MEPS 134:73-83 (1996)  -  doi:10.3354/meps134073

From a list of specific predictions put forward in 1992, we tested the proposition that, in physically benign environments where herbivorous fishes are abundant, consumer pressure will be the main structuring force in rocky shore communities. Manipulation experiments were done in an intertidal community situated 150 km east of Rio de Janeiro, Brazil. The presence or absence of basal species (2 types: red algal turf and colonial coelenterate) was tested in combination with the presence or absence of 3 common consumer groups (amphipods, crabs, fishes). The design allowed us to separate the effects of pre-emptive competition and consumer pressure on the abundances of species at the lowest trophic level. Although herbivores were abundant, most effects of consumer species were not significant. Of 16 data sets involving herbivores, 11 were not significant. Pre-emptive competition (detected as a response to presence or absence of basal species) had a major effect in all but 1 case. Further, the effects of grazers were not consistent. Herbivores had no significant effects on Enteromorpha abundance in clearings made in red algal turf patches. In similar clearings made in Zoanthus patches, grazers reduced the cover of Enteromorpha. In contrast, cover of Ulva was enhanced by fishes and crabs in both patch types. Amphipods also increased the abundance of Ulva, but only in clearings in Zoanthus patches. The roles of consumers were therefore inconsistent. Conversely, competitive effects were nearly always significant, consistent, and responsible for most of the variance in the dependent variables tested. Our results show that there is no basis for the generalization which proposes that, in benign tropical shore environments where herbivorous fish are abundant, the effect of consumers dominates community structure, and that competition is an unimportant structuring agent.

Competition . Community organization . Experimental . Grazing . Intertidal . Tropics