Global increases in biodiversity resulting from new species

Great horned owl in eucalyptus.  Courtesy
Great horned owl in eucalyptus. Courtesy

One of the most popular justifications for eradicating non-native plants is the claim that they will out-compete native plants, ultimately causing their extinction.  Innumerable studies have found no evidence to support that claim, but the belief persists amongst those who demand the eradication of non-native plants.

Islands have been considered particularly vulnerable to extinctions because they contain many endemic species (found only on that island) that have evolved in physical isolation from their ancestors from other places and become unique species.  And there were many animal extinctions–particularly of flightless birds–with the arrival of humans who were both their predators and brought predators with them.

However, despite the conventional wisdom that the introduction of new species of plants to islands would result in extinction of their predecessors, there is no evidence that this is indeed the case with introduced plants.  In 2008, Dov Sax and Steven Gaines published a study of species diversity on islands.  This is what they found:

Honeybee on wild mustard.  Courtesy
Honeybee on wild mustard. Courtesy

Predation by exotic species has caused the extinction of many native animal species on islands, whereas competition from exotic plants has caused few native plant extinctions…By analyzing historical records, we show that the number of naturalized plant species has increased linearly over time on many individual islands. Further, the mean ratio of naturalized to native plant species across islands has changed steadily for nearly two centuries. These patterns suggest that many more species will become naturalized on islands in the future.” (1)

In other words, the introduction of new plants to islands has not resulted in extinctions of the plants that preceded them.  Therefore, the result of plant introductions has been greater plant diversity on islands.

But what about the continents?

Painted lady butterfly on Weigela.  Courtesy
Painted lady butterfly on Weigela. Courtesy

Recently a new study was published that asked the same question on a global scale:  Has the introduction of new plants and animals resulted in the extinction of their predecessors?  The answer is a resounding NO!  (2)

The study was conducted on a huge scale by an international team of scientists:

  • “6.1 million species occurrence records from 100 individual time scales”
  • “35,613 species were represented…including mammals, birds, fish, invertebrates, and plants”
  • “The geographical distribution of study location is global, and includes marine, freshwater, and terrestrial biomes, extending from the polar regions to the tropics in both hemispheres.”
  • “The collective time interval represented by these data is from 1874 to the present, although most data series are concentrated in the past 40 years.”

Like most scientists who expect to find evidence of decline, this team of researchers was surprised to find little evidence of loss.  Here are some of their key findings:

  • “Surprisingly, we did not detect a consistent negative trend in species richness or in any of the other metrics of α diversity.”
  • “There is no evidence of consistent loss of biodiversity among terrestrial plants.”
  • “Time series for terrestrial plants exhibit, on average, a positive slope for species richness.”
  • “Collectively, these analyses reveal local variation in temporal α diversity but no evidence for a consistent or even an average negative trend.”  (Alpha diversity is species richness at the local level.)
  • “An analysis of slopes by climate regions reveals that temperate time series have a significantly positive trend…”

In other words, new plants result in more plants, particularly where we live, in the temperate zone.  There is no empirical evidence that new plants have resulted in the loss of the plants that were there before they arrived.

So what’s the beef?

Song sparrow in wild radish.  Courtesy
Song sparrow in wild radish. Courtesy

You might think that this huge new study would put the controversy to rest.  You would be wrong.  For every answer we find, there is a new question from nativists.  The response of native plant advocates to the good news that the plants they prefer will not disappear if new plants are allowed to live in their company is that the plant world is being “homogenized.”  They say that if new plants are permitted to remain, all landscapes will become the same, resulting in the loss of unique landscapes that existed in the past.

They are, of course, mistaken.  Their dire prediction will not come to pass because the biotic and abiotic conditions of every landscape are unique.  The climates are different.  The soils are different.  The atmosphere is different.  The plants and animals that are there when they arrive are different.  If the new plant survives in its new home, it will be capable of adapting to these local conditions and over time it will change, ultimately becoming a unique species.  When the first family of monkeys made the voyage from Africa to South America, they were the same species as those they left behind.  Now they are unique species as a result of genetic drift and genetic divergence.

