The critical scale of small-scale spatial variation in ecological patterns and processes in intertidal macrobenthic seagrass assemblages

Earlier work on benthic faunal assemblages in intertidal seagrass beds has shown maximal structural variation at spatial scales <150 m. To examine more closely spatial variation of species assemblages within this range, numbers of target taxa in the beds on North Stradbroke Island, Moreton Bay, Australia, were sampled at nested spatial scales of 0.5, 5, 30 and 90 m at two localities 900 m apart. Variance components were largest at the smallest scales, that at 0.5 m comprising 55% of the total and that at 5 m 13%. Individual species were patchily dispersed and their assemblages were randomly structured at all studied spatial scales. Although assemblage composition differed significantly between the two localities, overall faunal abundance and species richness were relatively uniform across the whole study area. These seagrass beds show little turn-over of dominant species and little evidence of niche partitioning through space over scales of <1 km. In this they contrast markedly with the equivalent intertidal dwarf-eelgrass beds of Knysna in South Africa that form a high turn-over, strongly structured, deterministic system.