Several mechanisms are involved in shaping species' distributions, such as abiotic parameters of climate and substrate; biotic interactions with competitors, predators, and mutualists; and geographic considerations that constrain dispersal. Janzen, in 1967, proposed that mountain passes are effectively higher in the Tropics than in temperate areas in terms of their effectiveness in limiting dispersal, creating greater opportunity for isolation of populations, and differentiation into new species. Here, I analyze quantitatively predictions derived from Janzen's theory via 1000 virtual species across South America covering all major environments on the continent, taking into account effects of environmental contiguity and connectivity, effects of seasonality, and presence of known biogeographic barriers (rivers, principally). Virtual species' distributions were obtained by calculating Euclidean distances in ecological space to 1000 seed points, and applying different thresholds of ecological similarity within which the species is allowed to disperse into adjacent pixels. Distributional areas were measured taking into account only mean environmental similarity across the year, and considering the effects of seasonality and known geographic barriers. The results illuminate the situation: distributional areas are smaller in temperate areas when only mean similarity is considered (contra Janzen), but the pattern reverses when seasonality and barriers are considered, confirming and clarifying Janzen's ideas regarding generation of greater biological diversity in tropical areas as compared with temperate areas.

Keywords: landscape subdivision, species' distributions, biogeography, virtual species