Friend or foe? A behavioral and stable isotopic investigation of an ant-plant symbiosis.

In ant-plant symbioses, the behavior of ant inhabitants affects the nature of the interaction, ranging from mutualism to parasitism. Mutualistic species confer a benefit to the plant, while parasites reap the benefits of the interaction without reciprocating, potentially resulting in a negative impact on the host plant. Using the ant-plant symbiosis between Cordia alliodora and its ant inhabitants as a model system, I examine the costs and benefits of habitation by the four most common ant inhabitants at La Selva Biological Station, Costa Rica. Costs are measured by counting coccoids associated with each ant species. Benefits include patrolling behavior, effectiveness at locating resources, and recruitment response. I also compare the diets of the four ant species using stable isotope analysis of nitrogen (N) and carbon (C). Ants varied in their rates of association with coccoids, performance of beneficial behaviors, and diet. These differences in cost, benefit, and diet among the ant species suggest differences in the nature of the symbiotic relationship between C. alliodora and its ants. Two of the ant species behave in a mutualistic manner, while the other two ant species appear to be parasites of the mutualism. I determined that the mutualistic ants feed at a higher trophic level than the parasitic ants. Behavioral and dietary evidence indicate the protective role of the mutualists, and suggest that the parasitic ants do not protect the plant by consuming herbivores.