Differential predation on alternative prey, and the survival of rocky intertidal organisms in New South Wales

Abstract

An examination of the development of ideas involving predation as a major force organizing communities reveals that few experiments have been done on the rôle of the availability of different types of prey. This could be especially important where generalist predators are selective and fluctuations occur in the relative abundances of alternative prey. From observations of the diets of a predator on a rocky intertidal shore in Australia, a simple model is devised to account for trends in diets by consideration of the presence or absence of prey. Where preferred prey are present, the effect of predation on alternative prey is small. Where preferred prey are absent, these alternative prey suffer intense mortality because of increased feeding by predators. Three experimental manipulations of predators and preferred prey tested this model. These confirmed that rankings in preference both between and within species of prey can influence the outcome of predation, and hence, the relative abundances and diversity of prey on the shore. Thus, the presence and activities of predators and prey at some range of natural densities are insufficient to account for patterns of abundance of intertidal prey species because the numbers of some types of prey surviving in an area may also be a function of the relative availability and relative attractiveness of the alternative types of prey in that area. It is concluded that predators are not affecting this community in the way described for rocky shores elsewhere. The implications of the described effect of predation are critically discussed in comparison with more familiar rôles ascribed to predators, especially with regard to hypotheses about the causes of visible patterns of community structure. Experimental designs that consider a number of alternative hypotheses about patterns of predation are suggested and briefly described.