Alien threat.

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 broadest type of pressure, the authors broke habitat loss down into a set of 14 smaller categories. These categories overlap, but taking them in their fullest sense, the most important ones are agriculture (which affected 38 percent of the species surveyed), commercial development (35 percent), and water development (30 percent, including the 13 percent threatened by irrigation, which are also counted under agriculture).

Habitat loss is usually linked directly to human activity but exotics, once established, can continue to spread on their own. The study suggests that their mobility could undercut the U.S. Endangered Species Act, at least as it applies to private property. While the Act prohibits activities that would do direct harm to listed species, it does not require that property be managed in a fashion conducive to their long-term survival. As the study notes, where development plans have been blocked by the presence endangered species, landowners may sometimes have the option of simply waiting for invaders to erase the obstacle. On the other hand, landowners interested in conservation may be faced with expensive exotic control chores that are usually not even tax deductible.

The authors surveyed 2,490 types of organisms that were either officially "listed" under the U.S. Endangered Species Act, or that had been identified as endangered by the Nature Conservancy in conjunction with several major biological survey programs. They found data sufficient to assess the threats faced by 1,880 of these. The study did not attempt to quantify exotic pressure that has not yet resulted in endangerment, although exotics frequently suppress native species without pushing them to the brink of extinction. Nor did the study attempt to extrapolate from the U.S. experience to the global dimensions of the threat. But a study just released by Worldwatch, Life Out of Bounds, argues that in many other parts of the world, the ecological, economic, and social effects of exotics are at least as bad and in some cases worse than in the United States.