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.
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.
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.
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.
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.