The equilibrium theory of biodiversity dynamics: a general framework for scaling species richness and community abundance along environmental gradients

Large-scale temporal and spatial biodiversity patterns have traditionally been explained by multitudinous particular factors and a few theories. However, these theories lack sufficient generality and do not address fundamental interrelationships and coupled dynamics between resource availability, community abundance, and species richness. We propose the Equilibrium Theory of Biodiversity Dynamics (ETBD) to address these linkages. According to the theory, equilibrium levels of species richness and community abundance emerge at large spatial scales due to the population size-dependence of speciation and/or extinction rates, modulated by resource availability and the species abundance distribution. In contrast to other theories, ETBD includes the effect of biodiversity on community abundance and thus addresses phenomena such as niche complementarity, facilitation, and ecosystem engineering. It reveals how alternative stable states in both diversity and community abundance emerge from these nonlinear biodiversity effects. The theory predicts how the strength of these effects alters scaling relationships between species richness, (meta)community abundance, and resource availability along different environmental gradients. Using data on global-scale variation in tree species richness, we show how the general framework is useful for clarifying the role of speciation, extinction and resource availability in driving macroecological patterns in biodiversity and community abundance, such as the latitudinal diversity gradient.