EARLY PATTERNS OF ALGAL SUCCESSION IN A MIDLITTORAL COMMUNITY OF THE MEDITERRANEAN-SEA - A MULTIFACTORIAL EXPERIMENT

Early patterns of algal succession were investigated from March 1991 t o March 1992 in a midlittoral community on the west coast of Italy (Li gurian Sea). A multifactorial experiment on algal colonization in arti ficially cleared plots was designed to test for the effects of patch s ize, time of patch formation, herbivore density and position within th e large gaps. The experimental units were arranged in six replicate bl ocks, each representing a stretch of shore 10-15 m long, distributed a long 1 km of the coast. Each block consisted of eight plots randomly i nterspersed within stands of macroalgae. The clearings were large (22 x 22 cm) and small (12 x 12 cm) quadrats, both exposed to natural and reduced densities of herbivores. Four plots, representing all size x h erbivore combinations, were set up in March and four in September 199 1. Results indicated that the abundance of the early colonists was hig hly variable both in space and time. Algal cover was greater in the la rger than in the smaller plots, while no differences were found among the edge and the centre positions of the large gaps. In general, the r ate of recovery of patches was greater in the quadrats cleared in Sept ember than in those scraped in March, the only exception being the fil amentous red algae for which the opposite trend was evident. Patella b ecame more abundant in the small plots than in the large ones. Chaetom orpha and the filamentous red algae were dominant in the gaps where li mpet density was reduced. By contrast, these species were strongly dep ressed in patches where Patella had free access. Rissoella developed m ainly in the large plots exposed to grazers, suggesting that the seque nce of species replacement was herbivore-mediated in these areas. Resu lts were discussed in relation to the patchy nature of the community, to differences in species' life-history traits and to their vulnerabil ity to herbivores. The necessity for further experiments to clarify so me unexplained results was also acknowledged.