| Mutualisms are ubiquitous in nature, yet they are arguably the least well understood of all ecological interactions. My dissertation investigates several aspects of mutualism, using obligate ant-plant symbioses in the Amazon as model systems. Because mutualisms rarely involve a single pair of interacting species, mutualisms are often better described as multi-species networks of interacting partners, than as simple pairwise interactions. In multi-species networks, the dynamics of mutualistic interactions depend on the relative costs and benefits of interaction among alternative partners. The myrmecophytic plants Cordia nodosa and Duroia hirsuta associate with several species of obligately symbiotic ants. The ants Azteca depilis and Myrmelachista schumanni associate with both plant species, while two undescribed species in the genus Azteca and Allomerus octoarticulatus associate only with C. nodosa. In Chapter 1, I compare the ant partners of C. nodosa and D. hirsuta with respect to two benefits known to be important in ant-plant interactions: protection against herbivores and protection against encroaching vegetation. I find that ant species differ in how they treat their myrmecophytic hosts, and that these differences are reflected in several measures of host plant fitness. In Chapter 2, I describe in more detail the protection against encroaching vegetation provided by M. schumanni to D. hirsuta. I show that M. schumanni creates pure stands of D. hirsuta, called "devil's gardens," by poisoning all plants except its host plants with formic acid. I also show that this behavior has an associated cost, increasing herbivore pressure on D. hirsuta living in devil's gardens. In Chapter 3, I investigate the synchronization of life cycle events between mutualists. I show that the reproductive phenology of A. octoarticulatus closely matches the seasonal availability of its nest sites, C. nodosa saplings. In Chapter 4, I investigate how mutualism shapes demography and life history. I find that ants do not benefit plants in the early stages of mutualism. I also find that ant colony survival and plant growth are mutually reinforcing. Finally, I describe how positive feedback between partners intertwines the life histories of separate species as they engage in mutualism. |
