The Problem of Invasive Species

When a plant or an animal is carried, whether on purpose or entirely by accident, to a place where it does not naturally live, the results are sometimes completely harmless. Occasionally, however, the newcomer thrives so successfully in its new surroundings that it causes a great deal of damage. Such "invasive species" can spread astonishingly rapidly in their new home, partly because the predators, parasites, and diseases that once kept their numbers firmly in check back home are now entirely （ 1 ）.

The harm done by invasive species can be remarkably severe. They may outcompete native plants and animals for limited food and space, sometimes driving long-established local species toward extinction. They can also badly damage farms, forests, and waterways, costing enormous sums of money to control or repair. （ 2 ） their dramatic and far-reaching effects, invasive species are now widely considered to be one of the leading threats to biodiversity all around the world.

Once an invasive species has become firmly established in a new region, removing it again is usually extremely difficult and expensive, and complete success is genuinely rare. （ 3 ）, experts strongly stress that prevention is far more effective, and far cheaper, than any later cure. Stopping harmful species from arriving in the first place—by carefully checking goods, cleaning equipment, and controlling trade—is almost always easier and wiser than trying to undo the damage after it has already been done.