Ants, Plants Benefit From Mutual Relationship, Researchers Find

FAYETTEVILLE, Ark. - University of Arkansas researchers have found that some species of tropical ants feed the plants that house them, refuting current theories of mutual relationships in nature that espouse reciprocal exploitation.

Cynthia Sagers, associate professor of biological sciences, and Shauna Ginger, an undergraduate student on a Science Information Liaison Office SILO Undergrarduate Research Fellowship, report their findings in the upcoming issue of Oecologia.

"This study changes our understanding of this relationship between ant and plant," Sagers said. She and Ginger studied Azteca ants, some of whom live their entire lives in Cecropia trees, nesting in the hollow stems and foraging for food among their leaves.

The lack of complete reciprocation in ant-plant interactions challenges current ecological thinking that each organism must receive the same net benefits as the other to form a sustainable mutual relationship, Sagers said. Previous studies overestimated the amount of food the ants take from Cecropia and failed to account for the ant-supplied plant food.

Cecropia trees sport huge lobed leaves that form in spirals on slender stems and frequently grow in disturbed landscapes of lowland tropical moist forests. Male and female cigar-shaped flowers grow on separate trees. The Azteca ant queens colonize the hollow Cecropia stems and the worker ants forage among the leaves and stems for beetles, caterpillars and specialized food bodies produced by the plant.

Sagers and Ginger set out to discover what the ants ate and what the plants used for nitrogen sources.

"We knew that the ants live only on the trees," Sagers said. "So the only things they could eat would be parts of the plant itself or insects that land on the plant."

To discover the ants’ dietary secrets, the researchers looked at the ratios of carbon stable isotopes in the insects and potential diet items. Because plants and animals have distinct isotope signals, the contribution of each item to the ant diet can be assessed. At the same time, they looked at the nitrogen stable isotope ratios found in the host plants.

This study marks the first time the exchange of nutrients between ant and plant have been studied simultaneously, Sagers said.

They spent six weeks hopping around 11 Caribbean islands in pursuit of two-meter tall Cecropia saplings and the insects that dwell within their stems. They collected samples and measured the carbon and nitrogen stable isotope ratios using the University of Georgia’s Stable Isotope/Soil Biology Laboratory.

They found that the worker ants get only about 19 percent of their food from the plant, and the larvae get about 42 percent of their food from the plant. The rest of their food source comes from insects that land on the Cecropia’s leaves or stems.

To learn about the plants’ food sources, the researchers also compared the nitrogen isotope signatures of colonized stems with un-colonized stems. The colonized Cecropia stems received 93 percent of their nitrogen from the ants.

"The ants are feeding the plants," Sagers said. "Nutrients are exchanged in both directions. Theses results suggest that mutualism may persist in this system, not through mutual exploitation, but by a reduction in the sources and degrees of conflict between parties."

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