Aim Variation in species richness has been related to (1) environmental conditions (water, energy and habitat characteristics) and (2) regional differences (contingent historical events and regional particularities that result in differences between regional faunas acting at broad extents). Whereas climatic factors have been widely studied, the effects of regional differences are less often quantified. This work aims to characterize global trends in the species richness of mammal assemblages with respect to both current and historical influences.
Location All terrestrial biogeographical realms except Antarctica.
Methods Species richness in checklists from 224 sites distributed worldwide were investigated by partitioning the variation between a general set of habitat/climate factors, biogeographical regions, and their overlaps. Additional analyses studied the specific overlaps of region, water and energy. Data were also divided according to area to determine if the strength of these effects varies according to the size of sites.
Results Environmental effects explained 38% of richness variation across all sites, whereas environmentally independent regional effects explained 11% and the overlap between region and environment explained 13%. Results were similar when only larger sites (between 1000 km2 and 10,000 km2) were considered. However, the importance of the overlap between region and all environmental variables was greater in smaller sites (between 100 km2 and 1000 km2). In contrast, the specific importance of water and energy variables and their overlap with region was greater in larger sites. The strength of the independent effect of region remained almost invariant regardless of the size of the sites studied.
Main conclusions The relationship between species richness and climate varies with scale and among regions. Although environmental variables are the strongest correlates of richness, the unique history and physiographic characteristics of a region produce differences between the richness of mammal assemblages and their response to environmental gradients. The importance of environmental variables varies with scale: climatic gradients are more important at coarse grain (larger sites), possibly as a result of their effects on species ranges, whereas habitat type is more important at the smaller sites, where the importance of ecological interactions increases. Therefore, regional differences and the scale at which richness is measured should be taken into account when evaluating species richness–energy hypotheses.