by Whit Gibbons

April 15, 2007

Aliens are invading the world. You would be hard pressed to find any substantial land area anywhere that is not under siege by at least one form of alien life. But these aliens have not come from outer space but from our own world.

The list of introduced exotics causing someone some kind of problem somewhere is immense. Some are old and familiar--rabbits in Australia, fire ants in the South, brown tree snakes on Guam. Introduced rats destroy native wildlife on islands throughout the world. Kudzu and chestnut blight are species that most people wish had never left their native habitats. Some U.S. invaders are relatively recent arrivals, such as Burmese pythons in the Everglades and killer bees in Texas.

Unwanted plants and animals come from places all around the globe. The Eurasian tamarisk tree has invaded California deserts; South American water hyacinth, Australian pine, and Brazilian pepper have dramatically altered southern Florida; European invaders such as zebra mussels and the purple loosestrife plant are inflicting widespread natural and economic damage elsewhere. Alien species reportedly cost our country more than $100 billion dollars a year.

Whether intentionally or accidentally introduced, nonnative species can become pests when no natural enemies are present to control their numbers. Through competition or outright predation, these species alter habitats and community structure, often eliminating some native flora and fauna. A common concern with invasive plants is that the introduced species will take over a habitat and crowd out naturally occurring plants.

But is a habitat necessarily ruined simply because one or more species dominate the others? The seemingly endless acres of coastal salt marsh grass are not viewed as aberrant. Nor have the miles and miles of native prairie grasses that once populated the Great Plains been considered unacceptable. As evolution accompanies environmental changes, the relative abundance of species changes. Some are successful; some are not; and many are relegated to minor roles in a habitat.

What should we do when we realize that yet another introduced species is taking hold? What if we didn't do anything? What if we abandoned our attempts at eradication and simply accepted the new order of ecosystems and their changing array of species distributions, both native and introduced? Evolutionary processes will continue to operate as they have for the past billion years, ultimately resulting in a different composition of species.

Although some people view investing time and money to fight introduced exotics as a hopeless exercise, others would not even consider the option of just letting the aliens have their way without trying to stop them. The idea of taking no action against potential extinctions of native plants and animals because of an alien species is anathema to many environmentalists. But are we winning a few battles while losing the war against alien species? Are the environmental benefits of preventing and controlling alien invasions worth the economic costs?

Whether an introduced species will be challenged and efforts made to eliminate it often depends on who or what is affected and how much political clout the affected parties have. For instance, the aquatic weed hydrilla was introduced into U.S. lakes from Africa, presumably through the home aquarium trade in the 1960s. Hydrilla is likely to have high levels of taxpayer dollars dedicated toward its eradication. True, the plants compete with native species, but that alone would not merit all-out war. These weeds, however, clog outboard motors of people engaged in recreational water activities.

In contrast, many introduced species have become an integral part of our culture and society. The European honeybee almost certainly competes with native bees for pollen but is considered desirable. Likewise, hundreds of popular horticultural plants are nonnative species whose continued existence is not merely tolerated but encouraged.

Introduced exotic species clearly create a dilemma. What should we do to combat what is becoming a national problem? Should we do anything? Dov Sax of the University of Georgia is a well-known research scientist who has conducted research on invasive species. I asked him for his perspective on exotic species invading the country. His answer will appear next week.

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