A relationship between species richness and evenness that depends on specific relative abundance distribution

Environmental Seasonality Regulates Community Evenness in Neotropical Bat Communities

Evenness is a key community property that provides insights into resource acquisition and ecosystem functioning. However, it is unclear how other community properties influence evenness after integrating the effect of environmental gradients. Using 49 neotropical bat communities, we tested the hypothesis that evenness increases in communities that have low species richness and low biomass because the negative effect of richness and biomass on evenness is regulated by environmental seasonality. We selected among path models to determine how temperature seasonality, the most important gradient across study sites, affected richness and biomass as drivers of evenness. Employing three indices of evenness, we found that more seasonal climate reduces species richness, and lower richness increases evenness. Moreover, a decline in biomass with increasing seasonality also increases evenness. A decrease in resource specialization and rarity as sites become more seasonal may explain the negative relationship between seasonality, richness and evenness. Moreover, the negative effect of biomass on richness and evenness may be due to an expansion of niche space and a positive effect of smaller body size on diversification rates, which may allow more species packing and greater richness. We believe our results bring us closer to a unified theory of which factors control evenness in a community.

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.