for over 700 years. 

New Zealand intends to be “predator free” by 2050.  As you might expect, many people in New Zealand object to this program because rodenticides are an indiscriminate killer of animals, such as the native species of parrot, the kea, and many domestic animals such as dogs.  There are other concerns as well, such as the feasibility of such an undertaking and the toxicity of rodenticides to the environment and to humans.

Defending New Zealand’s native parrot, the kea

One of the authors of that aggressive program is an academic at University of Auckland in New Zealand, James C. Russell.  His defense of his program and the academic discipline of invasion biology on which the project was based was published by an academic journal(1)  It is an unusual defense, one that we wouldn’t expect to find in an academic journal, because it does not use scientific evidence to defend the annihilation of non-native animals.  Rather it accuses those who question those projects of having ulterior motives:   “Where evidence is disregarded, or motivations are disingenuous, arguments against [the negative impacts of] invasive alien species take the form of science denialism,” which Russell defines as “the rejection of undisputed scientific facts” such as the causes of climate change or the risks of smoking tobacco.

Russell then tells us what he believes motivates critics of invasion biology:

  • “Science denialism typically originates from groups with a vested interest in opposition to the scientific consensus…” In other words, the profit motive explains the criticism of invasion biology, in Russell’s opinion.
  • “…there is a strong correlation with support of free-market ideologies such as laissez-faire. ” Russell paints critics of invasion biology into a right-wing corner.

Finally, Russell advises invasion biologists how to respond to “denialism” of their projects:  “engage the criticisms but shift the debate from questions of scientific fact to questions of policy response.”  And THAT is at the heart of the matter.  Russell advises his colleagues to emphasize the policy goals, such as exterminating all wild mammals from New Zealand, rather than debate the scientific justification for that project.  Since there is little scientific justification for this project, that seems like good advice.  So, what is this advice doing in a scientific journal?  That is the final question.

Academic scientists respond to Professor Russell

A few months after Russell’s ad hominem attack on academic critics of invasion biology, the same scientific journal published four rebuttals to Russell and Blackburn, written by 11 academic scientists.

  • “We disagree that there is scientific consensus around invasive species, and propose that much debate in this field stems from legitimate disagreement and not from disingenuous rhetoric.” (2)
  • “Constructing an ostensible category of ‘denialists’ reflects invasion biology’s traditional reliance on inflammatory exaggeration to impose and enforce a dichotomous doctrine.” (3)
  • “…society’s spectrum of diverse perspectives, aspirations, and personal trade-offs, which effectively constitute what Russell and Blackburn impugn as ‘vested interests,’ could and should influence society’s debates rather than be discredited.” (4)
  • The organizations and individuals that continue to bemoan biodiversity loss are misleading the public and are directing conservation support away from the foremost problem, the precarious existence of species with remnant populations that are the result of habitat destruction and overexploitation.” (5)

Threats to mammal species. Source: International Union for Conservation of Nature

Another academic publication also published a response to Russell and Blackburn:  “Superficial understanding of the relationships between evidence and values creates exactly the dichotomization between science ‘believers’ and ‘denialists’ that Russell and Blackburn ostensibly seek to avoid.  Rather than ‘standing up for science’ such dichotomization undermines it, rendering aspects of scientific enterprise ‘off limits’ to the kind of rigorous critical (self) examination fostered by science at its best.”  (6)

Update:  James Russell has come to the attention of the US military, according to a press report published on December 4, 2017.  Russell says of his collaboration with the US military, “’And obviously we’re in the business of eradicating entire populations of animals from an island and so they have cocked their ear towards me once or twice.  You don’t have to be a genius to see that there’s potential military application in that.’  In this instance, Russell’s work was being measured for suitability against a US$100 million research pot made available by the United States’ Defense Advanced Research Projects Agency (Darpa).”

As psychologists have informed us for decades, it is a short step from animal abuse to human abuse.  The child who kills animals often becomes a killer of humans as an adult. 

