Biodiversity:

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 20 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... “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.