PARENT SESSION
Oral Session 67: Mutualism - Parasitism III: Ants.
Presiding: K Mooney
Wednesday, August 4, 8:00 AM to 11:30 AM, Meeting Room D 135.
Interspecific competition among ant mutualists: Consequences for a cactus producing extrafloral nectar.
Morris, William*,1, Ness, Josh2, Bronstein, Judith2, 1 Duke University, Durham, NC, USA2 University of Arizona, Tucson, AZ, USA
ABSTRACT- Mutualism is often caricatured as a pairwise interspecific interaction, but it more frequently involves a suite of species of one type (e.g., flowering plants) interacting with a suite of species of a second type (e.g., pollinators). Consequently, interactions between species of the same type can influence the strength of the interaction between particular pairs of mutualists. The fishhook barrel cactus Ferocactus wislizeni possesses extrafloral nectaries that are commonly visited by four species of ants in Tucson, Arizona. We present evidence that ants compete for plants: fewer cacti have more than one ant species present simultaneously than would be expected if ants were distributed at random. Ant turnovers are frequent and display a seasonal pattern, such that the fraction of cacti tended by a given ant species undergoes an annual cycle. Moreover, one species of ant is absent from some sites, causing ant species composition to vary both temporally and spatially. Ant species may also differ in their quality as plant protectors, as judged by the number of individual ants present on tended cacti and by the per-capita rate at which they remove herbivorous insects placed on the plants. We use a state transition model combined with estimates of potential costs and benefits of ant tending to examine how the seasonal cycle in ant frequencies driven by competition may affect the strength of the ant-cactus mutualism. Our results suggest that the local species composition of the ant community can substantially influence the benefits that cacti may receive by engaging in mutualisms with ants.
Key words: extrafloral nectar, cactus, mutualism, ant