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A Reversed Latitudinal Diversity Gradient in Avian Communities of Eastern Deciduous Forests

This paper describes a major and previously unrecognized reversal in the arctic-to-tropics gradient in avian species diversity within habitat types. The latitudinal zone within which a gradient reversal occurs encompasses the eastern deciduous forest of North America. It is bounded by two thresholds that probably represent reversals in the effect of seasonality on community diversity. Within this region strong seasonality fosters increased summer diversity, owing to strong resource oscillations combined with winter limitation of consumers. Temporarily superabundant resources in summer relax limits to ecological overlap and enhance diversity. Detailed studies of resource partitioning in coniferous forests support this conclusion. Historical events linked with glaciation of the northeast have probably also favored the evolution of migratory northeastern breeders able to exploit the northern vernal bloom that is underutilized by residents. The "species-packing" characteristics of the pattern of diversification in this reversed gradient are markedly different from those of tropical diversification. I propose that this nonmonotonic gradient demonstrates a complex relationship between environmental stability and community diversity in which diversity can be enhanced by variability within certain bounds of severity and predictability.