ABSTRACT: 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 April 1994. Sea urchins (Paracentrotus lividus and to a lesser extent Arbacia lixula) were removed by hand from 6 pools, chosen at random out of 12, throughout the study. The other 6 pools remained subject to natural densities of herbivores and served as controls. Two permanent plots 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 was marked 50 to 60 cm away. The following algal groups were monitored in these plots until March 1994: (1) articulated corallines, (2) coarsely branched algae,
(3) Cystoseira spp., (4) filamentous algae, and (5) foliose algae. Extensive sampling of macroalgae was carried out in April 1994. Four replicated 100 cm² plots were randomly located at each 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 groups but the coarsely branched algae. In contrast, the large variability between pools often masked the importance of interactive effects between physical and biological factors at the end of the experiment. Nevertheless, reliance on shelter apparently constrained the effect of sea urchins on the filamentous algae and on Cystoseira spp., and thus on a conspicuous portion of the algal assemblage of the pools. The results of this study provide an example of how spatial heterogeneity and habitat complexity may affect the outcome of interactions between herbivores and algae.

KEY WORDS: Habitat heterogeneity . Sea urchin grazing . Macroalgae . Littoral rock pools . Field experiments

Published in MEPS Vol. 126 (1995) on October 5
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1995