Ant invaders strand seeds without rides. (Biology).

Argentine ants crowding out native ones in South Africa are disrupting plants there, too, by failing to disperse seeds.

The effects of Argentine ants make a great test case for the importance of mixed-species partnerships, or mutualisms, says Caroline E. Christian of the University of California, Davis. Ecologists have worried for years that disrupting mutualisms could start a cascade-of misfortunes. However, Christian points out, experimental data have been scarce.

To fill this gap, she turned to South Africa's chaparral-like fynbos, where fires maintain a boom-and-burn cycle in vegetation. About a third of plant species rely on ants to protect their seeds. The ants eat nubbins off the seeds' outsides and leave remainders in underground nests, safe from fire and rodents.

The small, aggressive Argentine ants are now sweeping into the region. Some of the native ants are holding their own, but other species have already vanished.

In experiments, Christian found no signs that the Argentine ants dispersed any seeds. She also discovered that two native ants that resist the invaders are relatively poor dispersers of large seeds. Two native species that perish in invasions collect more than four times as many large seeds as the tougher pair does.

After an experimental fire, Christian found only a tenth as many large-seeded plants in invaded areas as in uninvaded plots. In the Oct. 11 NATURE, she contends that her findings illustrate the critical ecological importance of mutualisms. Christian warns that the analysis highlights the dangers for ant invasions, "one of the most overlooked aspects of the global biodiversity crisis." --S.M.