Professor Russell seems to be proud of his collaboration with the US military.  In any case, he stands to profit from that collaboration.  That’s ironic, given that one of his criticisms of the critics of invasion biology is that they have “vested interests” in their criticism.  It seems that a grant from the US Military should be viewed as a “vested interest” in his advocacy for killing animals.  And, as he says, he is “in the business of eradicating” animals.

Little scientific basis for invasion biology

Invasion biology as presently defined by academic science originated in the late 1950s.  It began as a collection of hypotheses about the harm that non-native plants and animals were doing to native plants and animals.  Like all hypotheses, it was based on speculation that had to be tested in the real world.  In the past 25 years, many studies have been conducted that were designed to prove that non-native species are harmful to native species.  With few exceptions, these studies came up empty. More often than not, studies found pros and cons to introduced species, just as we would expect of similar studies of native species.  There is little evidence that invasion biology is an accurate description of how ecosystems operate. 

When academic scientists are forced to resort to name-calling to defend invasion biology it no longer deserves the status of scientific hypothesis.  And when it is discredited as a scientific discipline, it must be just a matter of time before the public realizes that there is no legitimate reason to kill non-native plants and animals.

We don’t see any sign of that paradigm shift, but we are hopeful that public policy will eventually be revised to reflect the reality that has been revealed by scientific studies.  In the absence of scientific justification for eradication projects, they must be treated as public policy decisions.  In a democracy, public policy decisions must reflect the public’s wishes.  In the absence of public support, these projects will continue to cause conflict.


  1. James C Russell and Tim Blackburn, “The Rise of Invasive Species Denialism,” Ecology & Evolution, January 2017
  2. Sarah Crowley, Steve Hinchliffe, Steve Redpath, Robbie McDonald, “Disagreement About Invasive Species Does not Equate to Denialism: A Response to Russell and Blackburn,” Ecology & Evolution, April 2017
  3. Mark Davis and Matthew Chew, “’The Denialists Are Coming!’ Well, Not Exactly: A Response to Russell and Blackburn,” Ecology & Evolution, April 2017
  4. Jacques Tassin, Ken Thompson, Scott Carroll, Chris Thomas, “Determining Whether the Impacts of Introduced Species are Negative Cannot be Based Solely on Science: A Response to Russell and Blackburn,”  Ecology & Evolution, April 2017
  5. John Briggs, “Rise of Invasive Species Denialism” A Response to Russell and Blackburn,” Ecology & Evolution, April 2017
  6. Susanna Lindstrom, “An Interdisciplinary Perspective on Invasive Alien Species,” PLoS Ecology Blog, October 2017

Where is the invasion biology debate headed?

Mark Davis speaking at Beyond Pesticides conference, April 2017

Mark Davis is Professor of Biology at Macalester College in St. Paul, Minnesota.  He is one of the first academic ecologists to publicly express skepticism of invasion biology.  His book, Invasion Biology, was published by Oxford University Press in 2009.  It was the first critique of invasion biology written by an academic scientist. Professor Davis cites the many empirical studies that find little evidence supportive of the hypotheses of invasion biology. 

In 2011, Nature magazine published an essay written by Professor Davis and 18 coauthors entitled, “Don’t Judge Species on their Origins.”  This essay suggested that conservationists evaluate species based on their ecological impact, rather than whether or not they are natives.  The essay initiated an intense debate in the academic community of ecologists that continues today. 

Professor Davis spoke at the Beyond Pesticides conference in Minneapolis at the end of April 2017. (Video available HERE) He described invasion biology as an irrational ideology that is based on nostalgia for the past and a belief that wildlands are being damaged by “alien invaders.”  In fact, the perceived damage is largely in the eye of the beholder, depending largely on one’s membership in a group benefiting from the nativism paradigm, such as chemical manufacturers, conservation organizations, government agencies, and employees.  Some academic careers are also at stake.  Futile attempts to re-create historical landscapes always have the potential to make things worse.  In many instances, it is more sensible to change one’s attitude about the changing landscape than trying to change nature.

