Skip to content

Doug Tallamy refutes his own theory without changing his ideology

August 14, 2012

In our debates with native plant advocates, the scientist who is most often quoted to support their beliefs is Doug Tallamy who wrote an influential book, Bringing Nature Home:  How Native Plants Sustain Wildlife in our Gardens .    Professor Tallamy is an entomologist at the University of Delaware.

Professor Tallamy’s hypothesis is that native insects require native plants because they have evolved together “over thousands of generations.”  Because insects are an essential ingredient in the food web, he speculates that the absence of native plants would ultimately result in “ecological collapse” as other animals in the food web are starved by the loss of insects. (1)

Professor Tallamy freely admits that his theory is based on his anecdotal observations in his own garden, not on scientific evidence:  “How do we know the actual extent to which our native insect generalists are eating alien plants?  We don’t until we go into the field and see exactly what is eating what.  Unfortunately, this important but simple task has been all but ignored so far.”  (1)

This research has now been done to Professor Tallamy’s satisfaction by a Master’s Degree student under his direction.  The report of that study does not substantiate Professor Tallamy’s belief that insects eat only native plants.  In his own words, Professor Tallamy now tells us:

“Erin [Reed] compared the amount of damage sucking and chewing insects made on the ornamental plants at six suburban properties landscaped primarily with species native to the area and six properties landscaped traditionally.  After two years of measurements Erin found that only a tiny percentage of leaves were damaged on either set of properties at the end of the season….Erin’s most important result, however, was that there was no statistical difference in the amount of damage on either landscape type.” (2)

Corroborating Evidence

This finding that insects are equally likely to eat native and non-native plants may be new to Professor Tallamy, but it isn’t new to the readers of Million Trees.  We have reported many studies which are consistent with this finding.

Anise Swallowtail butterfly in non-native fennel

The English garden, where plants from all over the world are welcome

Specialists vs. Generalists

When debating with native plant advocates, one quickly learns that the debate isn’t ended by putting facts such as these on the table.  In this case, the comeback is, “The insects using non-native plants are generalists.  Insects that are specialists will not make that transition.”  Generalists are insects that eat a wide variety of plants, while specialists are limited to only one plant or plants in the same family which are chemically similar.   

Professor Tallamy offers in support of this contention that only “…about 10 percent of the insect herbivores in a given ecosystem [are not specialists],” implying that few insects are capable of making a transition to another host plant. 

However, categorizing insects as specialists or generalists is not a dichotomy.  At one extreme, there are some insects that choose a single species of plant as its host or its meal.  At the other extreme, there are insects that feed on more than three different plant families.  It is only that extreme category which has been estimated at only 10% of all phytophagous (plant-eating) insects.  The majority of insects are in the middle of the continuum.  They are generally confined to a single plant family in which the plants are chemically similar.   

Putting that definition of “specialist” as confined to one plant family into perspective, let us consider the size of plant families.  For example, there are 20,000 plant members of the Asteraceae family, including the native sagebrush (Artemisia) and the non-native African daisy.  In other words, the insect that confines its diet to one family of plants is not very specialized. 

Soapberry bug on balloon vine. Scott Carroll. UC Davis

Professor Tallamy offers his readers an explanation for why specialist insects cannot make the transition from native to non-native plants.  He claims that many non-native plants are chemically unique and therefore insects are unable to adapt to them.  He offers examples of non-native plants and trees which “are not related to any lineage of plants in North America.”  One of his examples is the goldenrain tree (Koelreuteria paniculata).  This is the member of the soapberry (Sapindaceae) family to which the soapberry bug has made a transition from a native plant in the soapberry family in less than 100 generations over a period of 20 to 50 years.  Professor Tallamy’s other examples of unique non-native plant species are also members of large plant families which probably contain native members.  Professor Tallamy is apparently mistaken in his assumption that most or all non-native plants are unique, with no native relatives. 

The pace of evolution

Even if insects are “specialists” we should not assume that their dependence on a native plant is incapable of changing over time.  Professor Tallamy’s hypothesis about the mutually exclusive relationships between native animals and native plants is based on an outdated notion of the slow pace of evolution.  The assumption amongst native plant advocates is that these relationships are nearly immutable. 

In fact, evolution continues today and is sometimes even visible within the lifetime of observers.  Professor Tallamy provides his readers with examples of non-native insects that made quick transitions to native plants:

  • The hemlock wooly adelgids from Asia have had a devastating effect on native hemlock forests in the eastern United States.
  • The Japanese beetle introduced to the United States is now eating the foliage of over 400 plants (according to Professor Tallamy), some of which are native (according to the USDA invasive species website).

These insects apparently made transitions to chemically similar native plants without evolutionary adaptation. If non-native insects quickly adapt to new hosts, doesn’t it seem likely that native insects are capable of doing the same?  That is both logical and consistent with our experience.    For example, the native soapberry bug mentioned above has undergone rapid evolution of its beak length to adapt to a new host.

