Competition and facilitation among coastal dune plants in the Southeastern United States
Franks, Steven Joseph
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Competition has dominated ecological theory and the design and interpretation of experiments, but there is growing evidence that interactions among organisms can be facilitative, and that interactions range along a spectrum from negative (competitive) to positive (facilitative). One current goal in ecological research is understanding what factors affect the strength and direction of interactions within and among species. Theory suggests that facilitation may be especially important in stressful or frequently disturbed habitats and may particularly benefit individuals at the earliest life stages. The main objective of the research presented in this dissertation was to investigate factors shaping interactions among several coastal dune plant species. Vegetation and environmental sampling as well as several experiments were conducted to meet this objective. Vegetation sampling indicated positive association among several species, which is consistent with possible facilitative interactions. Seed bank sampling and a seed planting experiment in vegetated and open microsites showed that facilitation was important at early life stages, with adult plants increasing seed accumulation and seedling emergence in the field. In an experiment in which dune plant density, species richness, and a burial treatment were varied, burial disturbance shifted interactions from neutral to facilitative, supporting the hypothesis that facilitation increases with increasing stress and disturbance. An experiment examining interactions within and among two dune species showed that the presence of neighbors increased survival but decreased growth, indicating that neighbors may protect each other from sources of mortality, leading to facilitation but still compete for resources, reducing growth. These results emphasize that the outcome of interactions depend on the environmental conditions in which they occur and give further evidence that facilitation is important in communities occurring in stressful and frequently disturbed habitats.