Abstract
Using data on the distribution of birds in Southern Africa, we provide an explicit determination of the structure of species distribution along the productivity gradient (which is correlated to species richness), showing that the majority of species exhibit increasing occupancy toward more productive regions, but decreasing average reporting rates per species within occupied grid cells. The mean geographical range size of species in Southern Africa decrease along the energy gradient, as most species are present at high productivity levels but only some can extend their ranges toward lower levels.
Species turnover among grid cells consequently decreases toward high energy levels. These patterns support the hypothesis that higher productivity leads to more species by increasing the probability of occurrence of resources that enable the persistence of viable populations, without necessarily affecting local population densities.