Effects of species interactions on resource heterogeneity [electronic resource]
Keywords: Biology.

Despite the understanding that the dynamics of mutualisms are intrinsically dependent on the external system in which they exist, most studies of positive interactions have focused on benefits, costs, and outcomes for each of the individual interacting partners, while relatively little work has focused on examining how mutualistic interactions affect and interact with broader-scale ecological processes. In this dissertation, I address that gap by examining nutrient exchange in positive interactions and how reciprocal interactions and feedbacks between individuals affect nutrient dynamics at broader scales. I have focused specifically on ant-plant interactions, which have been used by many researchers as model systems for examining a broad range of evolutionary and ecological questions relating to mutualisms and species interactions. Ant-Acacia interactions are classic and well-studied examples of obligate protective mutualisms in which symbiotic ants defend trees against herbivores while using specialized rewards from the trees as food and/or shelter; ants that use these rewards but impede tree reproduction or do not defend their host trees have been presumed to be parasites of the mutualism. In chapter 2, I use a Kenyan ant-Acacia mutualism to explore how differences in the interactions between a host tree and different symbiotic ants influence resource availability and disturbance, and in turn, life-history traits of the host tree. Using an ant-removal experiment, I show that different ant mutualists manipulate nutrient levels and water stress of their host trees, as well as influence herbivory, beetle damage, host-tree growth rate, reproduction, and mortality. Though only one of these ant species previously has been considered a "true mutualist" and two have been deemed parasitic, these data suggest that together, this guild of ants creates divergence in the life-history strategies of the host trees that may benefit the populations of ants and trees as a whole. In chapter 3, I explore the direct and indirect effects of the different Kenyan --Acacia-ant partners on host-tree foliar nutrients and available nutrients in surrounding soils. Using an ant-removal experiment and a 15N-tracer experiment, I show that the different ant partners have divergent effects on foliar nutrient concentrations in their host tree, and these effects are attributable to species-specific differences in the ant-plant interactions; one species of ant appears to provide a nutrient subsidy its host tree. Different ant species also influence soil nutrient availability, leading to the creation of small-scale soil nutrient heterogeneity in this system. Lastly, in chapter 4, I examine the generality of my results from the previous two chapters by examining a similar ant-plant system in Costa Rica. My results demonstrate that the different Costa Rican ant species differ in the degree to which they clear encroaching vegetation around their host trees; this clearing behavior in turn affects foliar nutrient concentrations and soil moisture, available N, and available P. These findings are similar to those from the Kenyan ant-plant system, and suggest that differences in how individual members of plant-ant guilds interact with their host trees may lead to effects on both foliar and soil nutrients across a broad range of ant-plant interactions. These results may be broadly applicable across a range of other ant-plant mutualisms, and suggests that species interactions such as these are important for the creation and maintenance of resource heterogeneity.