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Intruders Overcome Natives
California has about 200 species of ants. Over a dozen of those live in Stanford University’s Jasper Ridge Biological Preserve, a hilly expanse of grassland and woodland just upslope from the main campus. There, for the past 15 years, biology professor Deborah Gordon has been surveying the distribution of Argentine ants and native ants (and other arthropod species), trying to understand population dynamics and patterns of aggression and competition.

“We find that the Argentine ants are much more abundant around the edges [of the preserve] where there are residential areas, partly because people provide refuge where there are houses, but also because people carry Argentine ants in landscaping plants, including natives,” she said. In those areas where the Argentine ant has become established, it has completely changed local ant distributions. “In places where the Argentine ant hasn’t reached there are certain configurations of ants that tend to go together,” Gordon said. The harvester ant (Messor andrei)--the big black ant that collects seeds and adorns its nest with piles of chaff--and the carpenter ant (Camponotus semitestaceus), which feeds on dead insects, sugar-producing vegetation, or decaying substances, for example, often share higher-elevation territory. Each species moves out from a central nest to forage, remaining separate from other ant species in a manner consistent with competitive dynamics. Wherever the Argentine ant has been introduced, however, the previously distinct populations begin to mill around together, forming a random aggregation that weakens each group’s competitive advantage.

This disruption is not only bad for the ants; it affects the entire ecosystem. Native ants carry seeds around and, “when the Argentine ant comes along, it eliminates the other ants that are dispersing the native-plant seeds,” Gordon said. This can benefit invasive plant species such as star thistle, which, she pointed out, does better in areas where there are Argentine ants, possibly because there is less distribution of the native grass seeds that compete with the thistle. And although we think of bees and butterflies first when it comes to pollination, ants participate in that crucial activity too. So when their patterns of movement are disrupted, vegetation patterns follow suit.

The Argentines tend to establish long highways off which they forage. They work together so well that they are formidable opponents to any native ants in their path. Although they are relatively small--the wingless workers are 2-3 millimeters long, compared to the native harvester ant at 4.5-6 millimeters--size doesn’t impede them. They’ve got numbers on their side, as well as efficiency.

“It seems as though the Argentine ant isn’t succeeding because it’s more aggressive,” said Gordon, “but only because it’s such an effective searcher. So it finds the food first,” before the native species. “And once it gets there, the native ants don’t try to take it away from them.”

The invader ants will eat anything, and they’re relatively tolerant of both higher and lower temperatures; as a result, they can cover a wider range of seasonal conditions and habitats than native species. “The native species have evolved to deal with each other, but they haven’t evolved to deal with these intruders,” said Gordon.

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