Biotic invasions: Causes, epidemiology, global consequences, and control

Biotic invaders are species that establish a new range in which they prolif erate, spread, and persist to the detriment of the environment. They are th e most important ecological outcomes from the unprecedented alterations in the distribution of the earth's biota brought about largely through human t ransport and commerce. In a world without borders, few if any areas remain sheltered from these immigrations. The fate of immigrants is decidedly mixed. Few survive the hazards of chron ic and stochastic forces, and only a small fraction become naturalized. In turn, some naturalized species do become invasive. There are several potent ial reasons why some immigrant species prosper: some escape from the constr aints of their native predators or parasites; others are aided by human-cau sed disturbance that disrupts native communities. Ironically, many biotic i nvasions are apparently facilitated by cultivation and husbandry, unintenti onal actions that foster immigrant populations until they are self-perpetua ting and uncontrollable. Whatever the cause, biotic invaders can in many ca ses inflict enormous environmental damage: (1) Animal invaders can cause ex tinctions of vulnerable native species through predation, grazing, competit ion, and habitat alteration. (2) Plant invaders can completely alter the fi re regime, nutrient cycling, hydrology, and energy budgets in a native ecos ystem and can greatly diminish the abundance or survival of native species. (3) In agriculture, the principal pests of temperate crops are nonindigeno us, and the combined expenses of pest control and crop losses constitute an onerous "tax" on food, fiber, and forage production. (4) The global cost o f virulent plant and animal diseases caused by parasites transported to new ranges and presented with susceptible new hosts is currently incalculable. Identifying future invaders and taking effective steps to prevent their dis persal and establishment constitutes an enormous challenge to both conserva tion and international commerce. Detection and management when exclusion fa ils have proved daunting for varied reasons: (1) Efforts to identify genera l attributes of future invaders have often been inconclusive. (2) Predictin g susceptible locales for future invasions seems even more problematic, giv en the enormous differences in the rates of arrival among potential invader s. (3) Eradication of an established invader is rare, and control efforts v ary enormously in their efficacy. Successful control, however, depends more on commitment and continuing diligence than on the efficacy of specific to ols themselves. (4) Control of biotic invasions is most effective when it e mploys a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders. (5) Prevention of invasions is mu ch less costly than post-entry control. Revamping national and internationa l quarantine laws by adopting a "guilty until proven innocent" approach wou ld be a productive first step. Failure to address the issue of biotic invasions could effectively result i n severe global consequences, including wholesale loss of agricultural, for estry, and fishery resources in some regions, disruption of the ecological processes that supply natural services on which human enterprise depends, a nd the creation of homogeneous, impoverished ecosystems composed of cosmopo litan species. Given their current scale, biotic invasions have taken their place alongside human-driven atmospheric and oceanic alterations as major agents of global change. Left unchecked, they will influence these other fo rces in profound but still unpredictable ways.