We invited Professor Davis to write a guest post for publication on Million Trees.  We asked him to express his opinion on these questions: 

  • Has the status of invasion biology changed much since Nature published your essay 2011?
  • Has increased knowledge of climate change had an impact on the status of invasion biology in academia?
  • What do you think is the future of invasion biology both as an academic discipline and as public policy?

Professor Davis’s guest post addresses these questions.  We are grateful to Professor Davis for his many contributions to our understanding of the fallacies of invasion biology and for his thoughtful guest post.

Million Trees


Competition to define nature

In the past few years, a new perspective has been taking hold in the field of ecology.  Referred to as ‘ecological novelty’ it emphasizes that many factors are producing ecologically novel environments.  Climate change (which includes changes in temperatures and patterns of precipitation), increased atmospheric carbon dioxide, which affects photosynthetic rates, increased atmospheric deposition of nitrogen (the whole earth is being fertilized due to the increased nitrogen we are pouring into the atmosphere), and the introduction of new species are all rapidly changing our environments.

A strength of the term ecological novelty is that unlike the invasion vocabulary it is simply descriptive.  It simply states that ecosystems are changing and are different than they were in the past, even the recent past.  It says nothing about whether this change is good or bad.  In this paradigm, species can be referred to as novel species, new arrivals, or long-term residents.

The less biased ecological novelty paradigm differs dramatically from the more ideological nativism paradigm.  It differs in the language it uses and it differs in the implied direction that land management should proceed.  More generally, it forsakes the normative atmosphere that permeates restoration ecology, conservation biology, and invasion biology, all of which have been substantially guided by the nativism paradigm.

The Sutro Forest in San Francisco is a good example of a novel ecosystem. It is a thriving mix of native and non-native species. Much of it will be destroyed by the irrational belief that native species are superior to non-native species.  Million Trees

Currently, invasion biologists are trying to discredit ecological novelty as a valid or valuable perspective.  This is hardly surprising since the ecological perspective would displace the nativism paradigm, and many stakeholders have much to lose if the nativism paradigm were abandoned, e.g. chemical companies, restoration and management companies, local, state, and national agencies, to name just a few.  Not surprisingly, articles trying to shore up invasion ecology and to keep it relevant have been common in recent years.

While the public may not be aware of it, there exists a heated competition to define natureWhich side wins will significantly determine how nature is managed.  Given that the redistribution of species is only going to increase in upcoming decades, it is hard to imagine that people will still be so preoccupied with origins by the middle of the century.  Like the notion of wilderness, the nativism paradigm is more of a twentieth century concept, while the construct of ecological novelty is more fitting for the twenty first century.

Undoubtedly, nativist groups will still exist and will still be preoccupied with trying to restore their vision of the past.  But, due to the number of species being moved to new regions, much more attention likely will be given to the function of species than their origins, if only for pragmatic reasons.  For people coming of age now, cosmopolitanization is the new normal, both with respect to people and other species.  We will still carry our predispositions to divide the world into us and them, but it should be clear to most that the nativism perspective will be obsolete and that beyond the creation of museums, restoring the past will not be possible, whether a city or a forest.

Currently Earth is the only planet we know of where life exists.  In this context, the desire and practice of declaring some species as aliens, exotics, or invaders seems sadly provincial and even unseemly.  Roman playwrite Publius Terentius Afer (aka Terence) wrote in his play Heauton Timorumenos, “Homo sum, humani nihil a me alienum puto”, or “I am human, and nothing of that which is human is alien to me.” To those who still see such value in distinguishing native from alien species, I say, “I am of the planet Earth and nothing of that which is earthly is alien to me.”

Mark Davis

Status report on the invasion biology debate

Mark Davis, Macalester College

Mark Davis is Professor of Biology at Macalester College in St. Paul, Minnesota.  He is one of the first academic ecologists to publicly express skepticism of invasion biology.  His book, Invasion Biology, was published by Oxford University Press in 2009.  It was the first critique of invasion biology written by an academic scientist. Professor Davis cites the many empirical studies that find little evidence supportive of the hypotheses of invasion biology. 

