Carrying capacity for species richness as a context for conservation: a case study of North American breeding birds

Aim To demonstrate that the concept of carrying capacity for species richness (SK) is highly relevant to the conservation of biodiversity, and to estimate the spatial pattern of SK for native landbirds as a basis for conservation planning. Location North America. Methods We evaluated the leading hypotheses on biophysical factors affecting species richness for Breeding Bird Survey routes from areas with little influence of human activities. We then derived a best model based on information theory, and used this model to extrapolate SK across North America based on the biophysical predictor variables. The predictor variables included the latest and probably most accurate satellite and simulation-model derived products. Results The best model of SK included mean annual and inter-annual variation in gross primary productivity and potential evapotranspiration. This model explained 70% of the variation in landbird species richness. Geographically, predicted SK was lowest at higher latitudes and in the arid west, intermediate in the Rocky Mountains and highest in the eastern USA and the Great Lakes region of the USA and Canada. Main conclusions Areas that are high in SK but low in human density are high priorities for protection, and areas high in SK and high in human density are high priorities for restoration. Human density was positively related to SK, indicating that humans select environments similar to those with high bird species richness. Federal lands were disproportionately located in areas of low predicted SK.