No Mercy From Argentine Ants

Argentine ants, the scourge of California kitchens, might rival humans in their relentless domination of natural habitats. When the ants invade a new territory, they drive out native ants, and populations of insect scavengers skyrocket. The findings, published in this month's issue of Conservation Biology, support the growing belief among biologists that nature preserves should be large enough to provide a buffer zone around the edges to protect a central core.

The aggressive South American ants have marched north into California at least as far as Davis, spreading as fast as 300 meters a year. The ants apparently establish beachheads in areas disturbed by human activity and then invade neighboring undisturbed areas. To study the ants' impact on native species, Stanford University ecologists Deborah Gordon and Kathleen Human surveyed insects and arthropods on the 1200-acre Jasper Ridge Biological Preserve near Palo Alto, California. They set up traps in 20 sites dominated by Argentine ants and 20 others that were at least 100 meters from the invasion front.

Overall numbers of ants were 100 times greater in the areas dominated by Argentine ants. At the same time, the number of native ant species dropped from 18 to none in most cases. Other research has suggested that Argentine ants outcompete native ant species for food, and head-to-head fighting crops up, too. Human says that the Argentine ants sometimes drive out natives in ferocious battles that last up to a week. "The Argentine ants pull them apart," she says. "There is a lot of dismembering." The domination affects more than ants. Sowbugs and other scavengers increased by up to 70% in number. At the same time, the populations of other species in the war zone fell dramatically. This was especially true of many flies, wasps, ticks, and mites, some of which prey on the native ants.

As the first quantification of damage inflicted by Argentine ants, the study "gives us a good idea of the way native ants interact with exotic species," says Terry McGlynn, an ant ecologist at the University of Colorado, Boulder. He adds that Argentine ants mainly follow human development. "It's not as though they are taking over all of California," he says.

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