In 2011, Nature magazine published an essay written by Professor Davis and 18 coauthors entitled, “Don’t Judge Species on their Origins.”  This essay suggested that conservationists evaluate species based on their ecological impact, rather than whether or not they are natives.  The essay initiated an intense debate in the academic community of ecologists that continues today. 

Professor Davis spoke at the Beyond Pesticides conference in Minneapolis at the end of April 2017. (Video available HERE) He described invasion biology as an irrational ideology that is based on nostalgia for the past and a belief that wildlands are being damaged by “alien invaders.”  In fact, the perceived damage is largely in the eye of the beholder, depending largely on one’s membership in a group benefiting from the nativism paradigm, such as chemical manufacturers, conservation organizations, government agencies, and employees.  Some academic careers are also at stake.  Futile attempts to re-create historical landscapes always have the potential to make things worse.  In many instances, it is more sensible to change one’s attitude about the changing landscape than trying to change nature.

Mark Davis speaking at Beyond Pesticides conference, April 2017

We invited Professor Davis to write a guest post for publication on Million Trees.  We asked him to express his opinion on these questions: 

  • Has the status of invasion biology changed much since Nature published your essay 2011?
  • Has increased knowledge of climate change had an impact on the status of invasion biology in academia?
  • What do you think is the future of invasion biology both as an academic discipline and as public policy?

Professor Davis’s guest post addresses these questions.  We are grateful to Professor Davis for his many contributions to our understanding of the fallacies of invasion biology and for his thoughtful guest post.

Million Trees


Competition to define nature

In the past few years, a new perspective has been taking hold in the field of ecology.  Referred to as ‘ecological novelty’ it emphasizes that many factors are producing ecologically novel environments.  Climate change (which includes changes in temperatures and patterns of precipitation), increased atmospheric carbon dioxide, which affects photosynthetic rates, increased atmospheric deposition of nitrogen (the whole earth is being fertilized due to the increased nitrogen we are pouring into the atmosphere), and the introduction of new species are all rapidly changing our environments.

A strength of the term ecological novelty is that unlike the invasion vocabulary it is simply descriptive.  It simply states that ecosystems are changing and are different than they were in the past, even the recent past.  It says nothing about whether this change is good or bad.  In this paradigm, species can be referred to as novel species, new arrivals, or long-term residents.

The less biased ecological novelty paradigm differs dramatically from the more ideological nativism paradigm.  It differs in the language it uses and it differs in the implied direction that land management should proceed.  More generally, it forsakes the normative atmosphere that permeates restoration ecology, conservation biology, and invasion biology, all of which have been substantially guided by the nativism paradigm.

The Sutro Forest in San Francisco is a good example of a novel ecosystem. It is a thriving mix of native and non-native species. Much of it will be destroyed by the irrational belief that native species are superior to non-native species.  Million Trees

Currently, invasion biologists are trying to discredit ecological novelty as a valid or valuable perspective.  This is hardly surprising since the ecological perspective would displace the nativism paradigm, and many stakeholders have much to lose if the nativism paradigm were abandoned, e.g. chemical companies, restoration and management companies, local, state, and national agencies, to name just a few.  Not surprisingly, articles trying to shore up invasion ecology and to keep it relevant have been common in recent years.

While the public may not be aware of it, there exists a heated competition to define natureWhich side wins will significantly determine how nature is managed.  Given that the redistribution of species is only going to increase in upcoming decades, it is hard to imagine that people will still be so preoccupied with origins by the middle of the century.  Like the notion of wilderness, the nativism paradigm is more of a twentieth century concept, while the construct of ecological novelty is more fitting for the twenty first century.

Undoubtedly, nativist groups will still exist and will still be preoccupied with trying to restore their vision of the past.  But, due to the number of species being moved to new regions, much more attention likely will be given to the function of species than their origins, if only for pragmatic reasons.  For people coming of age now, cosmopolitanization is the new normal, both with respect to people and other species.  We will still carry our predispositions to divide the world into us and them, but it should be clear to most that the nativism perspective will be obsolete and that beyond the creation of museums, restoring the past will not be possible, whether a city or a forest.

