Aim Geographic variation in the species richness of birds has been shown to be strongly associated with annual water and energy levels (actual evapotranspiration, AET) at the global scale. However, the gradient in eastern North America appears to be anomalous, because richness is greatest around the Great Lakes, whereas AET is highest in the south-eastern US. Here I examine if birds may be responding to vegetation produced during the breeding season rather than to annual production.
Location North America east of longitude 98° W.
Methods The bird richness pattern was examined using climatic variables, remotely sensed estimates of annual and seasonal plant biomass, and time since areas were exposed by the retreating Laurentide ice sheet from 20,000 to 6000 yr bp.
Results Average summer GVI (Global Vegetation Index, derived from NDVI) was found to be positively linearly associated with richness, explaining 82% of the variance, whereas the relationships between richness and annual measures of both AET and GVI were curvilinear. The pattern of retreat of the Laurentide ice sheet explained an additional 6% of the variance in richness, consistent with a previous analysis of Canadian birds.
Main conclusions In eastern North America, a seasonal variable associated with plant production explains the diversity gradient rather than the annual measures, but it does not undermine a general conclusion that bird diversity is closely linked with plant biomass. Further, both contemporary and historical factors appear to influence the gradient, and an association between bird richness and the geographic pattern of glacial retreat is detectable in both climatic and plant-biomass models of bird diversity.