PATCH RECOLONIZATION THROUGH MIGRATION BY THE ECHINOID PARACENTROTUS-LIVIDUS IN COMMUNITIES WITH HIGH ALGAL COVER AND LOW ECHINOID DENSITIES

We examined the recolonization of patches which had been artificially cleared of the echinoid Paracentrotus lividus. Our aim was to mimic an extraction event such as those produced by harvesters acid to examine how long it took for the sea urchin to recolonize the cleared areas. We selected a community in which there was a low echinoid density (ca. 2 ind.m(-2)) and high algal cover. These conditions are typical of ma ny rocky Mediterranean assemblages at depths below 4-6 m. The experime ntal patches varied in the degree of isolation from neighbouring sea u rchin. In these circumstances we found that recolonization was slow, a nd only in the most accessible zones did the sea urcchin abundance rec over within 18 months. After 4 and a half years, the population had re covered to less than 46% of the original abundance in the most isolate d area. Settlement and growth of urchins, rather than migration, may a ccount for recolonization in isolated zones. We conclude that extracti on of this species may have a long-lasting effect on the local populat ions of echinoids which, in turn, may influence algal communities.