Although Professor Tallamy tells us that the relationship between insects and plants evolved over “thousands of generations,” he acknowledges much faster changes in plants when he explains why non-native plants become invasive decades after their arrival:  “Japanese honeysuckle, for example, was planted as an ornamental for 80 years before it escaped cultivation.  No one is sure why this lag time occurs.  Perhaps during the lag period, the plant is changing genetically through natural selection to better fit its new environment.”  Does it make sense that the evolution of plants would be much more rapid than the evolution of insects?  Since the lifetime of most insects is not substantially longer than the lifetime of most plants, we don’t see the logic in this assumption. 

Beliefs die hard

Although Professor Tallamy now concedes that there is no evidence that insects are dependent upon native plants, he continues to believe that the absence of native plants will cause “ecological collapse.”  In the same book in which he reports the study of his graduate student, Professor Tallamy repeats his mantra:  “…our wholesale replacement of native plant communities with disparate collections of plants from other parts of the world is pushing our local animals to the brink of extinction—and the ecosystems that sustain human societies to the edge of collapse.”

This alarmist conclusion is offered without providing examples of any animals being “pushed to the brink of extinction.”  In fact, available scientific evidence contradicts this alarmist conclusion. (3)

*********************

(1)    Tallamy, Doug, Bringing Nature Home, Timber Press, 2007

(2)    Tallamy, Doug, “Flipping the Paradigm:  Landscapes that Welcome Wildlife,” chapter in

Christopher, Thomas, The New American Landscape, Timber Press, 2011

(3)    Erle C. Ellis, et. al., “All Is Not Loss:  Plant Biodiversity in the Anthropocene,” http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0030535

13 Comments leave one →
  1. Richard Drechsler permalink
    August 14, 2012 8:34 am

    Thank you. Very informative with useful citations.

  2. August 14, 2012 12:17 pm

    Interesting and important article. Nativists sometimes make it difficult for any insect or animal to dine on native plants when they insist on caging them, after they plant them, to “protect” them from being eaten. Strange but true, and hard on any animal or insect who is looking consciously for food, or “unconsciously” for a flower to pollinate, especially if the area has previously been cleared to plant the natives.

  3. August 16, 2012 12:38 am

    why do you call them alarmists

    Webmaster: Predicting “ecological collapse” in the absence of evidence seems alarmist to us.

  4. September 2, 2012 2:21 pm

    The trouble with using secondary sources is that the probabilities for misinterpretation grow exponentially. Such is the case with Erin Reed’s work cited here. It is better to check her original work:
    http://dspace.udel.edu:8080/dspace/bitstream/handle/19716/9778/Erin_Reed_thesis.pdf?sequence=1
    These are some of her conclusions: “I found moderate evidence to support the prediction that pest populations will remain lower and more stable in native-based landscapes. Native properties supported a higher diversity of herbivores and natural enemies in the herbaceous layer than properties landscaped with non-native plants.”
    This may not be strong evidence, but it is the opposite of refuting Doug Tallamy’s arguments. In the meantime evidence keeps accumulating to support the fact that native plants are better for native wildlife. Take these two examples:

    Webmaster: I read Erin Reed’s thesis before writing this post, but I did not quote from it in this post. Rather I quoted verbatim Professor Tallamy’s description of the thesis. Since Professor Tallamy was Erin Reed’s thesis advisor, we can safely assume that he is accurately describing its content and conclusions. I could cherry pick several quotes from the thesis which would confuse and obfuscate the issue as the thesis is all over the map, including unsubstantiated speculation about why more evidence of greater insect populations in native gardens wasn’t found.

    Amanda M. Conover. The Impact of Non-native Plants on Bird Communities in Suburban Forest Fragments http://ag.udel.edu/enwc/faculty/Williams/Amanda%20Conover%20MS%20Thesis.pdf
    “The proportion of native plants was the best variable in predicting Wood Thrush occupancy. Forest structure variables were the strongest predictors of presence for American Robin, Carolina Chickadee, and Gray Catbird. Both forest structure and native plant proportion were important variables in predicting the occupancy of Eastern Towhee, Northern Cardinal, and Ovenbird. For Carolina Wren, invertebrate abundance was the most important variable in predicting occupancy.”

    Beal Christy, Ph.D. Bird foraging preferences and caterpillar biomass on suburban landscape trees. http://gradworks.umi.com/34/40/3440456.html
    “I found that (1) native trees supported significantly more foraging than non-native trees; (2) migratory bird foraging choices were best explained by the amount of caterpillar frass collected in drop cloths but foraging choices of residential birds were best explained by tree nativity; and (3) bird foraging was highest on the Fagaceae in both studies.”

    This is just the tip of the iceberg. I listed additional evidence published by many authors here: http://nativeplantwildlifegarden.com/the-science-behind-our-passion-for-native-plants/

    Webmaster: We could play dueling citations here, but we won’t because we have provided our readers with plenty of scientific studies that reach other conclusions. We clearly won’t change your mind by engaging in such an exercise.

