The tiny black Argentine ant is well known as a household pest. But by replacing native ants, they could also be disrupting natural ecosystems. A study by a University of California, Davis, graduate student, published this week in the journal Nature, has for the first time shown that when key beneficial species are removed by an invader, the destructive effects can reverberate through the ecosystem.

Caroline Christian, a student at the UC Davis Center for Population Biology, studied the fynbos shrublands of South Africa, an area similar in climate and vegetation to the chaparral of California. The fynbos is renowned worldwide for its high level of biodiversity. Wildfires sweep the fynbos every 15 to 30 years, killing most mature plants. New plants grow from seeds buried in the ground by native ants. Christian found that when Argentine ants displace native ants, plants that depend on those ants to bury their seeds do not regenerate after fire.

"There's been a lot of concern that invasive species may disrupt mutually beneficial interactions between plants and animals," said Maureen Stanton, a professor of evolution and ecology at UC Davis and Christian's thesis supervisor. If those interactions are crucial, there might be cascading effects on the whole community, she said.

Seed burial by ants is key to survival for about a third of fynbos plant species, Christian said. When fresh seeds fall, ants are attracted to them and carry them off to bury in their nests. Different ant species specialize in seeds of different sizes: Ants that work cooperatively deal with bigger seeds, while ants that tend to work alone bury smaller ones. If the seeds are not picked up quickly, virtually all are eaten by rodents.

Argentine ants do not bury seeds at all. But they do wipe out two fynbos ant species, Anoplolepis custodiens and Pheidole capensis. Two others, Meranoplus peringuey and Tetramorium quadrispinosum, coexist with the invader. It turns out that Anoplolepis and Pheidole ants prefer large seeds, while the others go for small seeds.

Large seeds placed in invaded areas were less likely to be buried by ants and more likely to be eaten by rodents, compared to large seeds in uninvaded areas, Christian said. Small seeds were much less affected.

Christian carried out controlled burns of areas in fynbos to see whether the invading ants had a real effect on the plant community. Seeds of many fynbos plants need fire to germinate, so most new growth happens in the year after a fire.

After burning, invaded areas showed a tenfold drop in the number of new plants from large-seeded species, compared to uninvaded areas, Christian said.

"It's sobering, and a wake-up call," said Stanton. The study showed the threat from invasive species both to the fynbos and to ecosystems in general, she said.

"This is the first work to show not just the immediate effects of an invasive species, but the larger effects on an ecosystem," Stanton said. Potentially, there could be further effects. For example, animals that eat large-seeded fynbos plants may also decline as a result of the Argentine ant invasion, she said.

The study was funded by the National Science Foundation and conducted in Kogelberg Biosphere Reserve, South Africa, with assistance from the South African Museum and Cape Nature Conservation. It is published in the Oct. 11 issue of Nature.

Editor's note: Images of the ants and the fynbos are available. Contact Andy Fell for details.