Are the latitudinal gradients in taxa turnover? A worldwide study with Sciuridae (Mammalia: Rodentia)

Changes in taxa composition among different communities in a landscape or along an environmental gradient are defined as ²-diversity. From a biogeographic point of view, it is interesting to analyse patterns of ²-turnover across latitudinal bands, and to understand whether ²-diversity is significantly associated with endemism at lower latitudes, as predicted by theory. We inspected these issues by using squirrels (Rodentia, Sciuridae) as a study case. Distribution data for each genus were obtained from literature and mapped. The two hemispheres were subdivided into 23 latitudinal bands of equal area, and we calculated a ²-turnover index between latitudinal bands with two formulas: Wilson and Shmida’s (1984) and Lennon et al.’s (2001) indexes. We found that the peak of number of Sciuridae genera significantly corresponded to the peak in ²-turnover scores at the same latitudes (25-31°N) with Wilson and Shmida’s (1984), but not with Lennon et al.’s (2001) index. We also found that the turnover between ground and tree squirrels corresponded to the grassland vegetation latitudinal bands (around 40° N), and the beginning of the latitudinal bands characterized by tropical and subtropical forests is accomplished with the occurrence of tree and flying squirrels.