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.