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The gamma diversity patterns of California vegetation including the total flora and its various life-form and taxonomic subdivisions can be largely explained in the statistical sense by climatic and topographic variables, and thus serve as a reasonable test of several of the proposed environmental hypotheses of diversity regulation. The actual relationship between environment and diversity patterns is more complex than ordinarily supposed. Topographic heterogeneity has a strong effect on patterns of the total flora and most subdivisions. Weather variables account for the bulk of the diversity patterns in the models used, but the results are unexpected in that favorableness indices (mean weather variables) are generally more important than indices of seasonality or irregularity. The results obtained in this study together with similar observations in other situations suggest that favorableness hypotheses deserve more theoretical and empirical attention.
Richerson et al. (Wed,) studied this question.