Currently Earth is the only planet we know of where life exists.  In this context, the desire and practice of declaring some species as aliens, exotics, or invaders seems sadly provincial and even unseemly.  Roman playwrite Publius Terentius Afer (aka Terence) wrote in his play Heauton Timorumenos, “Homo sum, humani nihil a me alienum puto”, or “I am human, and nothing of that which is human is alien to me.” To those who still see such value in distinguishing native from alien species, I say, “I am of the planet Earth and nothing of that which is earthly is alien to me.”

Mark Davis

Unseen City: A tribute to urban nature

It was pure pleasure to read Unseen City (1).  Unlike most nature writing, Nathanael Johnson asks readers to notice and appreciate the urban nature that we tend to take for granted.  Ironically, the plants and animals that we see every day and in great numbers do not get the attention they deserve.  Most nature writing tends to focus on rare and remote species to which we have little access and often laments their absence where we live.  Conservationists often advocate for expensive programs to reintroduce rare species to urban centers where they haven’t lived for decades, if not centuries.

Johnson’s focus on the ordinary species around us is refreshing.  We were happy to take a break from the usual hand-wringing about loss of biodiversity and instead enjoy the richness and beauty of the nature we have.  It is our loss when we ignore the nature we have. Johnson’s intense focus on urban species reveals that they are every bit as interesting as the rare species we seldom see.  Johnson’s approach to nature is analogous to the optimist’s “glass-half-full” approach to life.

Another appealing aspect of Johnson’s approach is that his story is told from the perspective of a young father, introducing his toddler daughter to the mysteries of nature.  One of our primary concerns about the museumification of our parks by native plant advocates is that children are being deprived of the opportunity to interact with nature.  Being required to stay on trails or observe from behind fences is no way for children to appreciate the complexity and beauty of the natural world.  Johnson takes his daughter deep into the weeds to experience nature in a physical, tactile way. 

A few examples of the homely creatures in our cities

Johnson wrote his book while living in San Francisco and then in Berkeley.  So, the species he encounters and studies are those with which we are all familiar.  Here are some of the creatures he tells us about, with a few of the interesting things we learn about them.

  • Pigeons are reviled by many serious bird watchers. In fact, they are remarkable creatures in many ways.  They mate for life and they are extremely devoted parents.  They tend to nest in the same place and their ability to find and return to that nest from long distances is one of the reasons why humans have formed intense relationships with them.  There is a long tradition of keeping homing pigeons that are raced by their keepers in competitions that occur all over the world.  The pigeons are taken long distances from their nests and then timed on how long it takes them to return home.  Johnson tells remarkable stories about how pigeons overcome challenging attempts to prevent them from finding their way home.
  • Eastern grey squirrel. Creative Commons

    Squirrels are both extremely agile and very resourceful. Here is an example of how squirrels defeated an attempt to keep them out of a bird feeder: “…squirrels had to climb up through a vertical pipe, leap onto a blade of a spinning windmill, cling to it, and then sail off on the right trajectory to land on a platform.  Then they had to go paw over paw upside down along a suspended chain that passed through a series of spinning disks, negotiate a revolving door, run through a slack canvas tube, and keep their balance while crossing a pole covered with slick spinning rollers.  From there, it was a six-foot jump to another tunnel, through which they had to ride a sliding vehicle made to look like a rocket ship by pushing it along with their paws.  Finally, there was an eight-foot jump to the food.” (1)  I retell this to story to spare our readers the pointless effort of trying to prevent squirrels from raiding their bird feeders.

