MODELS AND MECHANISMS OF SUCCESSION: AN EXAMPLE FROM A ROCKY INTERTIDAL COMMUNITY'

An investigation of the processes that cause succession was performed in an intertidal community on the central Oregon coast. The community was dominated by barnacles and several species of macroalgae. The successional sequence was determined at three different sites by clearing sets of plots in a way that mimicked natural disturbance. Succession at each of these sites followed the same general sequence. A barnacle, Chtham- alus da/li, first colonized the plots and was later replaced by a second barnacle, Balanus glandula. The macroalgae Pelvetiopsis limitata, Fucus distichus, and Endoc/adia muricata colonized the plots only after Balanus was established. While the order of species arrival and departure was consistent, the rate of succession varied greatly among sites. The sequence of community development that was observed at one site over a 36-mo period occurred in < 12 mo in a nearby area. Differences in the rate of succession appeared to result from variation in the timing of successful Balanus recruitment. The mechanisms of succession were investigated in a series of field experiments. An experiment with Balanus-removal, Chthamalus-removal, and control plots was used to assess the interactions between barnacles. A direct interaction, competition for space with Balanus, caused Chthamalus to decrease in abundance as succession proceeded. Chtham- alus did not affect the establishment of Balanus. Successful Balanus recruitment depended on occasional periods of larval settlement followed by periods of favorable weather. At all three sites, algal colonization was dependent on the presence of barnacles. Balanus greatly facilitated algal colonization, while Chthamalus only weakly facilitated algal colonization. Facilitation of algal colonization by epoxy-filled barnacle tests indicated that facilitation resulted from barnacle tests altering the substrate, rather than the activities of the living animals. A factorial experiment involving manipulations of barnacle and consumer (limpet) abundances demonstrated that the facilitation of algae by barnacles is an indirect interaction that is mediated by limpets. Barnacles decreased limpet foraging activity and thereby increased algal abundance. Succession in this community is complicated by several processes that are not included in traditional views of succession. (1) Not all early successional species have the same effect on the establishment of later successional species. This results in spatial variation in the rate of succession. (2) The model of succession is different in each step in the successional sequence. The Chthamalus-Balanus interaction is an example of the tolerance model, while the barnacle-algae interaction is an example of the facilitation model. (3) Both direct and indirect interactions between species determine the course of succession. The results of this study support a general model that predicts the effect of consumers on the rate of succession. Consumers slowed succession in this community in two ways. First, limpets delayed the establishment of Balanus and the competitive exclusion of Chthamalus. Second, limpets delayed the establishment of macroalgae. Previous studies in marine and terrestrial habitats have found that consumers may slow, accelerate, or have no effect on the rate of succession and these observations are consistent with the predictions of this general model.