Biotic Invasions: Causes, Epidemiology, Global Consequences and Control

Summary

Humans have accomplished an unprecedented redistribution of the earth's living things. Both incidentally and deliberately, through migration, transport, and commerce, humans are continuing to disperse an ever-increasing array of species across previously insurmountable environmental barriers such as oceans, mountain ranges, rivers, and inhospitable climate zones. Among the most far-reaching consequences of this reshuffling is a sharp increase in biotic invaders — species that establish new ranges in which they proliferate, spread, and persist to the detriment of native species and ecosystems. In a world without borders, few if any areas remain sheltered from these wholesale immigrations, and for some areas such as oceanic islands, the trend has become an onslaught.

Despite ubiquitous arrivals of new plants, animals and microorganisms, the fate of immigrants is decidedly mixed. Few survive and only a small fraction become naturalized. Most that do become naturalized exert no demonstrable impact in their new range. However, some naturalized species do become invasive, and these can cause severe environmental damage. There are several potential reasons why immigrants succeed: Some escape constraints such as predators or parasites, some find vacant niches to occupy, some are aided by human-caused disturbance that disrupts native communities. Whatever the cause, successful invaders can in many cases inflict enormous ecological damage.

The scientific literature reviewed by the panel makes it clear that:

• Animal invaders can cause extinctions of vulnerable native species through predation, grazing, competition, and habitat alteration.

• Plant invaders can completely alter the fire regime, nutrient cycling, hydrology, and energy budgets in a native ecosystem, greatly diminish the abundance or survival of native species, and even block navigation or enhance flooding.

• Many non-native animals and plants can hybridize with native species.

• In agriculture, the principle pests of temperate crops are non-native, and the combined expenses of pest control and crop losses constitute an onerous "tax" on food, fiber, and forage production.

• The global cost of virulent plant and animal diseases caused by organisms transported to new ranges and presented with susceptible new hosts is currently incalculable.

Identifying future invaders and taking effective steps to prevent their dispersal and establishment is an enormous challenge to ecology, agriculture, aquaculture, horticulture and pet trades, conservation, and international commerce. The panel finds that:

• Identifying general attributes of future invaders has proven difficult.

• Predicting susceptible locales for future invasions seems even more problematic, given the enormous differences in commerce among various regions and thus in the rate of arrival of potential invaders.

• Eradication of an established invader is rare and control efforts vary enormously in their efficacy. Successful control depends more on commitment and continuing diligence than the efficacy of specific tools themselves (trapping or spraying insecticides, releasing biological control agents).

• Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders.

• Prevention of invasions is much less costly than post-entry control.

Revamping national and international quarantine laws by adopting a "guilty until proven innocent" approach, instead of the current strategy of denying entry only to species already proven noxious or detrimental, would be a productive first step. The global consequences of failing to address the issue of invasions effectively would be severe, including wholesale loss of agricultural, forestry and fishery resources in some regions and disruption of the ecological processes that supply us with natural services on which the human enterprise depends. Given their current scale, biotic invasions have also taken their place alongside human-driven atmospheric and oceanic change as major agents of global change, and left unchecked, will influence these other forces in profound but still unpredictable ways.