Invading Ants Alter Regional Ecosystems
Copyright 2001
Cosmiverse from University of California -- Davis
October 11, 2001
The tiny black Argentine ant is a well-known household pest, and by replacing native ants, it could be disrupting natural ecosystems. A study in this week's Nature by a University of California, Davis, graduate student has shown for the first time that when an invader removes key beneficial species, 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 famous around the world for its high level of biodiversity. Wildfires sweep the fynbos every 15 to 30 years, killing most mature plants, and new plants grow from seeds buried in the ground by native ants. Christian discovered that when Argentine ants displace native ants, the 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 supervisor of Christian's thesis. If those interactions are critical, it could cause cascading effects on the whole community, she said. About one third of fynbos plant species depend on seed burial by ants for their survival, 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 usually work alone bury smaller ones. If the seeds are not picked up quickly, rodents eat them.
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 not as likely to be buried by ants and were more likely to be eaten by rodents, as compared to large seeds in uninvaded areas, Christian said. Small seeds were much less affected.
Christian did controlled burns of areas in fynbos to see if 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 during 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," said Stanton. And the effects could be even further reaching. For example, animals that eat large-seeded fynbos plants may also decline as a result of the Argentine ant invasion, she said.