Invasion of the invasive species! Local biodiversity is increasing because of man, not despite him.

• Author: Bailey, Ronald
• Position: Column

HERE'S A FACT I suspect most people don't know: Wherever human beings have gone in the last two centuries, we have increased local and regional biodiversity. Biodiversity, in this case, is defined as species richness. For example, more than 4,000 plant species introduced into North America during the last 400 years now grow wild here; they now constitute nearly 2o percent of the continent's vascular plant biodiversity.

Yet "the popular view [is] that diversity is decreasing at local scales," as the biologists Dov Sax of Brown University and Steven Gaines of the University of California at Santa Barbara noted in a 2003 paper published by the journal Trends in Ecology and Evolution. And one alleged culprit for the purported loss of diversity is competition from invasive species--that is, plants and animals introduced into ecosystems where they are not native.

Opponents of invasive species fear that aggressive outsiders will wipe out native species. That might seem reasonable, since there are a few species, such as kudzu, purple loosestrife, and water hyacinth, that grow with alarming speed wherever they show up. But that doesn't mean other species are necessarily in danger. "There is no evidence that even a single long term resident species has been driven to extinction, or even extirpated within a single U.S. state, because of competition from an introduced plant species" the Macalester College biologist Mark A. Davis noted in the journal BioScience in 2003.

Yet this spurious threat of extinction is one of the chief reasons given for preventing the introduction of exotic species to new areas.

There are plenty more examples of increasing biodiversity associated with the introduction of new species. Vascular plants brought in from all over the world have doubled the species richness of the plant life on most Pacific islands. The biodiversity of some islands has increased so much that they now approach the richness of continental areas. In New Zealand, since European settlement began 160 years ago, 2,000 introduced plant species have joined 2,000 native species, and only three native plant species have gone extinct. The opening of the Suez Canal introduced 250 new fish species from the Red Sea into the Mediterranean Sea. Only a single extinction resulted.

Researchers find increases in species richness on the local level as well. Sax and Gaines cite studies finding that a corner of West Lancaster in Great Britain has seen a dramatic rise in...