The process of adaptation and evolution is often more rapid than we expect.  Sometimes such changes have occurred within the lifetimes of scientists who were able to witness these changes.  More often, the changes occur more slowly and are only visible in museum collections or fossil records.

Consider the consequences

Garter snake in eucalyptus leaf litter.  Courtesy
Garter snake in euclypatus leaf litter

It is physically impossible to prevent the arrival of new species.  Even when they are not intentionally introduced they find a way to piggy back on the daily activities of humans.  They arrive on our airplanes and cargo ships.  We aren’t going to stop importing or exporting our products all over the world.  Nor are we going to quit traveling.  We must accept the consequences of the way we live and quit blaming plants and animals for their passive participation in our movements.

Aside from the question of whether or not it is physically possible to stop the arrival of new plants and animals, let’s acknowledge that at least in the case of plants no great harm has come from their introductionSince we now enjoy more plants than were here when they arrived, just what is it that we’re griping about?  We seem to be griping about change.  Change will occur whether we like it or not.  We can’t prevent change, so we must quit fighting against something that we are powerless to prevent.  That is the definition of wisdom.

Finally, we must consider the consequences of trying to eradicate non-native plants that are firmly entrenched in our landscapes.  Huge amounts of herbicide are being used in the futile attempt to eradicate them.  Fires that pollute the air and endanger our homes are set for the same purpose.  Trees that are performing valuable ecological functions are being destroyed.  The animals that use these plants and trees for food and cover are being deprived of their homes and their food.  We are doing more harm than good.


  1. Dov Sax and Steven Gaines, “Species invasions and extinctions: The future of native biodiversity on islands,” Proceedings of the National Academy of Sciences, August 12, 2008
  2. Maria Dornelas, et. al., “Assemblage times series reveal biodiversity change but not systematic loss,” Science, April 18, 2014

Past & Present Botanical Myths

The conventional wisdom is that human knowledge, especially scientific knowledge, moves inexorably forward.  We are sometimes amused by the misconceptions of the past and marvel at the primitive knowledge of our ancestors.  However, we rarely stop to think that our descendents will probably do the same when they consider our current state of knowledge.

Historical botanical beliefs

"The heads of cowslips, which trembled in the wind, were signed for Parkinson's disease, the 'shaking palsy.'" (1)

The Doctrine of Signatures seemed a logical botanical belief at a time when plants were one of man’s few medicinal tools and religion was a powerful influence in human society.  The Doctrine of Signatures, which was actively promoted by the church in 17th century Europe, was based on a belief that God had “signed” plants with certain suggestive shapes and colors to inform humans of their medicinal properties.  For example, a heart-shaped leaf was considered God’s message to us that a particular plant would be beneficial to the human heart and this message was strengthened by a flesh-colored flower. Every plant was believed to be useful in some way if man could only discern its use.  Else why would they have been created, since the Garden of Eden was created for the benefit of man?  The church encouraged man’s study of plants as a way to worship God’s creation. (1)

Many botanical myths originated from ancient Roman and Greek horticultural treatises and persisted for hundreds of years.  For example a belief in the influence of the moon on plants is first found in the writings of Pliny in first-century Rome and also found in writings as late as 1693:  “[w]hen you sow to have double Flowers, do it in the Full of the Moon.”  (2) 

The origins of many horticultural myths are unfathomable but probably began with a particular event because we often confuse coincidence with causal relationships. (2)

  • Planting bay trees and beeches near  homes will prevent lightning strikes
  • An apple tree that fruits and flowers at the same time is a bad omen
  • The parents of a child who picks red campion will die
  • A pregnant woman who steps over cyclamen will miscarry

Modern botanical beliefs

Now we will turn to the theoretical underpinnings of the native plant movement to see how they are holding up to the scrutiny of current science and ask the rhetorical question, Is it time to relegate invasion biology to the dust heap of discredited science?

