Effects of patch shape in intertidal algal mosaics: roles of area, perimeter and distance from edge

Shape is considered an important attribute of patches, but little attention has been paid to its influence on the structure and dynamics of rocky shore assemblages. Effects of patch shape were investigated from September 1997 to June 1998 in algal assemblages on intertidal shores at Cape Banks, south of Sydney (NSW, Australia). Three experiments were established, in order to separately investigate the importance of three different characteristics of patch shape, namely area, perimeter and distance of the central zone from the edge. In each experiment, responses for sets of replicate patches that differed on average for only one shape characteristic were compared. Patches of different shapes were produced by scraping off all resident organisms, and subsequent colonisation was recorded. The range of shapes naturally occurring at the study sites was also measured over 1 year, and the shapes of plots used in the experiments were chosen to cover this range of natural variability. Shapes of natural patches within assemblages were very variable. Consistent differences were observed in patch size, geometry and characteristics of the substratum among sheltered and exposed shores and among different times of the year, suggesting that different disturbances influence the dynamics of assemblages at these sites. The results of the three experiments showed that shape may have important effects on patch colonisation. Area, perimeter and distance of the central zone from the edge significantly affected growth of algae in experimental clearings. Responses to different patch characteristics were species specific. Effects of different attributes of patch shape also varied over time. This spatial and temporal variability of responses suggests that colonisation processes of patches of different shapes are complex, probably involving effects that can produce contrasting results. Results are relevant to the design and interpretation of field experiments, as they suggest that the shape of plots may affect the perception of patterns and processes in intertidal assemblages.