UBC Theses and Dissertations
Factors governing interactions among species in a complex marine ecosystem Wood, Spencer A.
Species interactions take a variety of forms, from weak to strong, and negative to positive, each with unique consequences for local community structure. These interactions are contingent on biotic, abiotic, and scale-dependent variables, and this makes them difficult to predict. In this dissertation I describe three empirical intertidal studies aimed at understanding the dependency of species interactions on environmental and ecological context. First I present a test of the relative importance of six variables known to affect the sign and strength of species interactions, using perturbation experiments across a range of habitats in New Zealand and North America. I observe that species interactions in communities are typically weak, with a balance of positive and negative effects. Interaction strengths are related to traits of species, such as body size and trophic level, but are relatively insensitive to environmental conditions. The second study has two parts. First, I examine the prevalence of nonlinear species interactions, using experimental gradients in abundance of three focal species. Then I assess how well linear and nonlinear interaction coefficients, estimated in the first experiment, predict the outcome of a second set of perturbations. I find that interactions are typically linear across focal species densities, and that predictions regarding the outcome of short-term perturbations are not significantly improved by nonlinear estimates. Lastly, I investigate the relative impacts of biotic interactions, abiotic stress, and the interactive effects of the two factors, on community diversity. The results demonstrate that although environmental stress alters certain species interactions, it does not change the overall distribution of weak and strong effects. This is, in part, because biotic processes mitigate the effects of abiotic stress and allow the community to maintain its functioning even under harsh conditions. Together the results demonstrate how empirically-based interaction coefficients can be used to explain species diversity and the responses of ecosystems to environmental change. Interactions are directly related to population density, relatively insensitive to environment conditions, and strongly tied to species' allometric properties. This exhibits the potential for future research to predict ecosystem dynamics without detailed information on communities.
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