Some general propositions about the study of spatial patterns of species richness

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Abstract
Spatial patterns of species richness are very well studied, yet there is surprisingly little consensus about what factors or processes control those patterns. Hypotheses have accumulated in the literature, rather than being tested and replaced by better ones. We argue that this is because most studies are not designed to test hypotheses about what controls patterns of richness. Many hypotheses are weak (often because of non-operational variables) or badly formulated (often because they do not make mutually exclusive predictions). Support for many existing hypotheses is largely post hoc. In contrast, correlations have been used to test general hypotheses. The clearest general patterns over broad spatial scales in polar to temperate areas are that richness is related to variations in climate. At smaller spatial scales, richness is most consistently related to productivity. Mechanistic hypotheses that predict more than just patterns of richness would be desirable, but simple correlative hypotheses that make testable predictions about patterns of richness in nature are better than elegant mechanistic hypotheses that, in practice, make no predictions.