The field of invasion biology upon which the modern native plant movement is based, originates with the publication in 1958 of The Ecology of Invasions for Animals and Plants by Charles Elton.  Elton postulated that every plant and animal occupies a different ecological “niche” and plays a specific role within that niche: 

“…every species will have a slightly different role, or niche, and often, he believed, every niche will be filled.  Some animals eat grass, others leaves; some plants grow on wet soil and some grow on dry; some birds nest in dead trees, others in live ones.  When new species are introduced, the theory goes, they can get a foothold and start reproducing only by finding a vacant niche or by throwing some other species out of its niche…” (3)

Elton’s corollary to the exclusivity of the niche is that the introduced species will have a competitive advantage because its predators are absent in its new home.  The predicted result of Elton’s theory was that introduced species will exterminate previous occupants, mass extinctions will occur, and the result will be a simplified ecology composed of few surviving species.

The problem with Elton’s theory is that it doesn’t correspond with reality.  More and more scientists are finding that the frequency of introductions far exceeds the frequency of extinctions.

  • In 2002 Dov Sax reported that introduced species greatly outnumbered extinctions on oceanic islands:  “In the case of plants, islands are now twice as diverse as they were before humans started moving things around.” (3)
  • In 2012, Erle Ellis, et. al., reported that “…while native losses are likely significant across at least half of Earth’s ice free land, model predictions indicate that plant species richness has increased overall in most regional landscapes, mostly because species invasions tend to exceed native losses.” (4)
  • In San Francisco, the second-most densely populated city in the US, ninety-seven percent of the 714 plant species known to exist in San Francisco in 1850 are still found there, despite the fact that most plants and trees in the city are introduced.  (5)

The dire predictions of invasion biology have not come to pass nearly 60 years after their inception.   Many scientists are clearly ready to abandon invasion biology because it does not conform to reality.  Can we finally breathe a collective sigh of relief and move on to a less gloomy view of ecology?  Some day our descendents will look upon this episode in human history and laugh, as we laugh at the 17th century Europeans who examined plants, looking for the clues from God that revealed their purpose.


(1) Richard Mabey, Weeds:  In Defense of Nature’s Most Unloved Plants, Profile Books Ltd, London, 2010, page 87-91

(2) Andrea Wulff, The Brother Gardeners, Alfred Knopf, 2008, page 11-12

(3) Emma Marris, The Rambunctious Garden, Bloomsbury, USA, 2011, page 102-104

(4) Erle C. Ellis, et. al., “All Is Not Loss:  Plant Biodiversity in the Anthropocene,”

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

A dialogue about insects and non-native plants

We received a comment on our “Wildlife” page from “entomologist” that deserves a comprehensive response. 

 Conversation with “entomologist”

 “entomologist:”  “Adaptation to exotic species by specialist herbivores is unusual.  Those butterflies that switch to exotics tend to be generalists already.”  

Webmaster:  “Entomologist” is mistaken that the butterflies now using non-native plants are generalists, by which we assume he means that they use many plants, rather than a specific species.  According to Professor Art Shapiro (UC Davis), 26 of the 82 species of California butterflies now feeding on exotic plant species, are using only one plant species.  In other words, nearly one-third of California butterflies presently using exotic plant species are not generalists.(1)  When butterflies have made the transition from a native to a non-native plant, the plants are usually chemically similar. 

Anise Swallowtail, Sutro Forest
Anise Swallowtail, Sutro Forest, March 2010

The Anise Swallowtail is a conspicuous example of a California butterfly that is now dependent upon a particular exotic plant, fennel. This relationship between a specific native insect and a specific non-native plant is one of the reasons why the Million Trees blog was created.  Non-native fennel is being eradicated by every native plant “restoration” in the Bay Area.   

