Second Leading Cause: How a Runaway Idea became Invasive Species Dogma

One of the most commonly repeated assertions about introduced invasive/exotic species is that they are the second leading cause of biodiversity decline worldwide after habitat destruction.  It is clearly an idea that has captured the imaginations of those who strive to raise awareness of the invasives issue and build support for actions and policies which will help address its consequences. What disturbs me is not that invasives aren't a real problem worthy of attention - from my perspective they assuredly are - but instead that the claim itself has become canonical, an invasives cataclysm recited as dogma by believers in the true faith who willingly repeat it, often without attribution.  Its acceptance as fact simply because it has been said many times by various groups and authorities, rather than based on the measurable scientific data that should inform such a conclusion, is startling to say the least, and worrisome. 

Consider this random sample, plucked straight from the prevailing electronic media stream, of declarative derivations on this "second leading cause" theme:

"The second biggest threat to Connecticut's natural habitats is invasion by alien plants and animals (behind loss of habitat to sprawling land development.)" - Eastern Connecticut Forest Landowners Association

"the second greatest threat to the 500+ endangered & threatened plant species in Florida is adverse effects from invasive exotic plants (the greatest threat is direct habitat destruction via population growth, urban sprawl, etc.). " Florida Invasive Pest Plant Council

"Ecologists believe that biological invasions are second only to habitat destruction as a threat to biodiversity."  Commonwealth Scientific and Industrial Research Organisation, Australia

"In the past 25 years, exotic invasive species has risen from the 6th to the 2nd threat to biodiversity, second only to habitat destruction." -Great Lakes United

"The World Conservation Union has identified invasive alien species as the second most significant threat to biodiversity, after habitat loss."  Alberta Invasive Plant Council

"The renowned Harvard biologist Edward O. Wilson has claimed that the introduction of alien species is second only to habitat destruction as the leading cause of extinctions worldwide." Discover Magazine

There is remarkable consensus in the environmental community that these unsubstantiated statements should be accepted as fact, but startling vagueness as to their source material and originators.  Those versions that are actually attributed to someone, not simply repeated as mantra, are often themselves mere repetitions, reinforced with the added weight and authority of those who repeat them.  Thus the USDA or The Nature Conservancy declares invasive species to be the second greatest threat and becomes the cited authority by a local Exotic Pest Council or waterhsed group in its publications and pronouncements on the subject. There is no objective basis for evaluating such claims taken at face value or their implications if substantiated.  At issue is the misrepresentation and exaggeration of scientific data though careless repetition and popular imagination.  This is very shaky ground on an issue where we cannot afford sloppy science. 

So where did this idea originate, and what is the basis for its conclusions?  Apparently it was first proposed in The Diversity of Life (1992) by no less authority than biologist and Harvard Professor E O Wilson, who has compellingly addressed issues of biodiversity and the anthropogenic factors accelerating global extinctions.  According to a 2003 letter by two Arizona State Professors, published in Science Magazine, the basis of Wilson's reasoning on the biodiversity impacts of invasive species is "a vast intuitive extrapolation from unpublished data about North American fishes."  The authors contend that the idea's emotional appeal allowed it to proliferate "without rigorous quantitative support."

Wilson is concerned with biodiversity decline, and places invasive species after habitat destruction as the leading cause worldwide.  So far, so good, assuming you have read his work and accept the evidence Wilson marshalls in support of his argument.  Most of us do not read all the primary source material that informs what we accept as true, but scientists should be concerned with documentation and the assumptions behind hypotheses.   

With that admonition and taken at the surface level, this is still a compelling idea.  The second leading cause of anything must be, by implication, highly significant.  Yet according to Daniel Simberloff, in the department of Ecology and Evolutionary Biology at the University of Tennessee who writes in the March-April, 2002 edition of  The American Scientist, "habitat destruction contributes to the threat to 85 percent of all imperiled and federally listed species in the United States, and contributes similarly to the threat to imperiled birds alone."  If true, then at least in the United States invasive species are a far distant second leading cause of rare species decline.   

There is also a tendency for those who repeat Wilson's statement to substitute "plants" for "species" when speaking of leading causes of biodiversity decline.  In fact, I am unaware that there is strong evidence of even a single rare species that has gone extinct from causes attributed primarily to invasive plants: invasive animals and pests and pathogens have caused extinctions, certainly, but not plants so far as I know.  Invasive plants are a legitimate concern with extensive environmental, economic and cultural impacts that are worth developing strategies to address, but declaring them the second leading threat to rare species is just not supported by the available evidence.

E O Wilson has written since then that "Extinction by habitat destruction is like death in an automobile accident: easy to see and assess. Extinction by the invasion of exotic species is like death by disease: gradual, insidious, requiring scientific methods to diagnose."  I am in complete accord with this conclusion. We do need better measures and better data informing our understanding of the behavior and impacts of invasive species.  We also need better standards for declarations of scientific fact.  No matter how compelling it may be to repeat and how it fires the imagination, "the second leading cause" theme is just a meaningless mantra without them.

Comments

[Genevieve]

That was very interesting. Just from my own observation of some of the main invasive plants around here (mimosa tree, japanese honeysuckle, kudzu, bermudagrass, etc.), I would say that subdivisions, parking lots and highways are much more rapid and efficient consumers of the unspoiled land.

