Weeds cost U. S. farmers about $10 billion every year. Seventy percent of that cost goes to crop losses. The rest pays for herbicides to keep the weeds at bay. Up to 75 percent of those weeds have been brought to the United States from Europe, Asia, or other parts of the world. Some are brought to this country on purpose as crops or as garden ornamentals. Others come by accident, as seeds stowed away in shipments of grains and other goods. Such foreign weeds are known by a variety of names, including "aliens," "exotics," "escaped ornamentals," and "introduced species."

Unfortunately, the price that exotic plants exact from American agriculture makes up only a small fraction of the damage these organisms are inflicting on U. S. lands. Montana alone hosts 4 million acres of European knapweed. The Asian vine kudzu, introduced to control erosion in the South, has been described as "the vine that ate the South." Cheatgrass covers tens of millions of acres throughout the American West. Hawaii now has as many exotics as natives.

Some of these plant species are so successful in their new homes that they cover entire ecosystems, displacing nearly all of the native plants. Scotch broom, a beautiful yellow-flowered shrub that looks good in a garden, flourishes in California to the detriment of native ecosystems. Broom covers whole mountains, leaving virtually nothing growing beneath its thin stems. A single plant can be cut down to the ground every year or stripped of all its leaves by caterpillars, and it will still grow several feet, bloom, and produce thousands of fertile seeds, every year.

Because many exotic plants have no natural enemies, they interrupt the food chain. In California, eucalyptus trees from Australia can form nearly sterile forests, where little grows but the trees themselves. The alien trees produce secondary plant compounds that native plants and animals are not adapted to cope with. Because the leaves are unpalatable to native insects and other herbivores, the forests support few primary consumers and still fewer predators.

Return to Top
Return to Web Bits Index