Over ten years ago, a park advocate in San Francisco became enraged by the eradication of fennel in his park because he was aware of the dependence of the Anise Swallowtail upon the fennel.  He made every effort to convince the so-called Natural Areas Program to stop destroying the fennel in his park.  He enlisted the help of Professor Art Shapiro in that effort. His efforts were ultimately unsuccessful.  The Natural Areas Program considered the non-native origins of the fennel sufficient reason to eradicate it, regardless of the needs of a native butterfly.  They continue those eradication efforts to this day. 

It is such mindless destruction of non-native plants, regardless of their benefit to fauna (or other benefits) that has made the Natural Areas Program so unpopular with people with a broader view of nature. We value the Anise Swallowtail butterfly as much as any theoretical benefit from eradicating a non-native plant.

“entomologist:”  “This idea that exotic plants are as good for wildlife as natives is just plain pathetic, especially for anyone who knows about herbivory  patterns on native and exotic plants.”

Webmaster:  By “pathetic” we assume “entomologist” means that he does not believe that insects eat non-native plants.  He is mistaken that insects do not eat non-native plants.  Returning to Professor Shapiro, he reports that 82 of 236 (35%) total species of California butterflies feed on non-native plants

Professor Dov Sax (Brown University) compared insects living in the leaf litter of the non-native eucalyptus forest with those living in the native oak-bay woodland in Berkeley, California.  He found significantly more species of insects in the leaf litter of the eucalyptus forest in the spring and equal numbers in the fall.(2)  Professor Sax also reports the results of many similar studies all over the world that reach the same conclusion.

The California Academy of Sciences finds that several years after planting its roof with native plants, it is now dominated by non-native species of plants in the two quadrants that are not being weeded, replanted and reseeded with natives.  Their monitoring project recently reported that there were an equal number of insects found in the quadrants dominated by native plants and those dominated by non-native plants. 

Damselflies (probably Common Blue) mating on non-native ivy in Glen Canyon Park.

We also use our eyes when we walk in our parks.  We often find insects in non-native plants.  Those non-native plants are often targets for eradication.  The damselflies in a San Francisco park are another example of the contradictory strategies of the Natural Areas Program.  They have made several attempts to reintroduce the rare Forktailed Damselfly to one of the parks in San Francisco.  Although those attempts have not been successful, we see other species of damselflies in that park, using the non-native plants that are repeatedly sprayed with herbicides by the Natural Areas Program.  We wonder if the herbicide use in that park is contributing to the failure of attempts to reintroduce the Forktailed Damselfly. Does the right hand know what the left hand is doing?

“entomologist:”  “Insects eating plants are at the base of the food chain and native plants have more insect herbivores and support more native birds.”

Webmaster:  We can agree that many birds eat insects and those that do are likely to benefit from greater populations of insects.  But, there is substantial evidence that insects are as likely to be found in non-native plants as in native plants and we trust that the birds know where to find them.  However, unlike “entomologist” we are as interested in the welfare of non-native birds as we are in native birds. 

“entomologist:”  “Doug Tallamy’s work shows this in the eastern US conclusively.”

Webmaster:  Professor Tallamy’s (University of Delaware) publications do not seem to be available on-line, which prevents us from reading his publications.  However, since he studies the insects on the east coast we don’t think whatever he reports trumps the studies that we have cited of insect populations here in the Bay Area. 

“entomologist:”     “I certainly feel for the loss of trees, but the alternative is that we accept a homogenized set of urban-tolerant plants and wildlife.  Maybe that’s ok if you don’t know the difference, but for those of us who actually pay attention it is profoundly sad.”

Webmaster:   We don’t see the logic of “entomologist’s” vision of a “homogenized” ecology.  If we destroy non-native plants and animals, our ecology will be less diverse.  And we hope that the readers of Million Trees will agree that we are, indeed, “paying attention.” 

