My first real experience with exotic species was more than 40 years ago, as a young child riding in through the south with my family. In the early morning the countryside looked ghostlike to me. Especially disturbing were the trees, shrouded in wisps of mist and completely covered by a choking overgrowth of a green vine. It was, of course, kudzu vine, marching steadily and irrevocably northward and covering everything in its path like a crawling green glacier.
Kudzu, introduced in the US in 1876, is one of the most familiar exotic species in the south. It grows up to a foot a day, and covers an estimated four million acres in the southern US. Attempts to control its growth costs millions yearly, and meet with only limited success. But it isn’t the only widespread exotic plant. According to Peter Bryant (Biodiversity and Conservation: A Hypertext Book), the introduction of exotic species is threatening 42% of the plant and animal species on the US Endangered Species List, second only to habitat loss in its importance.
The familiar tumbleweed (Russian thistle) often seen in western movies isn’t actually a western plant at all, according to Bryant, but was introduced to the west and has flourished there. Water hyacinth, an invasive plant species originally from South America, is choking lakes and intakes for water treatment and power plants worldwide. Zebra mussels, introduced accidentally by ships entering the Great Lakes, is steadily spreading down the Mississippi and Ohio Rivers and their tributaries. Zebra mussels are prolific, difficult to destroy, clog intakes for water treatment plants, and are displacing native mussels.
Starlings, a common and destructive pest in parts of the US, reportedly were brought to the US by a Shakespeare aficionado, whose goal was to introduce to this country every songbird mentioned in the Bard’s writings.
Plant and animal species do tend to migrate naturally due to factors such as climate change and land bridges, but these are typically slower changes. Often the resident ecological communities are able to adapt, providing the necessary checks and balances that produce a population equilibrium. But when new species are introduced deliberately or accidentally by man, the results can be disastrous. Predators which normally control the exotic are likely missing in the new environment. Prey species in the new environment may be unable to handle the introduction. And the new species may outcompete existing species for food, threatening to replace the natural population with the exotic one.
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