Environment / Biodiversity

Invasive species seem unfettered by the normal rules of natural competition. Two groups of researchers may have figured out why: The new arrivals leave their natural parasites behind. Mark Torchin, an ecologist at the University of California at Santa Barbara, and his colleagues studied 26 invasive animal species and found that the alien species carried less than 20 percent of their usual number of parasites and adopted only a few new ones. Charles Mitchell and Alison Power, ecologists at Cornell University, obtained similar results when they looked at 473 alien plant species. Torchin speculates that the parasites may have been left behind because their complex life cycles depend on multiple and often species-specific hosts. The findings appear to support a strategy of introducing parasites from an invader's original habitat to rein it in, but Mitchell advises caution. "If you're going to use biological control, you have to ensure that the parasites or pathogens won't affect the native community," he says.