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

13 thoughts on “A dialogue about insects and non-native plants”

  1. The native plant proponent’s adherence to the religious dogma of the native-plant-only philosophy as a maintenance strategy for our urban natural ecology lacks all the basics of the Scientific Method.

    * Ask a Question
    * Do Background Research
    * Construct a Hypothesis
    * Test Your Hypothesis by Doing an Experiment
    * Analyze Your Data and Draw a Conclusion
    * Communicate Your Results

  2. I much agree with you and I am very pleased to have found your blog.
    The following is a link to a book reference the book is written after a 30 years observation in a garden on wildlife and native non native plants very interesting :
    On my own land i have made some observations as well but not this intense and certainly not over such a long period.
    Webmaster: Thank you for visiting the Million Trees blog. I have read your post about your experience with eucalypts on your land in Wales. I admire your commitment to observing the way the eucalypts function on your property. Although we read the scientific literature and report our findings to our readers there is no substitute for actual experience and the close observation of what is happening. We hope our readers will visit your blog.

  3. This group makes mountains made out of molehills, native plant advocates are ruling or ruining the world? What tripe.
    World agriculture is built on removing weeds for our benefit or planting trees alien to an ecosystem for ditto. Native plant advocates have very, very little influence by comparison.
    Webmaster: The Million Trees blog is reporting only on the destructive effects the native plant movement is having in the San Francisco Bay Area. For example, one third of all park acreage owned by the City of San Francisco has been designated for native plant restorations. This is not a trivial amount of public land in the second most densely populated city in the United States.

    San Francisco is not the only city in the country that has a strong native plant movement. The city of Chicago has lost many of its trees on its public lands in the effort to restore the area to grassland prairie. You are fortunate if you are not experiencing as much destruction wherever you live.

    Although the impact of agriculture in the United States is undoubtedly greater, that is not an argument in favor of inflicting comparable damage in an urban environment. It is no comfort to us that more damage is inflicted by agriculture. In fact it is an argument for doing less damage when our food supply does not provide an economic argument in favor of it.

  4. A preference for native over introduced plant species is not a “movement” but a reflection of the combined knowledge biologists have aquired over the years. Every PhD, MSc, or even BSc knows species richness and diversity relies on the equilibrium reached by different species co-evolving over time . As such, your attempt to categorize those who advocate natural species over introduced as coming from a fringe group of scientists reveals your own dogmatic attitude..
    Webmaster: We do not believe that support for the native plant movement originated with “a fringe group of scientists.” Nor do we believe that “every PhD, MSc, or even BSc” supports this ideology. The scientific discipline of invasion biology from which the movement arose was popular about 30 years ago. Some of its assumptions are now being questioned by many scientists, based on evidence from the field. We report some of their work here. For example, both the concept of “equilibrium” in nature and “co-evolution” are now being reinterpreted by science. We use the word “movement” here to describe the amateur following of the scientific discipline. What they often lack in information, they usually compensate for with passion.

    And the native plant movement is not about “preference” for natives. In the San Francisco Bay Area it is about widespread destruction of healthy, mature trees that are doing no harm.

    I also note with deep concern the lack of concern that you show for the disruptive influence that introduced species and climate regimes unquestionable have on an ecosytem. The same downplaying of climate change the Oil, Gas and Coal lobby promotes gives me pause to question your own creds. They too decry the concensus that states native plant species generally promote biota richness. The introductiion of a non-natuve species – knapweed – has now caused the destruction of millions of hectares of prairie wildflower and grasslands that Burrowing Owls eg. need to survive but are now threatened by the growth of this one plant to the near exclusionof all others.
    Webmaster: You misrepresent this blog’s opinion of climate change. We do not “downplay” the significance of climate change. Indeed, we believe that climate change is one of the reasons why destructive “restorations” in urban areas are usually futile. The fact is the climate has changed and some plants that existed here several hundred years ago are no longer well-adapted to changed conditions. Those with a sincere concern about climate change should not be advocating for the destruction of millions of trees that sequester carbon..

    And regarding your theory that because a third of insects found on non-native plants were themselves native….that this somehow means we shouldnt be concerned about the two-thirds of other insects that DO NOT appear on your beloved fennel and so have fallen from consideration as important to the overall biosphere simply beggars belief!
    Webmaster: We recommend that you read Professor Shapiro’s study which we hope would reassure you that we are not carelessly abandoning the needs of two-thirds of the butterfly population in California. Neither butterflies nor native plants are evenly distributed in California. There are fewer species of butterfly on the coast because of the cooler climate. Unrelated to that, this is where most non-native plants exist because this is where most people live. In those places where there are more butterflies, there are also more native plants. This is not a cause and effect relationship. All generalizations used to support the native plant movement require careful examination.

