Effects of interactions between predators, variable predation regimes, and species body size on rocky intertidal communities : comparative and experimental approaches

Graduation date: 1995The body size (W) of animal species is one of the best predictors of\ud population density (D) when large assemblages are considered. It has been\ud shown that theoretically the D-W relationship can be the consequence of two\ud other distributions: the log-normal distribution of body sizes of the species and\ud of their abundances. We show in Chapter II (Navarrete and Menge) that the DW\ud relation is independent of other community patterns and that its expression\ud can be under the control of ecological forces. In Chapter III I show that within\ud assemblages of gastropods the D-W patterns are similar to those observed in\ud entire local communities, suggesting that processes controlling the D-W\ud expression operate at the community level, regardless of taxonomic affiliation.\ud In Chapter IV (Navarrete and Menge) the strength of predation on\ud mussels by the keystone seastar Pisaster ochraceus and whelks of the genus\ud Nucella was studied under different environmental conditions. Predation\ud intensity by the keystone predator was strong under all site x wave exposure combinations and was unaffected by the presence of whelks. On the other\ud hand, whelks had ecologically important effects on mussel survival in the\ud absence, but not in the presence of seastars. These results support the idea that\ud in keystone-dominated systems other species have only minor, if any effects on\ud the rest of the community. However, after the loss of the keystone these species\ud can adopt a major role in the altered system.\ud In Chapter V I conducted two cage experiments to evaluate the impact\ud of constant and temporally variable predation by different densities of whelks\ud on a mid intertidal successional community. Direct and indirect effects of the\ud exclusion of predators led to several changes in the abundance of sessile species,\ud notably a rapid increase in the cover of the bay mussel Mytilus trossulus.\ud Variable predation produced community compositions different from those\ud observed under constant predation regimes or predator exclusions. Community\ud composition in unmanipulated control plots resembled closer the composition\ud observed under variable predation treatment. Temporal variability in predation\ud by whelks can increase spatial variability in the sessile community and create\ud distinctive community compositions