Abstract:

The Fildes Peninsula (Antarctic) food Web has been considered to be simple. The main producers in this food web were benthic macroalgae,variation in biomass and standing crops have been described by the authors in another paper. The major consumers were molluscs (gastropods and chitons), echinoderms (sea urchins and a seastar), crustaceas, fishes and seabirds. The most abundant mobil species included fishes and seabird which occupied the top trophic level, predaceous gastropods and herbivorous molluscs, which occupied intermediate trophic levels. The most abundant sessile organisms were foliose algae, encrusting algae, and bivalves. These were all tested in two intertidal sections. Field experiments demonstrate that (1)Seabirds and fishes reduced the abundance of herbivorous molluscs, and predaceous snails. (2) Predaceous gastropods reduced the abundance of herbivorous molluscs and sessile invertebrates. (3) Herbivous molluscs reduced the abundance of foliose algae, and altered relative abundance of the encrusting algae. The encrusting algae proved to be inferior competitors for space with other sessile organisms when consumers were experimentally excluded. In this paper we evaluate the influences of substratum heterogeneity and consumers on patterns of diversity of sessile organisms in this habitat,local substratum topography is highly variable, ranging from smooth to irregular surfaces. Algal crusts typically dominate all low zone rock surfaces,and most other sessile species (invertebrates and foliose algae) occur in holes and crevices. The number and diversity of sessile species are lower on homogeneous surface than on heterogeneous surfaces. When consumers are excluded from the general rock surface, algal crusts are overgrown by foliose algae and sessile invertebrates.