Competition, mortality and diversity in South Atlantic coastal boulder communities

Boulder shores are common at all latitudes and dominate the intertidal and subtidal zones of sub-Antarctic coastlines. The encrusting benthos of bould ers was examined on similar shore types at four locations: Tierra del Fuego , East Falklaud, West Falkland and Bird Island (off South Georgia). Bird Is land is unusual in experiencing high trampling and organic enrichment from fur seals. The results were compared to a Patagonian site and a non-trample d South Georgia site (Husvik) and other sites taken from the literature. Pr incipal Component Analysis revealed South Atlantic/Southern Ocean eucrustin g faunas formed a distinct cluster when compared to assemblages from elsewh ere at similar latitudes. Bray Curtis cluster analysis of the South Atlanti c-Southern Ocean sites showed the major division was between Southern Ocean and South Atlantic Ocean sites, beyond which there were three distinct clu sters centred around Patagonia (Magellanic), the Falklands and Southern Oce an sites. The organisation of competitive interactions between species was mostly determinate and transitive (essentially hierarchical). The transitiv ity index scores were higher than most similar assemblages studied to date. The diversity of encrusting assemblages ranged from Shannon Weaver H value s of 2.38-0.77 (East Falkland and Bird Island, respectively) in the interti dal to 1.27-0.73 (Patagonia and South Georgia, respectively) in the subtida l zone. Annual mortality (of bryozoan colonies) varied from 85-97% in the i ntertidal to 65-92% in the subtidal, being higher in the Southern Ocean tha n South Atlantic sites, largely due to ice scour and wave action. The Bird Island mortality values may be high even for a Southern Ocean site.