"Most ground-foraging ants are attracted to and will feed on these substances [elaisomes], yet not all ants will disperse seeds to their nests," said Christian. "For example, Argentine ants eat elaisomes, but do not bury seeds. So, in a sense, they are cheaters."
In the fynbos, various species of native ants handle seeds of different sizes. Ants that work cooperatively can handle bigger seeds, while ants that tend to work alone bury smaller seeds.
Anoplolepis custodiens and Pheidole capensis are cooperative ants that feed on elaisomes of larger seeds. "These more efficient dispersers also happen to be the ones that are decimated by Argentine ants," said Christian.
The Argentine ants coexist with Meranoplus peringuey and Tetramorium quadrispinosum, which specialize in smaller seeds.
Christian carried out controlled burns of fynbos areas to see whether the invader ants have had a noticeable effect on the plant community. The seeds of many fynbos plants need fire to germinate, so most new growth occurs in the first year after a fire.
After the controlled burns, the areas inhabited by the Argentine ants showed a tenfold drop in the number of new plants from large-seed species compared with areas that had not been invaded by the non-native ants, Christian said.
"It's sobering, and a wake-up call," Maureen Stanton, a professor of evolution and ecology at UC Davis, said of her student's research. Further research may show that animals that eat large-seed fynbos plants may also be at risk as a result of the Argentine ant invasion.
The Argentine ant invasion has disrupted ecosystems, and even economies, around the world. A fierce household pest, the tiny black insects march indoors in dense trails that resemble a highway at rush hour to swarm over food and moist bedding.
The insects have invaded orchards from California to Portugal, where they protect aphids and scalesap-feeding agricultural pests that can destroy trees if they increase unchecked.
Andy Suarez, an entomologist at the University of California at Berkeley, found that Argentine ants pose a major threat to California coast horned lizards. The lizard feeds exclusively on ants, yet will not eat Argentine ants.
"Because Argentine ants eliminate most native ants in their introduced range, there is great concern that coast horned lizards may decline in response to the invasion," said Christian.
Research such as Christian's and Suarez's raises awareness about the consequences invasive species can have on mutually beneficial relationships in nature, but does not provide answers on how to eradicate exotic species, said Christian.
"Support for greater regulations of exotic plant and animal species will require increased public awareness of the many threats exotic species pose," she said. "Such steps could be costly for some businesses, but highly beneficial to others."
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