HABITAT HETEROGENEITY, SEA-URCHIN GRAZING AND THE DISTRIBUTION OF ALGAE IN LITTORAL ROCK POOLS ON THE WEST-COAST OF ITALY (WESTERN MEDITERRANEAN)

The effects of habitat heterogeneity and grazing by sea urchins on the distribution of algae in littoral rock pools were investigated on the west coast of Italy (western Mediterranean) from February 1991 to Apr il 1994. Sea urchins (Paracentrotus lividus and to a lesser extent Arb acia lixula) were removed by hand from 6 pools, chosen at random out o f 12, throughout the study. The other 6 pools remained subject to natu ral densities of herbivores and served as controls. Two permanent plot s were marked with epoxy putty in each pool. One plot was located just in front of the crevices sheltering the sea urchins while the other w as marked 50 to 60 cm away. The following algal groups were monitored in these plots until March 1994: (1) articulated corallines, (2) coars ely branched algae, (3) Cystoseira spp., (4) filamentous algae, and (5 ) foliose algae. Extensive sampling of macroalgae was carried out in A pril 1994. Four replicated 100 cm(2) plots were randomly located at ea ch distance from shelter in each pool and sampled by a nondestructive method. The results from the permanent plots indicated that the impact of sea urchins was greater close to the crevices for all the algal gr oups but the coarsely branched algae. In contrast, the large variabili ty between pools often masked the importance of interactive effects be tween physical and biological factors at the end of the experiment. Ne vertheless, reliance on shelter apparently constrained the effect of s ea urchins on the filamentous algae and on Cystoseira spp., and thus o n a conspicuous portion of the algal assemblage of the pools. The resu lts of this study provide an example of how spatial heterogeneity and habitat complexity may affect the outcome of interactions between herb ivores and algae.