Ecology of a facultative ant-plant 'mutualism'

摘要

To investigate how ecological factors shape facultative mutualisms, I studied the ant-plant, Conostegia setosa (Melastomataceae), and its ant occupants in Costa Rica and Panama. Most ant-plant studies have focused on specialized systems that involve only obligate ant inhabitants. Studies of facultative systems, however, may reveal how mutualisms are established and maintained and may help identify selective environments in which specialization might evolve.;My studies revealed great temporal and spatial variation in the C. setosa ant-plant system. At both sites the interaction was clearly facultative, with six to twelve general nesting ant species occupying the leaf domatia of C. setosa. However, in Panama an obligate ant inhabitant was also present. Contrary to current views of stability in specialized ant-plant mutualisms, there was high turnover in C. setosa occupation for all ant species over time.;In this system, only the obligate ant inhabitant showed any attraction to C. setosa plants, typically colonized by founding queens. All facultative species nested in a variety of sites and colonized plants primarily by moving all or portions of mature colonies into them. I examined why only a small fraction of the available ant species at these sites frequently occupy C. setosa plants. The common facultative inhabitants were among the most abundant ants in the surrounding forest soil and leaf litter. In addition, they nest in low vegetation more frequently than other species and are strong competitors--two apparent advantages in finding and dominating C. setosa plants.;Although ant species may differ in the degree of benefits they provide to their host plants, I found no differences between ant occupants in their effects on plant growth or herbivory levels. Confounding effects (e.g., clonal plant growth, high ant turnover) may make such benefits difficult to detect on a short time scale. Small differences between ant species in activity levels and herbivore detection may be sufficient for natural selection to favor specialization but high ant turnover may prevent further specialization by minimizing time for selection to act.;The variable nature of this facultative ant-plant system highlights the need for comparative studies across sites and time.