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
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.”