What is the most serious cause of species extinction?

I think the answer is pretty clear. Just looking at my own Potomac River watershed I can see that it’s changed a lot over the past few centuries. For many creatures, the changes have not been for the better. The streams feeding into the river are much warmer, forcing the native trout to retreat up into the highest mountain brooks. The dam at Little Falls bars shad, herring, and sturgeon from their upstream spawning areas. Silt caused by erosion smothers insect nymphs and other creatures that live on the stream bottom.

So without hesitation I reply, “Habitat loss.”

Now, how about the second most serious cause? I know that most of the familiar fish in the river originally came from somewhere else. I tick them off: Smallmouth bass, channel catfish, sunfish, walleye pike, the toothy muskellunge, and now the equally toothy snakehead from China. These must have had a tremendous impact on native creatures when they moved into the neighborhood.

My answer for number two is, “Invasive species.”

An apparent consensus. It turns out I’m right, at least according to a quick Google search. Here’s a sampling of what I found:

“Invasive species represent the second leading cause of species extinction and loss of biodiversity in aquatic environments worldwide,” says the U.S. Environmental Protection Agency.

“Exotic invasive species are a major threat to many ecosystems worldwide and the second leading cause of extinction behind habitat destruction.” This is from the University of Michigan’s Botanical Garden.

“Invasive species are second only to habitat destruction as the major cause of extinction,” says the U.S. Park Service in a publication on the Point Reyes National Seashore.

“Invasive species are the second leading cause of animal decline and extinction, worldwide!!!” Red is the color of choice for Wyoming’s Natrona County Weed and Pest district.

“The introduction of exotic species that replace local and native species is cited as the second largest cause of biodiversity loss.” The United Nations Environment Programme said this as part of its observance of the “International Year of Biodiversity.”

“After habitat loss, nonnative species are the second leading cause of endangerment.” This appears in a “Solutions Paper” on forests and biodiversity loss in Latin America commissioned by the Inter-American Development Bank.

Fact or something else? Or at least I thought I was right about the number two cause. Now, after attending an American Association for the Advancement of Science meeting in Washington, D.C., I’m not so sure.

One of the speakers was Mark Davis, plant ecologist at Minnesota’s Macalester College and author of the book Invasion Biology. According to Davis, the claim that invasive species are the second greatest cause of species extinction has been cited more than 700 times. It’s anybody’s guess how many more times the same claim has appeared in technical papers, reports, journalistic articles, power point presentations, and essays written by earnest Environmental Studies majors.

According to Davis, few of these claims cite the source for this information, which was a 1998 paper published in BioScience magazine. Its lead author was David Wilcove, a respected professor of ecology and evolutionary biology at Princeton University.

It so happens that Wilcove’s paper said something very different than the “findings” that are so widely being cited. “We emphasize at the outset that there are some important limitations to the data we used,” stated the paper. “The attribution of a specific threat to a species is usually based on the judgment of an expert source.”

The Wilcove paper continues: “[The] 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.”

Again, the paper emphasizes: “. . . assessments of the threats to individual species are often based on the subjective opinions of knowledgeable individuals, rather than experimental evidence or quantitative data.”

In other words, the researchers asked people—albeit experts—what they thought. In most cases the judgements were not based on actual facts. They were opinions.

A snowballing ‘fact.’ I can imagine how this whole chain of misunderstanding may have happened. Hard-line defenders of native species may have seen the Wilcove paper and used its findings—minus Wilcove’s caveats—to add scientific oomph to their anti-non-native positions.

A block of misleading display text on the paper’s first page might also deserve some of the blame. Likely written by an editor with little interest in scientific nuance, it asserted, “Habitat loss is the single greatest threat to biodiversity, followed by the spread of alien species.”

As time went on, a mounting number of Google searches found the same “fact,” and it snowballed from there, along the way gaining authority and legitimacy.

Can anything now be done to clear this up? Not likely, unless someone can find a way to get a snowball to roll back up a hill all on its own. What’s done is done.