Predicting direct and indirect interactions during succession in a mid-littoral rocky shore assemblage

Increasing the predictive capabilities of ecological models is important fo r providing solutions to environmental problems. Progress in this direction relies on the understanding of basic ecological processes. Here, I used in teraction web models and natural history information to predict the direct and indirect interactions that regulated succession in a relatively unstudi ed rocky shore assemblage in the northwest Mediterranean. Natural changes i n abundance of organisms and general patterns of succession were examined d uring March 1991-September 1995. It was predicted that limpets enhanced suc cession by preventing the monopolization of the substratum by filamentous a lgae, indirectly facilitating the establishment of other colonists, such as the red alga Rissoella verruculosa, cyanobacteria (Rivularia spp,), and ba rnacles (Chthamalus spp.). This hypothesis was first tested by comparing su ccession in artificially denuded patches of substratum maintained at reduce d densities of herbivores, with similar patches exposed to natural densitie s. Variability in the effects of limpets was examined in relation to the si ze of patches and time of clearance. To unravel the pathways of interaction that influenced the outcomes of this long-term experiment, I manipulated s eparate components of the assemblage at early and late stages of succession . I tested whether the filamentous algae inhibited the establishment of Ris soella and Rivularia in the absence of limpets, and whether grazers could e xert direct positive effects on these algae. I also tested whether the colo nization of Rissoella and barnacles under the natural grazing regime could reduce the local abundance of limpets, thereby facilitating the establishme nt of filamentous algae at later stages of succession. The filamentous algae monopolized the substratum in the experimental absenc e of limpets, while in the presence of these grazers, Rissoella, Rivularia, and barnacles colonized. The filamentous algae inhibited the establishment of Rissoella (but not that of Rivularia) independently of the presence of limpets, and there was no positive direct effect of these herbivores on Ris soella. Interactions between barnacles and Rissoella were negligible, but t hese organisms jointly reduced the coverage of Rivularia and the local dens ity of limpets, eventually facilitating the colonization of filamentous alg ae late in succession. A main implication of the results of this study is that interaction web mod els integrated with a basic understanding of the mechanisms of interaction and supported by natural history information, may lead to correct predictio ns of the direct and indirect effects of species and their influence on suc cession. As an extension of this approach, I have presented a set of genera l, qualitative models of succession, applicable to different marine benthic habitats. It is argued that making these models more quantitative, through the analysis of specific alternatives to the null hypotheses, could substa ntially increase our capabilities to understand and predict the dynamics of these systems.