The influence of seabird guano on the biological structure of rocky intertidal communities on islands off the west coast of southern Africa

Abstract

On the rocky west coast of southern Africa, the growth rate of intertidal algae appears to be limited by the availability of inorganic nutrients. At offshore islands, where colonies of seabirds breed and roost, intertidal waters are enriched with nutrients from dissolved guano, and the growth rate of algae is enhanced. Comparison of mid-intertidal communities from island and nearby mainland shores showed that mainland communities were characterized by the absence of foliose algae and sessile filter-feeders, and the presence of large areas of "vacant", rocky surface. On island shores, mats of foliose algae dominated mid-intertidal communities and herbivorous limpets grew more rapidly and attained larger sizes. Dense populations of African black oystercatchers, which feed on limpets, occurred on island shores, but they were absent from mainland shores. The effects of nutrient enrichment at islands were evident, not only in the rate of primary production, but also in interactions between organisms at higher trophic levels and in the structure and functioning of the intertidal community as a whole.