Carbon and nitrogen isotopes trace nutrient exchange in an ant-plant mutualism

Mutualisms, biological interactions from which each species benefits, are thought to be evolutionarily stable only under a limited set of circumstances. Underlying all mutualisms is an intrinsic conflict between the parties, in that each is under selection to further exploit the other. Conflict between mutualists will lead to instability unless each species receives a net benefit from the interaction. To understand how mutualisms persist, then, it is essential to document the exact nature of the interaction. The relationship between Cecropia trees and Azteca ants has been a model for the study of mutualism since its description more than 100 years ago. Ants live in Cecropia's hollow stems and harvest specialized food bodies produced by the host. In return, ants defend the plant from leaf-feeding herbivores and encroaching vegetation. Central to modeling this relationship is understanding the exchange of nutrients between symbionts. Here we present evidence that the flow of nutrients has been misjudged. Although ants consume plant products, they provide more nutrients than they receive. Using stable isotope analysis, we calculate that only about 18% of worker ant carbon is derived from Cecropia, whereas 93% of the nitrogen in ant-occupied host plants is derived from debris deposited by ants. Ants rely on sources other than their host for food and, in doing so, make a rich source of nitrogen available to the host.