Is species richness limited by the total number of individuals? Towards an equilibrium theory of spatial diversity patterns

Background/Question/Methods

Species richness increases with energy availability. The most straightforward explanation of this species-energy relationship is the more-individual hypothesis that states that higher amount of resources supports higher total number of individuals which may be divided into more species with viable populations. However, spatial diversity variation does not seem to be driven by the variation of the number of individuals, and the number of individuals actually does not closely follow environmental variation. I will argue that the number of individuals can still mediate the negative feedback between diversity and extinction rate, thus affecting equilibrium number of species even when the diversity variation is driven by other factors, which may be unrelated to the number of individuals itself. Although a comprehensive theory of species richness variation must therefore be more pluralistic, it may still be able to predic wide range of patterns using a few simple principles.

 Results/Conclusions

Using data on birds and woody plants sampled at global to local scales, I will show (1) that while total number of individuals varies only weakly with energy-related environmental variables, species richness correlates with energy quite closely, and (2) that this relationship depends on phylogenetic scale, so that it is more predictable for larger taxa. Additionally, (3) phylogenetic patterns underlying observed diversity variation suggest that diversification is diversity-dependent, and (4) diversification rate is related to the number of species already present in an area. None of the observed patterns is in conflict with an equilibrium view of diversity, although factors responsible for diversity regulation seem to be decoupled from those responsible for diversity gradients. The relationship between these two groups of factors depends on spatial and phylogenetic scale and biological properties of the taxon in focus.