The role of herbivory, and direct and indirect interactions, in algal succession

Mechanisms of algal succession were studied by investigating the direct and indirect interactions among the component organisms in a barnacle- limpet-algal community in an upper rocky intertidal shore of British Columbia, Canada. A factorial design with barnacles and limpets removed singly and in combination was used for the sixteen 20 X 20 cm plots cleared in 1991 and monitored for 3 years. Results indicated that algal succession followed an inhibition model, in which the early colonist, ephemeral algae, inhibited the settlement of the later species, Fucus gardneri Silva and Pelvetiopsis limitata Gard., for the first 2 years. However, this inhibition occurred only in the absence of limpets. In the limpet-present plots, limpets grazed a substantial amount of ephemeral algae, and consequently fucoids occupied space in the early stage of succession, and these algae persisted. There was no obvious barnacle effect on algal abundance in the absence of limpets. However, in the presence of limpets, barnacles facilitated the colonization of F. gardneri; this alga had a very low percent coverage under similar conditions but without barnacles. This implies that the interaction strength between barnacles and F. gardneri was relatively weaker than those between limpets and ephemerals and between ephemerals and fucoids. In addition to the above mentioned direct interactions, indirect interactions among the component organisms appeared to be an important factor in understanding the complex interaction networks during algal succession. The positive effect of limpets on fucoids is an indirect facilitation mediated by ephemeral algae, which may occur because the limpets prefer to eat ephemerals rather than fucoids. In this barnacle-limpet-algal community, the indirect facilitation by limpets caused a bypassing of the early colonist stage and resulted in acceleration of the algal succession.