    How could anyone offering their opinion as a concerned naturalist fail to know the concern the rest of us have — “us” being those who actually work with plants and insects trying to gain even more insight into ecosystem dynamics — isn’t simply concern for the ability some insects have to utilize the introduced species, but rather is concern we must have if a species has some property about it that keeps native predators away…a predation cycle that all other plants must deal with to keep their numbers above extinction levels while not becoming so numerous they crowd out all other plants. And some certainly do not have enough native insect or grazing predators to keep them in check here. But back home they they for instance evolved over time to become the preferred plant that certain insects use to lay their eggs in..eggs that hatch into root-boring larvae that kill the plant and eventually leave their host.
    Webmaster: This generalization is unsupported by the facts on the ground. We recommend that you read the article in yesterday’s New York Times about evolution. The fact is that insects are capable of adapting to new plants within a relatively short time because their populations are large and their lives are short. The soapberry bug is an example of an insect that has evolved within 100 generations over 20 to 50 years to feed on an introduced plant with chemical composition similar to its native host plant. Many introduced plants have been here for hundreds of years. Equal numbers of insects are now found feeding on both native and non-native plants, as we have reported in this post on which you are commenting.

    A balance betwen predators continues for as long as there’s no outside influence to disrupt the balance native species all long ago reached. A plant that is too hard, bitter, acidic for any native predator to use will often result in it’s exponential reproduction. This soon crowds out everything else, even to the point of extinction for certain of the native organismsms .We know that a disruption of temp regimes has a deleterious effect on native organisms that co-evolved there. But AGW deniers try to tell us that plant life will become enhanced by the addition of CO2 to the environment. But we have numerous lines of evidence that point to no increase in tree trunk girth or height whatsoever. Nor has there been an increase to overall biomass over this last century when CO2 levels increased dramatically.

    So how can they say this to people who know better? Because they are actually speaking to people like you who show the same contempt for science and common sense that industry apologists like Lindzen and Seitz display when they denigrate the IPCC consensus. When you challenge the aversion the rest of us to introducing alien species to the local environment, Imust question your motives. Your knowledge has already been proven lacking.
    Webmaster: Again, you try to tie this blog to global warming deniers. This is a rhetorical device which is simply false.

    We are speaking to those who are willing to look at the scientific studies that challenge their assumptions. We do not have “contempt for science.” We believe only science can settle this debate. Our motives are simple and we state them repeatedly here: we do not want healthy trees destroyed needlessly; we do not want public parks to be sprayed with herbicides; we do not want our air polluted or our homes endangered by prescribed burns; we do not want to be fenced out of our public parks in order to create fenced native plant museums. We have nothing against native plants and we do not advocate for introducing non-natives. By all means, plant more natives. Just quit destroying those established non-native plants and trees that are not doing any harm.

    How do I know? Here’s my experience in the matter.

    To combat just one simple plant – Knapweed, we had to import from Eastern Europe several insect predators that controlled knapweed over there where it evolved. We then had to keep them isolated in labs until we could prove indisputably that these insects fed on nothing else but the knapweed scourge then forming knapweed monocultures out of once highly diverse meadows, grasslands and pastures.
    We had to know if these insects were “host-specific” – meaning they would eat knapweed and only knapweed, leaving all native plants and crops entirely alone.

    The process was so intense, and involved so many different fields, all with their commensurate PhDs needed to responsibly oversee progress, that I it occuured to me to calculate the expense that went into hatching each of 6 European weevils that were then all stacked* up on my forefinger.
    (* as they were prone to do since they mount and attempt to mate wirh each other indescriminately and without regard to the actual sex of the one being mounted thereby stacking up on each other as high as their own balance will allow )

    Anyhow. That particular stack I estimated at around $50,000. This I calculated by taking the approximate number of weevils we managed to raise for release into knapweed infestations, then divided that number by the overall cost of the bio-control project itself. We released THOUSANDS and thousands of them all over N. America!

    So what could prompt the need for such an expensive projectt?

    Endless fields of a thistle-like plant so bitter that merely brushing up against left a bitter taste in your mouth…no doubt the same feature that kept native insects or herbivorous grazers away.

    And THAT is also why we look suspiciously at you wanting to keep introduced plants.
    It is far, far more complicated than the simple matter you make it out to be. Ask any Australian what they think about introducing foreign toads they hoped would reduce another problem they had with a reproducting rodents.

    .Believe me, it has absolutely nothing to do with whether butterflies are better off with more fennel to land on.
    Webmaster: This explanation of your work speaks volumes. A lot of money is spent in this field. This creates economic interests that are highly motivated to retain funding for their projects. You could support your economic interest by urging your colleagues to make the necessary distinction between those non-native species that cause economic harm and those that do not. These destructive and needless projects are undermining your mission.

    We have no economic interest in our advocacy. As long as we see our urban parks being destroyed by pointless native plant “restorations” we will continue our advocacy. Our only reward is the preservation of our public lands.

