The imported fire ant invaded the southeastern United States some 70 years ago, with painful results for farmers, wildlife, and native insects. But a new study suggests that the invasion may have burned itself out.
Few American southerners grow up without feeling the sting of fire ants. The more aggressive of these are invaders from South America, ornery buggers that bite humans, livestock, wildlife, and even electrical equipment. Their large mounds disrupt crops and pastures. (The ants were introduced accidentally, probably in potting soil.) A 1990 study found that the imports also drive out local insects, reducing the number of native ant species by 70%, ant populations by 90%, and other arthropod populations by up to 75%. Although anecdotal evidence suggests the natives can rebound, no study has evaluated this finding rigorously.
It appears to be true. Entomologist Lloyd Morrison of the U.S. Department of Agriculture's Center for Medical, Agricultural and Veterinary Entomology in Gainesville, Florida, used the trapping procedures of the 1990 study at the same site, near Austin, Texas. In a study published in the August issue of Ecology, he reports that fire ants still dominate the area, but their numbers are down nearly 90%. Native arthropods, meanwhile, are now as diverse as before the invasion, and native populations have bounced back dramatically. The reason is unknown, but a recently arrived parasite from South America may have wiped out the imported fire ants, he says. Alternatively, the invaders may have overexploited the local food supply. These factors could have conspired to weaken the fire ants' hold and allow natives to recover, Morrison says.
It is also possible that insecticides caused the decline, says Ed Vargo, an entomologist at North Carolina State University in Raleigh who has worked at the Texas site--or maybe it was colony-swiping entomologists, who can impact local populations in their zeal to study the ants. Whatever the cause, Vargo is skeptical that all invasive species are that vulnerable. Even if the decline is natural, he says, it may be due to a reproductive oddity: Ant colonies with multiple queens (like the ones in Texas) produce mostly sterile males, so they depend on males from single-queen colonies, which are few and far between in Texas.