Here’s a fact that I suspect most people don’t know: Wherever we humans have gone in the past two centuries, we have increased local and regional biodiversity. Biodiversity, in this case, is defined as increasing species richness.
The fear among opponents of "invasive species" is the aggressive outsiders will cause a holocaust among the native plants. That might initially seem reasonable because there are a few species, like kudzu, purple loosestrife, and water hyacinth, that grow with alarming speed wherever they show up. But that doesn't mean other species are 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,” Macalester College biologist Mark Davis notes [PDF]. Yet this spurious threat of extinction persists as one of the chief reasons given for trying to prevent the introduction of exotic species.
As they say, read the whole thing.