A number of hypotheses for both the latitudinal diversity gradient and the species-area relationship center around the relationship between the total number of individual organisms that can be supported within a region and that region's species richness. Some authors have suggested that since the number of individuals should increase proportionally with the product of regional area and per unit area energy availability, that species-area and species-energy relationships are linked and should exhibit equivalent scaling. We present a theoretical framework that includes five different species-area-energy models and use a database on the global distribution of birds to test them in 107 regions across the globe. We also examine the extent to which the parameters of these models and the identification of a best model by information criteria methods can be explained by regional environmental variables.
Results/Conclusions
We find that in the majority of regions species-area and species-energy relationships cannot be viewed as equivalent, and species-energy slopes tend to be steeper than species-area slopes. This implies that increases in richness along productivity gradients cannot be due simply to the increase in individuals, and points to other factors which may facilitate the apportionment of resources among species such as the structural complexity of vegetation. We also find substantial variation in slopes across regions, and can explain two thirds of the variation in species-area slopes using regional estimates of net primary productivity and various measures of habitat heterogeneity. Finally, the most supported species-area-energy model in a region was related to the underlying pattern of beta diversity and the extent to which beta diversity varied along the productivity gradient.
