The role of topographic complexity in the structure and dynamics of rocky shore communities

Abstract

The overall aim of this work was to assess the importance of topographic complexity in structuring rocky shore communities. In order to achieve this the distributions of intertidal species were related to physically and biologically generated features of the habitat. In addition manipulative experiments were used to gain a more complete understanding of the processes in operation. Initially a review of the methods and indices used to measure habitat complexity was made. Three methods were compared using field trials: stereophotography, profile gauges and lengths of chain contoured over the substratum. Chains were the most efficient method to use, followed by stereophotography and profile gauges respectively. The results derived from the chain method and profile gauges were directly related, but stereophotography results were not comparable. Stereophotography, correlations between topographic features and the distribution of the overlying biota. This led to the development of an automated technique that directly linked these parameters. The strongest correlations between topographic features and biological distributions were typically observed for algal species. The influence of complexity, however, varied across a number of scales depending on the species examined. however, offered the additional benefit of allowing direct Both physical and biological complexity was demonstrated to have an important role in structuring rocky shore communities. Details of the recruitment of algae and sessile invertebrates to substrata of varying complexity were first examined. In order to achieve this the topography of the substratum was manipulated by the use of concrete blocks cast with differing surface features. The succession of intertidal communities was also observed on mussel beds. The recruitment of algae did not appear to be related to the complexity of the physical substratum, but was affected by biologically generated complexity. In contrast the settlement of barnacles was influenced by both habitat types. The distribution of mobile invertebrates in relation to topographic structures was highly variable; crevices therefore represented temporary habitats and refuges for these organisms. The shelter provided by both mussel beds and the physical properties of macroalgal canopy also influenced the distribution of such species. Results regarding species number and diversity on the concrete blocks were not consistent with the commonly held view that increased habitat complexity leads to increased richness and diversity. The movement patterns of intertidal predators were examined via the use of underwater camera technology. Biologically generated complexity was investigated in mussel beds in North Cornwall. Factorial manipulations of grazers and mussels were studied to examine their respective roles within a community. Predation and grazing pressure were influenced by both physical and biological structures. As a consequence the distribution of prey species were in part determined by the complexity of the habitat. Spatial and temporal scales of the structure and dynamics of mussel mosaics was also investigated. Habitat complexity was observed to have an impact on a number of community structuring processes. The generalities of these results and the requirements for future research are discussed throughout the thesis.

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