by Whit Gibbons

September 15, 2003

This past summer I saw newspapers with pictures of a man holding an ugly-faced piranha in Mississippi, a woman who found a tropical caiman in South Carolina, and college students standing in a patch of water hyacinth in Florida. The carnivorous fish, the crocodilian, and the aquatic plant are natives of tropical America, not the southeastern United States. In short, they are introduced species. Most environmentalists rate habitat destruction as the single greatest cause of biodiversity loss on every continent. Introduced, invasive species run a close second and are fast becoming a modern scourge.

An invasive species is one brought to a region, usually from another continent, that successfully establishes itself. The most nefarious invasive species are those we are unable to control while they proceed to eliminate native species, destroy habitats, or just become a nuisance. Pleas are made daily to federal and state governments to set controls on one invasive threat or another.

One fact about invasive species is that few generalizations can be made about what will determine success or failure. The findings can be contradictory, as in a review of three studies of invasive plants that identified certain seed characteristics common to successful invaders in Great Britain. In the three studies, one revealed that having large seeds made a plant more likely to be successful. Another study found that small seeds were the key, and the third said seed size did not matter. So much for figuring out which exotic plants will be most likely to populate England.

Another example is the Brazilian pepper tree, a major pest in southern Florida due to its supplanting many native varieties. According to one authority, Floridians had kept Brazilian pepper trees as ornamental plants for decades with no problems, until they suddenly began to grow wild and create environmental havoc. No one had predicted they might become a problem.

In another ecological paradox, one group of ecologists noted that introduced species did not fare well when biodiversity of native species was high. In contrast, another team provided evidence that the number of invading exotics increased in proportion to increasing numbers of native plant species. In other words, we have no idea whether any guiding ecological principles can be applied to predicting whether an introduced species will become a dominant part of the landscape or simply disappear.

Major laws and regulations are being proposed about how we should deal with the present-day pervasiveness of introduced species. The solutions are not going to be easy ones, biologically or politically, although if we do not do something, many environments will be changed in ways that are unquestionably negative from most perspectives.

A variety of issues confound the situation for anyone who cares to start philosophizing about introduced species. First, not all introduced species have bad reputations. A prime example is the honeybee, introduced from Europe. Second, most major environmental impacts on natural habitats of exotic species are a result of domesticated animals that we brought here on purpose. Feral burros destroy parts of the Mojave Desert; cattle overgraze native vegetation over millions of acres; and feral swine move like baby bulldozers through the vegetation of southern swamplands. All these qualify as introduced species that are invasive. Should we take care of these identifiable and controllable species before we worry about the new ones coming in?

Another viewpoint involves our perspective of when an introduced species constitutes a problem. Do we simply have a shortsighted view? Over evolutionary time, wouldn't all invading species eventually be incorporated into the habitats they have invaded? Won't all of them eventually be controlled naturally, without our intervention? Are we being too impatient in indicting some species simply because they have modified our view of the way we think the world should be right now?

A final point to consider is that most of us who are complaining about introduced, invasive species are really part of the problem. After all, our ancestors invaded the North American continent at least within the past millennium or so, which is not a long time on the evolutionary scale. Maybe then was the time to consider the exotic, invasive species problem.

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