Causality between abundance and diversity is weak for wintering migratory waterbirds

The species-area relationship, which is closely linked with the more general species-energy theory, is one of the most well-known patterns in geographical ecology, but the underlying causes remain contentious. The more individuals hypothesis (MIH) articulates a causal path from resource availability to population abundance to species richness. The MIH has been tested with a range of taxa including plants, invertebrates and land birds but never with migratory waterbirds. Using multiyear simultaneous survey data of wintering waterbirds in 10 lakes at Poyang Lake, China, and remotely sensed habitat condition measurements, we applied structural equation modelling (SEM) to test three causal paths: (A) good habitat conditions (e.g. habitat availability and heterogeneity) attract more species (high richness); (B) habitat conditions promote abundance (more individuals); and (C) habitat conditions promote abundance, which in turn increases richness. We also modelled responses of species richness and abundance to habitat conditions using generalised additive mixed modelling (GAMM) to assess their co-variation. While our analysis confirmed the first two paths, we found no support for the third, which is the central postulate of the MIH. In addition, in agreement with GAMM, SEM indicated that species richness was more closely related to habitat quality than to abundance. Our findings suggest that wintering waterbird species richness and abundance are two intrinsic community indices that covary with environmental variables.