Ever since humans began traveling over land and sea, assorted livestock, crops, pets, pests, and weeds have tagged along. Nearly every region of the globe has benefited economically from introduced species. Yet, new arrivals that become invasive have also created major problems for agriculture and other human enterprises and disrupted distinct communities of native plants and animals. Today, almost 20 percent of the world’s endangered vertebrate species are threatened in some way by exotic invaders, including 13 percent of vulnerable mainland vertebrates and 31 percent of those on islands .
In fact, invasions of natural ecosystems by nonnative species now rank second to habitat loss as the major threat to biodiversity . The sparse data available so far suggest that the pace of invasions is accelerating in parallel with the growth of global trade. In the San Francisco Bay area, for instance, the rate of successful aquatic invasions has climbed from one new species every 36 weeks since the 1850s, to one every 24 weeks since the 1970s, to as many as one every 12 weeks in the past decade . Some ecologists predict that as the number of potential invaders increases and the supply of undisturbed natural areas declines, biological pollution by alien invaders may become the leading factor of ecological disintegration . (See Bioinvasions Represent Broadscale “Biological Pollution.)
Bioinvasions Represent Broadscale “Biological Pollution”
|Percentages of Introduced Plant Species in Selected Countries|
|COUNTRY/REGION||NATIVE SPECIES||INTRODUCED SPECIES||PERCENTAGE INTRODUCED|
Source: Vernon H. Heywood, “Patterns, Extents, and Modes of Invasions by Terrestrial Plants,” in Biological Invasions: A Global Perspective, SCOPE 37, J.A. Drake et al., eds. (John Wiley & Sons Ltd., Chichester, U.K., 1989), p. 40.
1. Ian A.W. Macdonald et al., “Wildlife Conservation and the Invasion of Nature Reserves by Introduced Species: A Global Perspective,” in Biological Invasions: A Global Perspective, SCOPE 37, J.A. Drake et al., eds. (John Wiley & Sons Ltd., Chichester, U.K., 1989), pp. 232-233.
2. Edward O. Wilson, The Diversity of Life (W.W. Norton & Co., New York, 1992), p. 253.
3. Andrew Cohen, “Stopping Ballast Water Invaders,” Native Species Network, Vol. 1, Issue 2 (Spring 1996), p. 1.
4. Jeff Crooks and Michael E. Soulé, “Lag Times in Population Explosions of Invasive Species: Causes and Implications,” in Proceedings of the Norway/UN Conference on Alien Species, The Trondheim Conferences on Biodiversity, July 1-5, 1996, O.T. Sandlund et al., eds. (Directorate for Nature Management/Norwegian Institute for Nature Research, Trondheim, Norway, 1996), p. 39.