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

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

Hybridization: “Genetic pollution” or a natural process?

 
Presidio variety of California poppy. NPS photo

The San Francisco Chronicle’s gossip columnist, Leah Garchik, recently published a story about California poppies in the Presidio.  Apparently, someone planted the “wrong” poppy, or it migrated there.  The poppy that is native to the Presidio is small and yellow.  This “alien” poppy is the large orange poppy that most of us consider the classic California poppy.  The historical record indicates that this classic poppy grew elsewhere in San Francisco, but since it didn’t grow in the Presidio it must be removed because the Presidio’s Vegetation Management Plan “contains the requirement to remove any plants that could jeopardize the integrity of the genetics of native plants in the Presidio.” 

The “wrong” poppy

This incident reminded us of an article published in the newsletter of the local chapter of the California Native Plant Society several years ago, entitled “Contaminating the Gene Pool.”  In this article Jake Sigg instructs gardeners to beware of planting the wrong variety of a native plant because it will cause “genetic pollution.”  The California poppy is one of the examples he gives of a variety of California native being planted in San Francisco that doesn’t belong here.  It isn’t sufficient in his opinion, to plant a California native if that specific variety of the species didn’t historically occur in San Francisco.  He asks gardeners to “think in terms of preserving the genetic integrity of the local landscape.”  And he speculates many negative consequences of selecting the wrong variety, such as “genetic swamping, you’ve got all these foreign genes that are going to overwhelm the native population.”  We were reminded of Mr. Sigg’s vocal opposition to human immigration. 

For the benefit of our readers who aren’t gardeners, we should explain what Jake Sigg and the Presidio are worried about.  In a word, they are worried about hybridization, defined as “to breed or cause the production of a hybrid,” which is defined as “the offspring of two animals or plants of different breeds, varieties, or species.”  Hybridization is as likely to occur between two native plants as two non-native plants, but native plant advocates are concerned about the possibility of a native and a non-native plant producing a hybrid variety that is distinct from the native plant.  Hybridization is not inevitable, but it does occur naturally as well as through human manipulation.  The “From the Thicket” blog recently told the fascinating story of the development of a valuable garden cultivar variety of a favorite California native, ceanothus or California Lilac. 

Aside from the unpleasant association with eugenics, Mr. Sigg’s advice raises several practical questions.  How is the gardener supposed to know exactly which variety of a native plant “belongs” in San Francisco or even in a specific neighborhood within San Francisco, such as the Presidio?  And, in the unlikely event that gardeners might have such esoteric knowledge, where would they get the seeds of this specific variety?  Jake Sigg acknowledges this practical obstacle, but advises gardeners to get their seeds and plants only from the annual plant sale of the local chapter of the California Native Plant Society.  One wonders how many gardeners will follow this rather restrictive advice.

However, the more important question is the scientific question.  Is hybridization an inherently harmful process that always reduces species diversity?    We turn to Mark Davis for a less gloomy view of hybridization.  Like most scientific questions, there is evidence of both positive and negative effects of hybridization on species diversity.  Since you’ve heard the negative view from Mr. Sigg, we’ll let Mark Davis speak for the positive view.  Professor Davis reports in his book Invasion Biology* that “the fossil record generally shows that following the invasion of new species, the number of species resulting from adaptive radiations and evolutionary diversification exceeds the number of extinctions.”   And he concludes his discussion of hybridization and evolution by saying, “…a fair appraisal must also acknowledge that species introductions can enhance diversity as well, through hybridization, and the creation of new genotypes.”

The native plant movement has a narrow view of nature, which we do not share.  Their ideology is based on dire predictions of ecological disaster if we don’t follow their restrictive advice.  And when the managers of public lands choose to follow their advice, the consequences are usually the destruction of plants and animals that we value, in this case a field of California poppies. 


* Mark Davis, Invasion Biology, Oxford University Press, 2009, page 78-82.

Broom: “I’m ba-ack”

Leona Canyon, Spring 2011

One of our first posts, nearly one year ago, was about attempts to eradicate broom (see “Broom:  I’ll be back!”).  It’s that time of year again, when the broom is blooming around the Bay Area, adding a dash of bright yellow color to our public lands.

Despite continuing efforts to eradicate broom, it makes this annual comeback, regardless of the method used to kill it.  Mechanical destruction is one of the methods used.  In the photograph taken in fall 2010 of a trail in Redwood Park we see the unsightly result of such an attempt to eradicate the broom shown in our first post about broom in spring 2010.

Redwood Park, Spring 2010
Redwood Park, Fall 2010

The UC Davis Statewide Integrated Pest Management Program  predicts the results of this effort to eradicate broom:  “Brush rakes and bulldozers often leave pieces of rootstalks that readily can resprout…using large equipment to clear land creates a perfect environment for new seedling establishment, making follow-up control essential.”  So, we shouldn’t be surprised when we revisit this same site the next spring to find that although it is still unsightly, it is also covered with the resprouts of the broom.

Broom resprouts, Redwood Park, Spring 2011

Herbicides are also used to eradicate broom.  The East Bay Regional Park District uses Garlon for foliar spraying of broom, despite the public’s concern about herbicide use in our parks.  In contrast, Marin Municipal Water District (MMWD) has not used herbicides on its properties since 2005, in response to public protests.  In December 2010, the Marin Independent Journal reported that MMWD is considering using Roundup (which is considered less toxic than Garlon) for broom control, but that a proposal will not be made until 2012 at the earliest, after an environmental impact study that will consider all alternatives for “pest management.”

