THE LATITUDINAL GRADIENT OF SPECIES-DIVERSITY AMONG NORTH-AMERICAN GRASSHOPPERS (ACRIDIDAE) WITHIN A SINGLE HABITAT - A TEST OF THE SPATIALHETEROGENEITY HYPOTHESIS

The spatial heterogeneity hypothesis predicts a positive relationship between habitat complexity and species diversity: the greater the hete rogeneity of a habitat, the greater the number of species in that habi tat. On a regional scale, this hypothesis has been proposed to explain the increase in species diversity from the poles to the tropics: the tropics are more diverse because they contain more habitats. On the lo cal scale, the spatial heterogeneity hypothesis suggests that the trop ics are more diverse because they contain more microhabitats. The posi tive relationship between habitat heterogeneity and species diversity, on the local scale, is well documented. In this paper, we test whethe r habitat heterogeneity on the local scale can explain the latitudinal gradient of species diversity on the regional scale. We determined th e latitudinal gradient of species diversity of 305 species of North Am erican grasshoppers using published distribution maps. We compared the slope of this multi-habitat (regional-scale) gradient with the slope of a within-habitat (local-scale) gradient in the prairie grasslands. Our results show no significant difference between the slopes at the t wo scales. We tested the generality of our results by comparing multi- and within-habitat latitudinal gradients of species diversity for ant s, scorpions and mammals using data from the literature. These results are in accordance with those from grasshoppers. We can therefore reje ct the local-scale spatial heterogeneity hypothesis as a mechanism exp laining the regional-scale latitudinal gradient of species diversity. We discuss alternative mechanisms that produce this gradient.