Can the mid-domain effect explain bat diversity across the Neotropics?

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Tests of mechanisms behind diversity gradients typically focus on a single dimension, such as species richness. But characteristics such as geographic range size, trophic status and phenotype are tied to individual species, so mechanisms driving broad diversity patterns should show up across multiple dimensions. The researchers developed a stronger-inference approach using multiple dimensions and applied it to a proposed mechanism, the mid-domain effect (MDE).

They overlaid geographic range maps for 133 noctilionoid bat taxa to determine species composition in grid cells of 10,000 km². For each cell in the Neotropics, they calculated species richness and three indices each of phylogenetic, functional and phenetic diversity to establish observed diversity gradients. They also generated 1,000 simulated gradients for each biodiversity metric using an MDE model, estimating patterns expected if species distributions were randomly placed within the Neotropics.

For each simulation, they regressed the observed gradient onto the MDE-expected gradient. If the MDE drives the observed gradients, the coefficients of determination should be high, the intercept should not differ from zero and the slope should not differ from one.

MDE predictions for species-richness gradients fit the observed patterns. The model also produced strong spatially structured gradients of taxonomic, phylogenetic, functional and phenetic diversity. Nevertheless, its expected values for most biodiversity dimensions fit most observed patterns poorly. The researchers concluded that the MDE cannot account for most empirical biodiversity patterns. They suggested that a fuller understanding of latitudinal gradients requires simultaneously examining the relative effects of random, environmental and historical mechanisms on the distribution and abundance of current biota.