Even if we are willing to accept the health and environmental risks associated with herbicides, they are not a guarantee of success.  According to UC Davis, “One application of an herbicide does not always completely control brooms…Watch treated areas closely for at least a year, and retreat as necessary.”

 

Since the seeds of broom are known to live in the ground from 50 to 60 years, we should expect that it is very difficult to eradicate.  Unless we make a commitment to kill the plant above the ground every year for as long as the seeds live in the ground, we cannot expect to be successful in that effort.

We were therefore amused by this exchange that Jake Sigg had with one of the readers of his “nature newsletter” (December 20, 2010):

Reader:  “…We have been up there every year for about 3-1/2 years now…It has been quite instructive about how flat out difficult it is to reverse the process [of “displacement” of native plants].  Like most invaders, the non-native plants like it here and want to stay.  The broom in particular are [sic] astoundingly persistent in holding whatever territory that they have gained.  It sounds strange to say, that sometimes I think the project is an exercise in futility while at the same time is extremely satisfying work and will continue it into the indefinite future.”

Jake Sigg:  “…It is such a lop-sided struggle…And yet, we all keep going.  It is a paradox that I don’t understand.  While working I’m fully aware of the seeming futility of what we’re doing, and yet keep going—and enjoying it!!”

Jake Sigg and his readers would benefit from the advice of Mark Davis in his book, Invasion Biology.*  Professor Davis suggests that unless a non-native species causes great health or economic harm, we adopt the “LTL approach” i.e., “Learn to love ‘em” or at least “Learn to live with them.” 

If it is futile to eradicate a plant we don’t like, if that plant isn’t doing us or any other plant any harm, and if we are damaging our environment in our futile attempt, let’s change our attitude toward that plant.  If we can’t change the plant, it is still within our power to change ourselves.  Surely we can find something more fruitful to do that is equally satisfying such as planting more native plants rather than destroying non-native plants.

Broom is an especially good candidate for LTL.  It is green all year around.  It requires no care whatsoever.  And in the spring it treats us to a lovely carpet of bright yellow.   What’s not to love?


* Mark Davis, Invasion Biology, Oxford University Press, 2009, page 150

Mark Davis, “A Friend to Aliens”

Mark Davis, Professor of Biology at Macalester College is interviewed in the February issue of Scientific American.  He tells us that invasion biology must distinguish between change and harm when labeling non-native species as “invasive,” a term which he believes should be used only in those rare cases when the non-native species pose “health threats” or economic harm.  With the exception of isolated places, such as islands, Mr. Davis tells us that non-natives have not been the cause of extinctions of native species.

 He believes it is irresponsible to label non-native species as “invaders” if they do not cause such harm because attempts to eradicate them are wasteful of scarce resources and often harm the environment more than the mere existence of non-natives.   He advises us to learn to live with those species that are not harmful. 

 He also points out that the eradication of non-natives is often futile and is likely to become even more futile in the future as global travel and commerce increase and the climate continues to change.  All species are going to move, both natives and non-natives and in fact, natives are as likely to cause problems in their expanded range as the non-natives in those regions.  He offers the example of the mountain pine beetle in Western coniferous forests, which is killing half the timber forest in British Columbia as it expands its range, probably in response to increasing temperatures.

Mr. Davis was also interviewed by Environment 360, a publication of Yale University, in November 2009.  In that interview, he is joined by Dov Sax, assistant professor of biology at Brown University, one of the growing number of biologists who are questioning the assumptions of invasion biology.  He provides a local example  of exaggerated claims of invasiveness:  “Dr. Sax says he began to question exotic species orthodoxy as an undergraduate at the University of California, Berkeley.  A professor leading a field trip described the Bay Area’s abandoned plantations of Australian eucalyptus trees as a “biological desert.”  Says, Sax, ‘There was all kinds of stuff growing in there.  I found there were really a similar number of species in both [native oak and eucalyptus] woodland types.  Exotics weren’t always doing the awful things people seemed to think they were doing.’” 

 
Owlets in eucalyptus, Claremont Canyon, Oakland

We attended a few lectures of an undergraduate course at UC Berkeley that fit with Dr. Sax’s experience.  Students in this undergraduate course were required to “volunteer” in a variety of different “restoration” projects in the Bay Area.  One of the projects on the property of UC Berkeley focused on the eradication of eucalypts.  The leader of this project and supervisor of the students who chose his project had an undergraduate degree in “natural resources” and an MBA in “operations management.”  He made a number of unsubstantiated claims to justify the eradication of eucalypts, but the most flagrantly stupid statement was this:  “The carbon sequestered in non-natives doesn’t count.  Only the carbon sequestered in natives counts.”  This statement has no scientific meaning.  We assume it is intended as a philosophical statement.  In any case, students aren’t learning any science from such a statement. 

Critics of native plant ideology are accustomed to criticism from true believers and Mark Davis is no exception.  In an interview available on the Macalester College website, Mr. Davis says he,  “…received rebuttals that, he felt, veered toward ad hominem attacks on his inexperience in the field.”  But he has not backed down and has come to view this debate as an example of the “values and age-old religious attitudes toward nature [that] frame scientific study and debates more than most scientists would acknowledge.”  He concludes that interview with this observation:  “People can get addicted to paradigms.  Then paradigms become an ideology.  Belief and conviction are very difficult adversaries since they are little affected by data and evidence.”