Spatial stability of macrobenthic seagrass biodiversity

The magnitude and patchiness of macrobenthic biodiversity were compared across spatial scales spanning 5 orders of areal magnitude (ca. 2 m(2) to ca. 1.5 ha) in an intertidal seagrass bed in the warm-temperate Knysna estuarine bay, South Africa. The 75 component species and their populations were highly variably distributed across the site, abundances of the individual dominant animals being significantly patchy and composition of the macrofaunal assemblage being significantly non-uniform. Nevertheless, emergent assemblage attributes exhibited spatial constancy: values of assemblage metrics (faunal abundance, species richness, species density and species diversity) did not differ across the site or across spatial scales, and neither did the patterns of dispersion of species diversity, species richness or observed species density. Distribution of the 2 latter through space was even significantly uniform. Ecological arguments developed for temporal constancy of biodiversity seem broadly applicable to the spatial dimension at Knysna, suggesting that although apparently paradoxical, it may be the varying assemblage composition that permits this spatial stability. There is no evidence, however, that the Knysna seagrass macrobenthos is a competitively-structured assemblage with rigid niche-partitioning, a condition suggested to be necessary for temporal constancy; rather, it is more likely to be open, non-equilibrium and below carrying capacity.