Alien threat.

• Author: Bright, Chris
• Position: Introduced species supplanting native ones - Environmental Intelligence

Forty-nine percent of the species threatened with extinction in the United States are in trouble at least partly because of non-native "exotic" species, according to a study published in the August 1998 issue of Bioscience. An exotic species is a plant or animal that has been released into an ecosystem in which it did not evolve. If the exotic finds conditions to its liking, it may undergo a population explosion, suppressing or even wiping out native species in the process. The study, "Quantifying Threats to Imperiled Species in the United States," supports the general view held by ecologists, that exotics arc second only to habitat loss as a cause of endangerment. But that view has yet to translate into effective policy. David Wilcove, senior ecologist at the Environmental Defense Fund and lead author of the study, called exotics "the least recognized threat to wildlife today."

The reported size of the exotic threat may surprise even some ecologists. Previous research had suggested that exotics were threatening 35 to 43 percent of the species officially listed as endangered in the United States. The authors of the present study ascribe their higher figure in part to a more thorough accounting of endangered species in Hawaii, where exotics are much more disruptive than on most of the mainland. (Native island species have often evolved without the full range of competition, disease, and other pressures to which their mainland counterparts are exposed; that tends to make them more vulnerable to exotics.) Another reason for the higher figure is the authors' interest in including the whole spectrum of threats for each species, rather than just the primary threat. (Surveys that register only the primary threat tend to mask the importance of every factor other than habitat loss.)

Apart from exotics, the study found that 85 percent of the species surveyed were threatened by habitat loss, 24 percent by pollution, 17 percent by over-harvesting, and 3 percent by diseases, caused by both exotic and native pathogens. (The percentages don't add up to 100 because most of the species are affected by several threats.) The relative importance of the threats varied somewhat from one kind of organism to another, but habitat loss always came out on top. Because it is by far the...