Evidence that species richness begets species richness

AimThe variability in regional species richness is determined by the balance between speciation, extinction, and migration, but the underlying causes of high species richness in some regions (e.g. tropics, Mediterranean ecosystems) are challenging to explain. Species richness variability may be amplified by biotic feedbacks that promote species richness in some areas, while decreasing elsewhere. Following the example of species abundance, we speculated that species richness per area in arbitrary geographic units (e.g. quarter-degree squares) follows a hollow-shaped curve with the ranks of species richness to yield a species richness rank (SRR) curve. The hollowness of such an SRR curve may indicate biotic feedbacks.LocationSouth Africa, South West Australia, and global.Materials and MethodsWe used vascular plant (VP) and bird data from distinct geographic regions and global syntheses of VP, bird, mammal, and amphibian species richness. These data were subdivided into geographic grids and the deviation from normality of the species richness quantified using skewness and kurtosis and the values ranked to yield SRR curves.ResultsThese SRR curves all had a consistent hollow shape (corresponding skewness 1.08-3.4, kurtosis 0.23-18.40) distinct from a normal distribution arising if species were allocated randomly to geographic units. The SRR curves indicate that relatively species-rich regions are rare, while species-poor areas are common.Main ConclusionsNeutral processes, abiotic determinants, environmental heterogeneity, historical factors (e.g. climate stability), and biotic feedbacks all likely play a role in contributing to the shapes of the curves. Although these curves for diverse taxa and regions integrate these multiple drivers of species richness, they indicate an acceleration of species richness as a function of species richness as a result of speciation and/or immigration, limited by increasing extinction likelihood. Biotic feedbacks provide a parsimonious explanation for the shapes of the SRR curves across taxa and regions.