Patterns in procellariiform diversity as a test of species-energy theory in marine systems

Biological diversity varies systematically in space with levels of primary productivity. Within regions, the relationship is hump-shaped, but across larger areas it tends to be monotonic and positive. However, the generality of this latter pattern, particularly within marine systems, is not clear. Here a global analysis across regions shows that the relationship between species richness of Procellariiformes (albatrosses, petrels, storm-petrels, diving-petrels) and remotely sensed chlorophyll concentrations, a surrogate for primary productivity, is hump-shaped with a single mode. The decline in richness at higher productivities is associated with an increase in the temporal variability in levels of productivity and a decline in the areal extent of these productivities. It appears that the relationship between the areal extent of productivity classes and species richness is, in part, a function of a positive relationship between local (grid cell) and regional (productivity class) procellariiform species richness. We argue that the relationship between the temporal mean and variance in levels of productivity may be of general importance in explaining differences in the response of species to increasing productivity. Wherever the relationship between productivity and its variance is negative, a positive monotonic species richness response may be expected, whereas a unimodal response may be expected where the productivity mean-variance relationship is positive. In the southern oceans, the relationship between productivity and species richness results in coincidence between areas of high procellariiform species richness and high resource harvesting by humans, to the detriment of the birds.