    There are specific instances in which a specific introduced species is causing significant economic or health damage. Sudden Oak Death in California is an example. In those cases, taking action is appropriate and necessary. In our urban setting we find no such justification for the damage that is being done in our public lands. There is no doubt that there is more harm than benefit in these urban “restorations.”

    1. What rationale is being offfered by local politicians for this effort? The spotty explanation I was able to extract from your version of events has me thinking along the lines of la move toward the less water-intensive (sometimes called “xeric hygroscaping” ) that indeed many towns have opted to pursue that are located somehere in the Senora rain-shadow that strecthes all the way to here in Kamloops, BC located at its most northerly tip and is also characterized by plants preffering a semi-arid moisture regime like those native to your area.
      Webmaster: Politicians do not offer any “rationale” for these “restorations.” They merely respond to the demands that are being made on them. They acquiesce to whatever group makes the most noise or the one that politicians perceive to have the most votes, such as the Sierra Club.

      The “rationale” for these projects varies. Different people use different arguments. A common one is the professed desire for greater biodiversity. That rationale only makes sense if you subscribe to the notion that all non-native plants must be excluded from the calculation, a notion which we obviously reject.

      Another common argument is the bogus claim that if all non-natives are not eradicated, all natives will become extinct. This claim would only make sense if you blamed all extinctions on non-native plants. Climate change will cause far more local extinctions than any non-native species.

      As for the drought tolerant argument…some people make that argument but those who are actually engaged in these projects do not because their projects must be irrigated for several years if they are to be successful. Ironically, these irrigation systems are being installed in places that were previously occupied by non-native plants that did not require irrigation. It is another argument that is contradicted by the facts on the ground.

      By the way, hostilty is gone. I’m listening Its just that I have an alter-ego that loves to do battle with Tea party pinheads and Glenn Beck fans alike.. I read your references to having a personal preference for the introduced species as comoong from the ame place that sees industry and unrestricted capitalism as inviolable because their world has always been one where business opportunities decide military and political policy toward other nations, people and cultures. A sensibility that shares a great deal with the Brits who genocidal wars against Africans as the colonial perogative born of those whose white Anglo features marked their own racial superiority. I love to hate these people, if you get what I mean.
      Webmaster: We are delighted that hostility is gone. We enjoy these dialogues and we always learn from them if only to understand the perspective of the “other side.” We are tempted by, but will resist a tangential discussion about politics. Suffice to say that we are yellow dog Democrats who are more likely to watch Jon Stewart’s Daily Show than Glenn Beck (who is now thankfully off the air).

  5. Or to state it more directly, I thought you were advocating against local biologists because your personal artistic sensibilities preferred large trees over small prickly=pear cacti, thereby rendering all science on the matter moot and unrelated to what was really important…yourself. For that, I apologize.

    Just don;t say something that might see me wishing to revoke my earlier display of reasonableness here being offered to someone who may still turn out to be allied with Kochs, Armey, and the US Chamber of Commerce plotters conducting a stealth coup of democratic forms of government. 😉

  6. Do you have a better reference for this?
    “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.”
    The link you provide doesn’t support your claims. Not even remotely.

    Webmaster: What better source of information about the roof on the Academy of Sciences than the Academy of Sciences? The link is to their report of their monitoring of the roof. The graphs on page 8 and 9 support the statement: There are more non-naitve than native plants in quadrants D and E and the number of insects in D and E are equal to those found in quadrants A and B where there are more native plants.

    1. Now I see the source of your confusion. Lack of familiarity with statistics and with taxonomy led you to interpretations that are not warranted by the data shown on those graphs.

      The difference in number of native and non-native species in quadrants D and E are not statistically significant. Moreover, in order to talk about dominance, other measures would be necessary: total ground cover or biomass, for instance. Number of species tells you nothing at all.

      Webmaster: Nor did I claim that the differences are statistically significant. The statements I made, however, are consistent with the graphs in the monitoring report. The monitoring report contains a map of the area which shows that the quadrants are of equal size.

      The next graph does not count numbers of insects and much less measure insect biomass; it merely counts the orders. Each order of insects includes many thousands of species. Thus one mosquito counts as one order, Diptera. Five thousand flies, mosquitoes, gnats and midges also count as just one order. In other words, you cannot say: “there were an equal number of insects found in the quadrants dominated by native plants and those dominated by non-native plants.” Both parts of that sentence are incorrect.

      Webmaster: “Equal number of insects” may be poorly worded. “Equal number of orders of insects” would be more accurate. The Academy reported the number of orders, not the number of species or the number of individual insects. We used the data provided, You have offered no evidence that the number of species or the numbers of individual insects would lead to a different conclusion. The fact remains that the diversity of orders of insects in all quadrants are roughly equal.

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