Invader Ants Hurting Ecosystems, Economies

By John Roach
for National Geographic News
November 9, 2001
When merchant ships embarked from the shores of Brazil and Argentina in
the early 1900s to carry coffee and sugar to South Africa, North
America, and the Mediterranean, they carried a mischievous stowaway:
Linepithema humile.

The tiny black insect, better known as
the Argentine ant, used burgeoning global trade to invade ant
communities around the world. Scientists are just now beginning to tally
the damage. The reports are grim. Entire native ant populations have

To examine the consequences of such invasion on mutually beneficial relationships between ants and plants native to an area, Caroline Christian, a biology student at the University of California at Davis, looked at the biologically rich fynbos shrublands of South Africa.

Her research, reported in the October 11 issue of Nature, shows that when key beneficial species are removed by an invader, the destructive effects ripple through the entire ecosystem.

Fire and Seeds of Regeneration

Wildfires sweep across the fynbos once every 15 to 30 years, killing most mature plants and starting a cycle of regeneration. New plants grow from seeds that native ants have buried in the ground.

The seeds have fatty coatings, called elaisomes, which are highly sought-after sustenance for ants. When the ants eat this coating, the seeds are left unharmed. To protect this source of food from rodent seed predators, native ants bury the seeds.

The relationship is mutually beneficial: The ants are fed, and plants will regenerate in the event of a fire because their seeds have been buried out of harm's way.

Rodents quickly eat any seeds that are left unburied. Christian hypothesized that when fire sweeps fynbos areas that have been invaded by Argentine ants, certain plant species whose seeds are normally dispersed by native ants would not regenerate.

Controlled Burn

"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.

Wide-Scale Impact

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 scale—sap-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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