Background/Question/Methods .  Conservation planning benefits from knowledge of the spatial distribution of species richness and the underlying causal factors.  A complex suite of factors is known to potentially influence species richness involving biogeography, historic factors, biotic interactions, and human activities.   Brown et al. (2001) asserted, however, that fundamental ecosystem properties often override other factors and regulate species richness such that ecosystems have a fundamental “carrying capacity for species richness”. Subsequent research has shown that ecosystem properties relating to kinetic energy (heat), potential energy (food), habitat heterogeneity, and available water are the leading predictors of species richness at continental to global scales. The goal of this study is to estimate the spatial patterning of ecosystem carrying capacity for species richness for native landbirds across North America as a basis for conservation planning.  We evaluated the leading hypotheses on biophysical factors affecting species richness for Breeding Bird Survey routes from areas little influence by human activities, derived a best multivariate model based on information theory, and used this model to extrapolate carrying capacity for species richness across North America based on the biophysical predictors.  The predictors included the latest and likely most accurate satellite and simulation-model derived products relating to climate, primary productivity, vegetation lifeform, and topography.  Human population density and land allocation were overlayed on predicted carrying capacity for species richness. 

Results/Conclusions .  The best model included mean and interannual variation in gross primary productivity and explained 73% of the variation in landbird species richness.  Predicted carrying capacity for species richness was lowest at higher latitudes and in the arid west, intermediate in the Rocky Mountains mid West, and highest in the eastern US and the Great Lakes region of the US and Canada.  Human density was positively related to predicted carrying capacity, indicating that humans selected environments that favored high bird species richness.  Protected areas were disproportionately located in areas of low carrying capacity and private lands were disproportionately located in areas of high carrying capacity.  We identify areas that are high in carrying capacity but low in human density as high priorities for protection and areas high in carrying capacity and high in human density as high priorities for restoration.