Latitudinal Gradients in Species Diversity: A Review of Concepts

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Abstract
Gradients in species diversity have long been a source of interest to ecologists, but little is known of the determinates of diversity. Six distinct hypotheses for the control of diversity (along with various mixtures) have been proposed in the literature: the time theory, the theory of spatial heterogeneity, the theory of climatic stability, the predation hypothesis, the competition hypothesis, and the productivity hypothesis. In the present paper, these 6 hypotheses are restated, examined, and some possible tests suggested; in addition, the available data relevant to each hypothesis is briefly described and discussed. Several of the proposed mechanisms could act in concert or in series to determine any particular diversity gradient; however, because of the preliminary state of knowledge on the subject, for the sake of clarity, and in order to suggest tests of the various hypotheses, each of the potential components of control of diversity has been considered and assessed separately in this review.