How anthropogenic changes to species composition and diversity are likely to affect the properties of the ecosystems of which they are an integral part, and by extension the goods and services humans derive from them, is a key question in ecology. Despite over a decade of vigorous empirical research and theoretical developments, there remain many unknowns. Using intertidal rockpools and laboratory marine mesocosrns, I used a variety of approaches to address several of these relatively poorly studied issues. In particular, the work presented here focused on the relative roles of species composition and richness, as well as the extent to which such effects are context-dependent. The first study (Chapter II) takes advantage of a successional gradient of macroalgal species composition and diversity resulting from the periodic addition of artificial rockpools to a coastal defense structure. The results show that the focal ecosystem properties (macroalgal biomass and productivity) were largely determined by species composition (and functional traits). Macroalgal species evenness, but not diversity, peaked at intermediate stages during the chronosequence, but no measure of diversity had a detectable influence on primary productivity. The results confirm the prediction that effects of species diversity will be outweighed by compositional changes during succession. I used an experimental approach in Chapters III to V, manipulating the composition and richness of intertidal molluscan grazers (Chapters III and V) and intertidal predatory crabs (Chapter IV) and measuring their effects on prey assemblages as focal ecosystem processes. In a 13-month field experiment (Chapter III) I found that effects on the composition and functioning of developing rockpool communities were determined by grazer composition, not the number of species. Laboratory mesocosm experiments show that the influence of species richness on ecosystem processes can be context-dependent. The effect of resource partitioning (of the multi-species prey assemblage) among predators was only detectable at high predator densities where competitive interactions between individual predators were magnified. A factorial experiment using the rate of algal consumption by molluscan grazers as a response variable, provides the first empirical test of the prediction that the balance between species richness and identity effects can be determined by the degree of spatial heterogeneity (Chapter V). Species identity had strong effects on homogeneous substrates, with the identity of the best-performing species dependent on the substrate. The strengths and limitations of the predominantly small-scale experimental approach employed here are discussed (Chapter VI).

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