 The Big Picture

 We suggest that “entomologist” and other native plant advocates step back from their deeply-seated prejudices against non-native plants and consider the big picture.  The fact is that insects are particularly vulnerable to climate change because they live in relatively narrow temperature ranges. (3) Although they are adjusting well to changes in vegetation, they are not likely to be able to make an equally successful adjustment to changes in temperatures.  Therefore, if our top priority is insects, we would be wise to reconsider destroying millions of non-native trees that are sequestering millions of tons of carbon, contributing to greenhouse gases and thereby to climate change.    


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

(2) Dov Sax. “Equal diversity in disparate species assemblages:  a comparison of native and exotic woodlands in California,” Global Ecology and Biogeography, 11, 49-52, 2002.

(3) “Mountain plant communities moving down despite climate change, study finds,” Los Angeles Times, 1/24/11

Biodiversity: Another eucalyptus myth busted

Native plant advocates use many arguments to justify the destruction of non-native species and we have debunked many of those arguments here on Million Trees.  Now we will examine the claim that non-native species must be destroyed because their mere existence reduces biodiversity by out-competing native plants and animals.  Because eucalyptus trees are one of the primary targets for eradication, we will focus on the specific claim that the eucalyptus forest is a “biological desert.”   We are frequently told that “nothing grows” under the eucalypts and that they are not providing food or habitat to insects, birds, and other animals.

Professor Dov Sax (Brown University) tested these claims while a student at UC Berkeley.  He studied the eucalyptus forest in Berkeley, California, and compared it to native oak-bay woodland.  He found little difference in the species frequency and diversity in these two types of forest.
Eucalyptus forest and its thriving understory, Mt. Sutro, June 2009


He studied six forests of about 1 hectare each, three of eucalypts and three of native oaks and bays.  The sites were not contiguous, but were selected so that they were of similar elevation, slope, slope orientation, and type of adjacent vegetation.  He conducted inventories of species in spring and autumn.  He counted the number of:
  • Species of plants in the understory
  • Species of invertebrates (insects) in samples of equal size and depth of the leaf litter
  • Species of amphibians
  • Species of birds
  • Species of rodents

 He reported his findings in Global Ecology and Biogeography*:

“Species richness was nearly identical for understory plants, leaf-litter invertebrates, amphibians and birds; only rodents had significantly fewer species in eucalypt sites.  Species diversity patterns…were qualitatively identical to those for species richness, except for leaf-litter invertebrates, which were significantly more diverse in eucalypt sites during the spring.” 

Professor Sax also surveyed the literature comparing biodiversity in native vs non-native forest in his article.  He reports similar findings for comparisons between non-native forests and local native forests all over the world:

  • In Spain, species of invertebrates found in the leaf-litter of eucalyptus plantations were found to be similar to those found in native forests, while species richness of understory plants was found to be greater in the native forests.
  • In Ethiopia the richness of understory species was found to be as great in eucalyptus plantations as in the native forest.
  • In the Mexican state of Michoacán, species richness and abundance of birds were found to be similar in eucalyptus and native forests.
  • In Australia species richness of mammals and of soil microarthropods were found to be similar in native forests and in non-native forests of pine.

The only caveat to these general findings is that fewer species were found in new plantations of non-natives less than 5 years old.  This helps to illustrate a general principle that is often ignored by native plant advocates.  That is, that nature and its inhabitants are capable of changing and adapting to changed conditions.  In the case of non-native forests in the San Francisco Bay Area, they have existed here for over 100 years.  The plants and animals in our forests have “learned” to live in them long ago. 

Anise Swallowtail, Mt. Sutro, March 2010

We recommend that you visit the SaveSutro website   for a description of the richness of the non-native forest that thrives on Mount Sutro in San Francisco.  It is the perfect illustration of these scientific principles.  We can discuss scientific principles in the abstract, but there is no substitute for a walk in the forest to confirm with our eyes what science tells us.

*Dov Sax, “Equal diversity in disparate species assemblages:  a comparison of native and exotic woodlands in California,” Global Ecology and Biogeography, 11, 49-52, 2002.



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.”