Abundance/occupancy/patchiness relations in estuarine seagrass macrobenthos

Using > 1800 macrobenthic samples from 25 representative intertidal seagrass sites in the Knysna estuarine bay (warm-temperate South Africa), abundance/occupancy relations were examined in the light of Foggo et al.'s (2003) earlier work concerning British estuarine faunas. In addition, the relationship of degree of variation in macrobenthic abundance across unit area, 'patchiness', to this macroecological picture was investigated. Across the group of 30 dominant faunal components as a whole, the most widely and abundantly distributed species had (a) the highest mean abundances and (b) the least spatially-patchy densities in each of Knysna's diverse regions and at all spatial scales from landscape to local; however no significant relationship was observed between mean abundance and degree of patchiness. Individual dominant species displayed considerable variation; some showing positive occupancy/patchiness correlations, and several displaying significant positive and others negative abundance/patchiness ones. As expected along the estuarine gradient, positive and negative correlations of both abundance and occupancy with distance upstream were found in different species (both metrics in a given species always responding in the same manner), although significant upstream relationships with patchiness were found in only two. Local patchiness clearly contributes macroecological information additional to that provided by abundance or occupancy.