Scales of variation in the effects of limpets on rocky shores in the northwest Mediterranean

Grazing is an important determinant of the spatial and temporal patterns of distribution of intertidal organisms on rocky shores. This process is Likely to interact with other environmental factors at different scales in space and time. Understanding these interactions is important before generalizations about the ecological role of grazing can be advanced. In the present study. the ecological significance of grazing was evaluated by excluding limpets from mid-shore levels on rocky coasts in the northwest Mediterranean. Alternative models were evaluated leading to different predictions about the interactive effects of limpets, slope of the substratum and season. These models were distinguished on the basis of spatial and temporal consistency in these interactions. The null hypothesis of consistent effects of limpets was examined at relatively small (0.1 to 5 km) and large (100s of km) spatial scales, between seasons (summer vs winter), between dates within seasons, and in relation to slope of the substratum (horizontal vs vertical substrata). Univariate and multivariate analyses indicated inconsistencies in the effects of limpets at different spatial and temporal scales. Interactions between grazing, slope of the substratum and season changed from shore to shore, and strong effects of limpets were observed only occasionally. This variability is discussed in relation to differences in productivity and intensity of recruitment of algae among shores.