Relative abundance and evenness patterns along diversity and biomass gradients

The distribution of species relative abundances and evenness have long been of interest to ecologists, but until recently evenness and dominance indices were confounded by species richness. We computed six indices of evenness or relative abundance for 92 quadrats of herbaceous wetland plants, two of which (E-var and E-1/DS) have been shown to be mathematically independent of species richness. The other four indices were positively correlated with species richness which supports the idea that they are biased measures. Plant assemblages with either high biomass or high species richness tended to have low evenness (E-var and E-1/DS). These relationships were weak and resembled triangular upper-limit functions. These results suggest that evenness is scale-dependent. The weakly negative relationship between biomass and evenness was also apparent when biomass values were randomly assigned to species. Therefore, the negative relationship may be due to the inherent distribution of biomass values among species(as suggested by Drobner et ai.). When biomass values were randomly allocated to species, the relationship between biomass and E-var was significantly stronger (in terms of r(2), p = 0.015) than the observed relationship. The lack of agreement between the actual data and the null model suggests that the underlying distribution of species abundances is not sufficient to explain the observed patterns and other ecological processes are likely at work.