What does measuring species diversity in estuarine seagrass systems actually assess?

Relationships between species diversity and other species-density and species-richness metrics were investigated in the seagrass macrobenthos of Knysna estuarine bay, South Africa. Although a wide range of species density occurred across sites, neither Hill-Shannon nor Hill-Simpson diversity showed any significant relationship with it, although they did with species richness. Instead species diversity was very closely related to relative evenness, and (negatively) to overall assemblage abundance. No significant relationship was found between species density and evenness. Whilst there was a clear and marked decrease in species density upstream along the main estuarine channel, only one of the species-diversity indices (the Hill-Shannon) showed a significant equivalent decrease. Relationships depended on how 'species richness' was assessed, and were very strongly influenced by the superabundant local occurrence of a few individual faunal components (three gastropod and one tanaid species). Species-diversity analysis contributes nothing new in such estuarine seagrass meadows and seems best avoided.non