The influence of predation and substratal complexity on recruitment to settlement plates: a test of the theory of alternative states

Abstract

There are radical differences between the benthic communities occupying subtidal hard substrata at Marcus and Malgas Islands on the South African west coast, even although the islands are closely situated and experience similar physical conditions. Experimental settlement plates were installed at both islands to examine the process of succession by the benthos and as a partial test of the hypothesis that the two communities are alternative states of the same ecosystem. The plates were of different types of substrata of varying complexity, and half were caged to exclude predators while the remainder were left unprotected. The results indicate that predation by the abundant rock lobster Jasus lalandii is the main force maintaining the sublittoral community at Malgas Island. Because this major predator is virtually absent from Marcus Island, wave action and inter/intraspecific competition are considered more important in determining community structure there. At Malgas Island, unprotected plates became covered by algae within 4 months, thereby resembling the natural biota. In contrast, the protected plates at Malgas developed a community closely similar to that on the plates at Marcus Island, showing that, when protection is provided from rock lobsters, the community at Malgas has the capacity to converge on that of Marcus. In the absence of rock-lobster predation, plates which had pre-established mussels on them experienced the highest rate of recruitment and initially supported the largest variety of species. In the case of plates that experienced either extremely high or extremely low levels of predation, the complexity of the substratum ultimately had no influence on species richness: after 6 months richness was uniformly high on plates that were exposed to virtually no predation, and uniformly low on plates subjected to intense predation. Plates experiencing intermediate levels of predation consistently supported a higher number of species if they had a complex biotic substratum. At Malgas, the early colonists always comprised sessile space-occupying species which were established by larval settlement. At Marcus, postlarval juveniles and adults of mobile species rapidly moved onto the plates and influenced subsequent recruitment. These contrasts reflect the structure of the two communities which are determined by differing levels of rock-lobster predation.