Bioinvasions: Stemming the Tide of Invasive Species

by Amy Wagener and Wendy Vanasselt

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 non-native species now rank second to habitat loss as a major threat to biodiversity.

Non-native fish introductions are common in most parts of the world, too, with serious consequences for freshwater and coastal ecosystems. A survey of 31 fish introductions in Europe, North America, Australia, and New Zealand found that in 77 percent of the cases, native fish populations were reduced or eliminated following the introduction of nonnative fish.

The pace of invasions is accelerating in parallel with the growth of global trade. 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.

Burgeoning world trade has a particularly great potential to increase bioinvasions by opening unintentional but major dispersal opportunities. Food- and water-borne disease organisms, agricultural pests and weeds, and other nuisance species hitchhike to new lands aboard ships, airplanes, and trucks, stowed in shipping containers and packing materials or riding on nursery stock, unprocessed logs, fruits, vegetables, and seeds.

Deliberate introductions of exotic plants and animals for commercial and agricultural purposes also can pose risks. The bulk of the diet of most of the world's population comes from crop and livestock species that originated elsewhere, and land managers, agricultural scientists, and other sectors of society have clear economic incentives to continue importing exotic species for food, timber, horticultural, and other uses.

Of course, not all newly arriving species become problems. About 10 percent of introduced species become established in nonnative environments, and about 10 percent of those become pests. But those nonnative species that do become problems cause economic as well as biological damage.

Estimates of economic losses, not including damage to native species or to ecological services, range up to several billion dollars per year in the United States alone. One recent attempt to quantify the economic damages and control costs of invasive species in five countries-the United States, South Africa, the United Kingdom, Brazil, and India-came to $336 billion a year.

What can be done to stem the tide of bioinvasions? For one, before intentionally introducing an exotic, it would be helpful to thoroughly analyze potential risks and trade-offs of the introduction. However, biologists cannot predict with certainty the invasive potential of any given plant, animal, or microbe.

For this reason, a few nations-such as New Zealand, where 47 percent of the flora is already exotic-have adopted the precautionary principle, banning importation of all exotic species except for a few clean-list species that are known to be benign. In contrast, most nations, if they have any import restrictions at all, use a dirty-list concept, only denying import of known problem pests or weeds.

In the case of unintentional invasions, the first line of defense is a system of quarantines and regulations designed to limit the free flow of species through trade, transport, aquaculture, agriculture, forestry, game farming, horticulture, the pet trade, recreation, tourism, and travel.

Some countries are raising the profile of the battle against invasives. For example, in 1999, President Clinton created a United States interagency Invasive Species Council charged with improving education, research, and action against invasives.

There are also signs that individuals are beginning to understand the importance of their role in curbing invasive introductions, given that many invaders reach new territory via people who import seeds for their gardens, shop at nurseries, or transport plants in their luggage. In a recent survey of 157 people, mostly United States citizens interested in horticulture, 83 percent expressed a desire to avoid buying invasive plants.

Once invasives are established, eradication is difficult and costly. As of 1999, South Africa was spending about $35 million a year on efforts to control invasions of woody tree species which, unlike native species, consume large quantities of the country's water. In South Africa, employees of the national Working for Water Programme physically cut down the problem species.

With globalization making the world smaller, stronger international cooperation and increased education on the issue of invasive species is necessary to protect native habitats and species. (WRI Features, 791 words)