Assessing the synergism among environmental gradients: Towards a better understanding of macroecological hypotheses

The role of environmental gradients as drivers of biological diversity has been the center of many discussions in ecology and evolution. Hypotheses proposed to explain broad-scale patterns of biological diversity have mechanistic bases that often overlap, at least partially. Consequently, it is often difficult to tease apart the potential effects of different hypotheses. Here I investigate the synergism among macroecological hypotheses commonly invoked to explain species diversity. More specifically, I address the role of ambient-energy, climatic stability, habitat heterogeneity, productivity, and topographic complexity in shaping broad-scale patterns of tropical vertebrates under three different aspects of biological diversity: species richness, species pools, and species composition. I show how differences in the degree of synergism among distinct types of environmental gradients can be used to improve our understanding of traditional macroecological hypotheses, highlighting the convergent findings across the three different aspects of biological diversity.