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Non-native species are NOT the “second greatest threat to species in peril”

May 22, 2011

One of many doom and gloom scenarios used by native plant advocates to frighten the public into accepting their destructive “restoration” projects is the claim that “non-native species are the second greatest threat to the survival of species in peril.”(1)  Although the statement originates with a scientific study published in 1998, the context in which it was originally reported has long since been lost as it has been cited more than 700 hundred times in scientific studies according to Mark Davis.(2)

The original 1998 article in BioScience by Wilcove clearly states that the claim is not based on any actual data:

“We emphasize at the outset that there are some important limitations to the data we used.  The attributes of a specific threat to a species is usually based on the judgment of an expert source, such as a USFWS employee who prepares a listing notice or a state Fish and Game employee who monitors endangered species in a given region.  Their evaluation of the threats facing that species may not be based on experimental evidence or even on quantitative data.  Indeed, such data often do not exist.”(3)

This caveat is rarely repeated when the claim is invoked by native plant advocates to justify their crusade against non-native plants and animals.  In fact, since the statement was originally made over a decade ago, it is now repeated without reference to the original source.  It has acquired the status of a mantra amongst native plant advocates that requires no citation to substantiate its “truthiness.” 

The Wilcove article in BioScience in which this statement was made was heavily influenced by selecting a geographic area which is not representative of the United States as a whole.  Although Hawaii is a part of the United States its rates of extinction are not typical of the contiguous states of the union.  Rates of extinction are substantially higher on islands because they contain many more endemic (unique) species that do not occur elsewhere. These endemic populations are small and vulnerable to the introduction of competing species.  Native populations on islands are not supplemented by immigrations as they are elsewhere.   

Coqui frog is being eradicated in Hawaii. USDA photo


If Hawaii is removed from the anecdotal information in the Wilcove article, the rates of extinction are comparable to those in Canada where introduced species are considered the least important of six categories of causes of extinction (habitat loss, over-exploitation, pollution, native species interactions, and natural causes such as storms) identified in a similar article in 2006(4).  This list doesn’t include climate change, which is now considered a serious threat for extinction.    Similar studies in the continental United States have reached similar conclusions.(5)

At the time the Wilcove et. al. article was published there was no evidence of a single extinction (or even local extirpation) of a native plant in the continental US resulting from competition from an introduced species of plant.  Clearly, the authors of this study were guilty of exaggeration.(6) 

Although native plant advocates have misused this publication by taking it out of context, the authors were complicit in its misuse by selecting a geographic area that is not representative of the United States.   Non-native species are NOT the second greatest threat to the survival of endangered native species.  In fact, they probably aren’t the third, fourth, or fifth greatest threat to native species. 

 We wish that native plant advocates would examine the origins of their assumptions more carefully.  We believe if they did so they would modify their destructive projects to reflect a more inclusive view of nature. 

(1) Wilcove, DS, Rothstein, D., Dubrow, J., Phillips, A., and Losos, E, “Quantifying threats to imperiled species in the United States,” BioScience, 48, 607-615, 1998. 

(2) Davis, Mark, Invasion Biology, Oxford University Press, 2009, page 181.

(3) Ibid.

(4) Venter, O, et. al., “Threats to endangered species in Canada,” BioScience, 56, 903-910, 2006.

(5) Ibid., page 182

(6) Ibid., page 183

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