Density dependent foraging of sea urchins in shallow subtidal reefs on the west coast of Italy (western Mediterranean)

The ecological role of the sea urchins Paracentrotus Lividus and Arbacia lixula was examined in shallow subtidal reefs on the west coast of Italy from December 1994 to July 1996. The density of sea urchins was manipulated experimentally in patches of vertical substrata without erect algae. The following propositions were examined: (1) that sea urchins were responsible for the persistence of patches of bare rock and encrusting corallines in shallow subtidal habitats, (2) that their effects were density dependent, and (3) that these effects were consistent in time. The experimental manipulations consisted of 3 replicate patches randomly assigned to one of the following treatments: (1) Ox (total removals), (2) 0.5x (50% of the average density found in untouched patches), (3) Ix (untouched patches), and (4) 2x (200% of the average density found in the untouched patches). The removal of sea urchins significantly increased the coverage of the filamentous and fleshy algae in permanent plots, and these effects were consistent over time. In contrast, grazing had no effect on the encrusting corallines, Peyssonnelia spp. and invertebrates (barnacles, bryozoans and limpets). The response of the filamentous algae was proportional to the density of sea urchins, with percent cover values in the 0.5x treatment being intermediate to those observed in the 0x and 1x treatments. Conversely, the response of the fleshy algae was non-linear: only when sea urchins were totally removed did the coverage of these plants increase significantly. Doubling the density of sea urchins significantly increased the erosion of the margins of the patches, while their removal caused no change along the perimeter. In general, there was considerable variability from patch to patch in the abundance of most of the response variables analysed, while temporal changes were less important. The results show that sea urchins can have significant but variable effects on vertical patches of barren habitat; alternative explanations are suggested to account for the persistence of these patches despite the removal of grazers.