Ant invaders strand seeds without rides

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.”