Abstract This study examined the interactive effects of grazing by limpets and inclination of the substratum in maintaining differences between mid-shore and low- shore assemblages of algae in the northwest Mediterra- nean, at different scales of space and through time. Al- ternative models leading to different predictions about these effects were proposed and tested. Limpets were ex- cluded by fences from areas of the substratum at mid levels on the shore. The response of algal assemblages to this manipulation was compared with control and enclo- sure plots at the same level, and with unmanipulated plots in the low shore where limpets are less abundant. The effects of limpets were examined at several replicat- ed sites (0.1–4 km apart) for each slope of the substra- tum (nearly horizontal vs vertical), at different locations (hundreds of kilometres apart) and at different times. In- dividual taxa responded differently to limpet exclusion. The percentage cover of the coarsely branched and fila- mentous algae increased significantly in exclosure plots, in some loser reaching values found on the low shore. These patterns, however, varied greatly from shore to shore and significant effects were found both on horizon- tal and vertical substrata. Multivariate analyses indicated that grazing by limpets accounted for about 20% of the differences between mid-shore and low-shore assem- blages. This effect was independent of substratum incli- nation and was consistent in space and time, suggesting that physical conditions were not as stressful for macro- algae on vertical substrata as initially supposed. Variable recruitment of algae is proposed as a possible explana- tion for the lack of consistency in the effects of limpets at the scale of the shore. The results of this study empha- size the need for multiple-scale analyses of the interac- tive effects of physical and biological factors to under- stand the organization of natural assemblages. Key words Grazing · Physical factors · Repeated experimentation · Rocky shores · Scales of variability Introduction Physical and biological processes interact to generate spatial and temporal patterns in natural assemblages of species. Understanding the interplay of physical and bio- logical processes and the scales at which these interac- tions take place are major issues in ecology (Menge and Sutherland 1987; Danielson 1991). Spatial and temporal variability are usually large in natural systems, making generalizations of results from studies conducted at par- ticular places and times unreliable (Underwood et al. 1983; Foster 1990; Underwood and Petraitis 1993). As the relative importance of different ecological processes is likely to change with scale, multiscale analyses are necessary to set the spatial and temporal limits within which any relevant ecological process is likely to operate (Wiens et al. 1993; Underwood and Chapman 1996). Un- derstanding the scales of variation in patterns and pro- cesses will increase our ability to predict the conse- quences of natural and anthropogenic disturbances to the environment. These goals can only be achieved by in- creasing the extent and scope of ecological studies (Schneider 1994; Thrush et al. 1997). A number of physical and biological processes are known to affect the vertical distribution of algae and ani- mals in intertidal rocky shores. Several studies have in- ferred, and in part elucidated, the importance of physical forces such as desiccation and exposure to waves (Stephenson and Stephenson 1949; Southward 1958; Lewis 1964), and of biological interactions including the effects of consumers (Paine 1974; Lubchenco 1980; Moreno and Jaramillo 1983) and competition (Connell 1961; Lubchenco 1980; Shonbeck and Norton 1980; Hawkins and Hartnoll 1985) in setting the upper and lower limits of distribution of intertidal populations. This research has enhanced our understanding of the relative importance of different processes in regulating local pat- L. Benedetti-Cecchi ( ✉ ) · F. Bulleri · F. Cinelli Dipartimento di Scienze dell’Uomo e dell’Ambiente, via A. Volta 6, 56126 Pisa, Italy e-mail: bencecc@discat.unipi.it Tel.: +39-50-500943, Fax: +39-50-49694 Oecologia (2000) 123:406–417 © Springer-Verlag 2000 L. Benedetti-Cecchi · F. Bulleri · F. Cinelli The interplay of physical and biological factors in maintaining mid-shore and low-shore assemblages on rocky coasts in the north-west Mediterranean Received: 22 June 1999 / Accepted: 15 November 1999