  • Turkey vulture in San Jose, California by Dan DeBold. Creative Commons

    The turkey vulture is another underappreciated bird. They eat primarily dead animals and many of those animals died of diseases or toxic chemicals and are rotten and maggot infested when they are finally found (by smell) and eaten by the vulture.  The digestive and immune system of the vulture is capable of detoxifying chemicals and killing bacteria and viruses in the dead animal.  In other words, the vulture is cleaning up the remains of dead animals.  India has learned the value of vultures the hard way.  They killed many of their vultures with an anti-inflammatory drug they were feeding to their livestock.  When their vulture population dwindled, they were buried in dead animals, many dangerously diseased and toxic.  We eradicate animals at our peril because we often don’t understand the roles they are playing in the ecosystem.

Defending novel ecosystems

In addition to asking his readers to appreciate the positive qualities of the creatures in our cities, he also asks us to reconsider the deep prejudice against them that has become the conventional wisdom.  Plants and animals that people believe were transplanted by humans into places where they did not exist in the distant past are considered “alien invaders” that dominate their predecessors, driving them out and reducing biodiversity.

This narrative, which originated in academic science as “invasion biology” in the 1960s, has become a popular story with the media, which is always attracted to scary stories.  The media is significantly less interested in the peaceful resolution of their horror stories.  With few exceptions, an introduced species that initially seems to be a problem eventually fades into the woodwork to become just another player in the ecosystem.  Johnson uses the Argentine ant as one of many examples of an introduced species that spread rapidly, but 20 years later has nearly disappeared.  In other cases, a species initially considered an unwelcome intruder becomes a valuable asset, such as zebra mussels which filter pollution from lakes and have become a source of food for diving birds.

Novel ecosystems are the future

Johnson concludes his book with this reminder that novel ecosystems have been created by human disturbance and that we should be grateful for the plants and animals that are capable of surviving our abusive treatment of the planet:

“The species that I’ve written about here are, at best, invisible, and at worst, reviled.  We honor least the nature that is closest to us.  As Courtney Humphries put it in Superdove, ‘We create and destroy habitat, we shape genomes, we aid the worldwide movement of other species.  And yet we seem disappointed and horrified when those plants and animals respond by adapting to our changes and thriving in them.’

“Because they are associated with human disruption, the organisms that spring up from our footprints look like corruptions of nature.  But I’ve come to see it the other way around:  These species represent nature at its most vital and creative.

“Nature never misses an opportunity to exploit a catastrophe.  When humans bulldoze and pave, nature sends in a vanguard of species that can tough it out in the new environment.  These invasive species are not nature’s destroyers, but rather its creators.  They begin setting up food webs, they evolve and diverge into new species.  Because humans purposefully import exotic plants—along with the insects, seeds, and microbes we accidentally bring in from around the world—cities are remarkable centers of biodiversity.  These creatures crossbreed, hybridize, eat one another, form cooperative relationships, and evolve.  And so, at a time when thousands of species are at risk of extinction because of our destruction of wilderness, new species are springing up in the new habitats we have created.” (1)

Worshipping the rare at the expense of the common

The ONLY known Raven’s manzanita plant is in the San Francisco Presidio. Its exact location is a secret to protect it.

Vast sums of money are being spent in often futile attempts to reintroduce rare plants and animals to urban environments where they have not lived for a long time.  The National Park Service and San Francisco’s Natural Resources Division are having little success with their efforts to reintroduce Mission Blue butterflies.  After over 30 years, the National Park Service has still not successfully germinated endangered Raven’s manzanita from seed.  These fruitless efforts are not just wasteful of resources, they also inflict damage on the environment by using pesticides and setting fires to eliminate competition and destroying trees to increase sunlight on rare plants and host plants of rare insects.

The veneration of rare plants and animals is often at the expense of the plants and animals that are adapted to present environmental conditions.  In Unseen City Nathanael Johnson invites us to place greater value on the ordinary creatures who are capable of living with us.  We can treat them with the respect they deserve by not destroying them in pursuit of a fantasy landscape populated by fantasy creatures that are not capable of surviving the changes we have made in the environment.


  1. Nathanael Johnson, Unseen City: The majesty of pigeons, the discreet charm of snails and other wonders of the urban wilderness, Rodale Wellness, 2016