    And last but not least, this is not ‘Professor Tallamy’s hypothesis that native insects require native plants because they have evolved together “over thousands of generations.”’ You should do well to heed the comments submitted in this very blog by people who know the subject:
    “A preference for native over introduced plant species is not a “movement” but a reflection of the combined knowledge biologists have acquired 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.” (mycos July 26, 2011)

    Webmaster: The phrase evolved together “over thousands of generations” is a verbatim quote from Tallamy’s first book Bringing Nature Home. If it’s not his hypothesis, why did he say that while explaining his theory to his readers?

    Every Ph.D., M.Sc. and B.Sc. does not know this because there is presently precious little support in the scientific community for equilibrium theory. It is a theory that has largely been overturned by more recent studies. Likewise, more recent research has found substantial evidence of rapid evolution in response to changes in the environment.

    Since I am not a scientist I submitted my draft for review by a distinguished professor of evolution and ecology at a major university. He said, “All of your arguments are valid.” He suggested a few qualifications and I made the revisions he suggested. I stand firmly on the arguments I made in this article.<

  5. Dr. Ron Obvious permalink
    November 24, 2013 5:41 pm

    Unfortunately, you have badly misrepresented the facts from the get go of this article and you do a disservice to the public by being uninformed or intentionally misleading. Yes, I could be more polite but I’m sorry but there is little reason to sugar coat this. The above article is rife with cherry picked facts or errors. I’ll just point out a few.

    1) “Professor Tallamy freely admits that his theory is based on his anecdotal observations in his own garden, not on scientific evidence”.

    You took his quote out of context because he regularly cites this large body of literature and has contributed articles himself. If you even visited his website you’d see this. Moreover, there are literally hundreds of recent articles demonstrating how native herbivores fare worse (or cannot survive in most cases) on non-native plants. A Web of Science or Google Scholar search will turn up a large number of hits on the subject using any relevant terms. This isn’t a matter of warring citations. The overwhelming majority of research on the subject demonstrates that introduced plants reduce biodiversity and ecosystem functioning.

    2) You gave several examples of evidence that non-native plants are equally good (or better) than native plants for butterflies etc. Each of these can be taken apart but we’ll start out with one that you unfortunately misinterpreted.

    “Professor Arthur Shapiro (UC Davis) reports that 82 of 236 (35%) total species of California butterflies have been observed either laying their eggs or feeding on non-native plants.”

    Even at a first glance this says that 2/3rds of California butterflies have not been observed laying eggs on or feeding on non-native plants. So the 1/3rd that do lay eggs or feed on non-native plants… how well do they fare? Not very well in many of those instances. I suggest you read some of the Lepidoptera literature (including Shapiro’s research which you must have just skimmed the abstracts). Since you told an earlier poster that you don’t want long lists of references, I’ll spare you, but I urge you to look into this. It’s not actually an unknown and it’s very well-documented that the majority of herbivores are specialists. And yes, some of this is obvious to evolutionary biologists, especially those that study co-evolution.

    3) Some of these studies don’t even show what you claim they do, as pointed out above. But even if they did, you can always cherry pick a small number of examples that show contrary results, but these are decidedly in the minority. This is literally akin to the intelligent design debate. One can always find a few scientists taking a contrarian point of view (prominently Dov Sax, and a few others), but the majority of theory won’t support the position that herbivores suffer no impacts or are generally going to benefit from species introductions. This is basic co-evolutionary theory and one need not go far to find the myriad reasons why it wouldn’t even make sense for the majority of herbivores to be generalists and why it’s the very small minority of species that can quickly adapt (see any of a number of papers by Futuyma. List can gladly be provided).

    Dr. Ron Obvious

    • November 24, 2013 6:46 pm

      We have read all the studies we have cited, including Mr. Tallamy’s book, the thesis of his student, and the chapter he wrote in another book. All the quotes from the cited studies are accurate and have not been “taken out of context.” We have discussed Dr. Shapiro’s study of California butterflies with him and have not misrepresented his study.

      Dr. Obvious has not provided any studies in support of his opinions, which contradict the scientific literature we have cited. His statements about co-evolution, specialization and other assumptions that are the underpinnings of invasion biology are not consistent with empirical evidence. Invasion biology is quickly becoming the “contrarian viewpoint.”

  6. jennifer permalink
    December 9, 2013 6:32 pm

    Well good, let me just tell the monarchs we are being alarmists and everything will be fine.

Trackbacks

  1. Insects Don’t Prefer Native Plants | Save Mount Sutro Forest
  2. Destroying Wildlife Habitats « Save the Trees of Glen Canyon Park, San Francisco
  3. Native Plants are a Moral Choice by Garden Rant | Greenhouses sale
  4. Invertebrates such as insects are plentiful in the eucalyptus forest | Death of a Million Trees
  5. Are non-native plants “ecological traps” for birds? | Death of a Million Trees

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 143 other followers

%d bloggers like this: