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 Like ly to interact with other environmental factors at different scales in spac e and time. Understanding these interactions is important before generaliza tions about the ecological role of grazing can be advanced. In the present study. the ecological significance of grazing was evaluated by excluding li mpets from mid-shore levels on rocky coasts in the northwest Mediterranean. Alternative models were evaluated leading to different predictions about t he interactive effects of limpets, slope of the substratum and season. Thes e models were distinguished on the basis of spatial and temporal consistenc y in these interactions. The null hypothesis of consistent effects of limpe ts was examined at relatively small (0.1 to 5 km) and large (100s of km) sp atial scales, between seasons (summer vs winter), between dates within seas ons, and in relation to slope of the substratum (horizontal vs vertical sub strata). Univariate and multivariate analyses indicated inconsistencies in the effects of limpets at different spatial and temporal scales. Interactio ns between grazing, slope of the substratum and season changed from shore t o shore, and strong effects of limpets were observed only occasionally. Thi s variability is discussed in relation to differences in productivity and i ntensity of recruitment of algae among shores.