Quantifying interspecific spatial overlap in aquatic macrophyte communities

Abstract

Levins’s asymmetrical α index quantifies between species overlap over resources more realistically than similar-purpose single-value indices. The associated community-wide $\bar \alpha$ index expresses the degree of “species packing”. Both indices were formulated upon competing animal (i.e., mobile) organisms and are independent of population densities. However, overlap over resources for nonmobile organisms such as plants may have an impact even below carrying capacity. The proposed $\hat \alpha$ index, based on Levins’s α index, quantifies spatial overlap for plants integrating information on species spatial distribution and crowding conditions. The $\hat \alpha$ index is specifically designed for plant distribution data collected in discrete plots with density expressed as percent coverage (%cover) of substratum. We also propose a community-wide ${\hat \alpha_{\text{c}}}$ index, conceptually analogous to $\bar \alpha$, but furnished with a measure of dispersion (se ${\hat \alpha_{\text{c}}}$). Species importance within the community is inferred from comparisons of pairwise $\hat \alpha$’s with ${\hat \alpha_{\text{c}}}$. The $\hat \alpha$ and ${\hat \alpha_{\text{c}}}$ indices correlate closely and exponentially with plant density, and correct apparent over- and underestimations of interaction intensity at low and very high crowding by Levins’s α and $\bar \alpha$, respectively. Index application to aquatic plant communities gave results consistent with within-community and general ecological patterns, suggesting a high potential of the proposed $\hat \alpha$ and ${\hat \alpha_{\text{c}}}$ indices in basic and applied macrophyte ecological studies and management.