Patterns of mammalian species richness in relation to temperature, productivity, and variance in elevation

In Texas, the relationship between environmental factors and species richness of all mammals, bats, rodents, and carnivores was investigated by multiple-regression methods. Variance in elevation was a strong, positive predictor of species richness for all mammals, bats (Chiroptera), and rodents (Rodentia). It was a significant but weak predictor for carnivores (Carnivora). Productivity had significant, negative regression coefficients for all mammals, bats, and rodents. Productivity was positive, significant, and relatively stronger for carnivores. Species richness of all mammals, rodents, and carnivores may follow a hump-shaped curve along a gradient of increasing environmental productivity. Mean-annual temperature range was significant with a negative sign for carnivores and with a positive sign for rodents. Mean intermonthly variability of precipitation had a positive and significant relationship to carnivores, but significant, negative coefficients for bats and rodents. Variation in all mammals and carnivore species richness, and in all mammals and bat richness, was significant, with positive signs, for maximum and minimum temperatures, of record, respectively. All mammals, bat, and rodent richness varied significantly, with negative signs, with the coefficient of variation of mean January temperature. Overall, the climatic-variability indices exhibited only weak control over density of mammalian species. With the exception of productivity their influence on current ecological hypotheses of species richness was complex. Statistical support, contradiction, and statistical lack of significance depended upon the particular combination of environmental factors and taxa under consideration. Results suggest that current ecological hypotheses of species richness often function in directions not concordant with predictions and are not satisfactory as general explanations of biotic richness for these vertebrate groups.