[GreenmanTim]

Thanks, Genevieve. It felt a bit traitorous to be exposing the nakedness of the emperor in this post when I am a strong supporter of finding strategies that can address the impacts of invasive species on things we value and are trying to conserve. I just can't abide empty rhetoric and unquestioned assumptions on issues of this import and with such associated costs.

I also freely admit that the synergistic impacts of invasives with other environmental stressors may in combination be a more devastating blow than as independant actors. The decline of certain bird species due in large measure to habitat loss is exascerbated by invasive plants replacing preferred native communities, but this is not the only factor in their decline.

[Charlottesvillain]

Well, the dogma may not be supported by the science, but I can certainly see why discussion on this issue becomes so hyperbolic. I war relentlessly against kudzu and mimosa, and both are nearly impossible to kill. I don't know whether they are displacing habitat or not, but efforts to eradicate them, or even contain them, seems damn near impossible.

The invasive threat seems perhaps more destructive at the marine level. Freshwater lakes seem highly susceptible to invasion, and it seems as though those environments are more easily thrown off balance. Zebra mussel, milfoil, and now the dreaded snakehead; these and other invasives seem to have enormously destructive tendencies.

[GreenmanTim]

No argument from me on that score, CV. The destructive capabilities of many invasive organisms are real threats and management nightmares. It was the unquestioned soundbite that distorts the underlying science that I got all twisted up about. Sort of reminds me of the "What is a Liberal" discussion you prompted yesterday at Tigerhawk and the thread about questioning authority that emerged from it.
Question assumptions but not your resolve to address the underlying problems. And as Mario Cuomo was fond of saying - and doubtless it does me good to remember - "Any Jackass can kick down a barn." :-)

[Jenn]

Well, I certainly cannot argue with your statement that "second leading cause..." has become meaningless mantra. Really, it's just fodder for leading off any paper about invasive species, in that first paragraph where you're supposed to cite all the seminal papers so the reader can go back and consult them. I've seen people use this line and cite papers that are actually citing others, and I've seen people cite reviews as the source. I cringe every time I hear or read it.

However, there is at least one study that has come out over the past ten years that scientifically supports the statement: Wilcove et al. in Bioscience in 1998 ("Quantifying threats to imperiled species in the United States." - v. 48, pp. 607-615). I read it but darnit I cannot find a copy of it right now. It's got graphs and everything :-). That's the main one I see cited when writers choose to repeat the mantra - I don't really feel that the Davis paper referred to in that Science letter addressed Wilcove et al., and I don't remember seeing Matt Chew's "in press" paper he cites.

The other source I have seen is a bit vaguer - a book published in 2001, edited by Sala and Chapin, titled "Future scenarios global biodiversity." It was cited repeatedly while it was still in press. I have not read it so I am not sure what work was done and whether it was by the editors or by a chapter in the book authored by someone else.

I had never heard the idea that E.O. Wilson was behind the statement until I read your post.

Setting aside the difficulties in truly teasing apart habitat loss from invasive species (in many cases one can't happen without the other), I find that we've come up against that same problem we always do. Do we take a few decades to attempt to settle the argument or do we instead direct our resources to prevent further invasive species introductions? Maybe we can just all agree that invasive species is just one of several threats to native biodiversity.

[GreenmanTim]

Thanks, Jenn, for your - as always - informed and thoughtful perspective. Invasive organisms threaten biodiversity along with other things we humans value, such as rangeland, scenic vistas, our backyards. Allocating resources and policy to address these impacts is the overwhelming challenge, and in my experience, their being a leading cause of biodiversity is but one selling point for such efforts, and not always the primary motivator. The "nuisance factor" of acres of barberry, tangles of bittersweet, fouled boat propellors and dead trees blighted by introducted pests and pathogens motivates a broader swath of the population to deal this this problem.

[Nuthatch]

In their massive publication, Threatened Birds of the World, BirdLife International places invasive species as the 3rd biggest threat to birds, impacting 25% of birds they categorize as at-risk globally. Habitat destruction is #1 (85% of species) and direct exploitation, mainly for food and cage birds, is #2 (31%; a species can have more than one threat).

[Rosie Redfield]

I think you may be misinterpreting the Simberloff quote. Saying that "habitat destruction contributes to the threat to 85 percent of all imperiled and federally listed species..." is not the same as saying that 85% of the threat is from habitat destruction.

[Lab Lemming]

Looking at biodiverity threats from an Australian/New Zealand POV, I would say that it is hard to overestimate the threat posed by introduced placental mammals. Or the cane toad.

And I seem to remember that a recent study showed that worldwide frog decline was caused by a disease carried by the invasive African clawed frog, and not by habitat destruction, as was previously believed. The internet gives me a popular article here:
http://www.msnbc.msn.com/id/11219037/
But no link to the original journal article on which this is based.

[GreenmanTim]

Rosie, I will concede the fine point but not the broad theme: namely, that the overall contribution of habitat destruction to the loss of federally endangered or threatened rare speces in ther United states is vastly greater than the next greatest threat posed by invasive species. Invasives are unquestionably significant, but on their own are a far distant second to habitat destruction. Invasives alter habitats: bulldozers and pavement eliminate them. Draining swamps has greater impacts overall on biodiversity than the simplification of native plant communities after invasion by invasive plants.

What irks me, and prompted this post, is not that invasive species are not a significant threat, but that the cavalier use of this one "statistic" mistates their impact, paints with too broad a brush -especially substituing "plants" for "species" - , and has been repeatedly uncritically restated to the point of meaningless.