Patterns of turtle species' geographic range size and a test of Rapoport's rule

There has been a recent resurgence of interest in documenting and explaining patterns of species' range sizes with the goal of determining if general patterns exist. Much recent debate has centered on whether Rapoport's rule, the tendency for range size to increase with increasing latitude, is a general rule or a local effect. I calculated the sizes of turtle species ranges from distribution maps and used published natural history data to examine how range size varies with latitude, and to determine if differences in range size exist among continents, and if correlations with body size, available land area, habitat and diet breadth exist. The distribution of turtle range sizes extended over six orders of magnitude and formed a lognormal distribution with many species having moderate or small ranges and few species having large ranges. Range size was positively correlated with available land area, habitat breadth, diet breadth, and body size. Multiple regression accounted for only 39% of the variance in range size indicating that other important factors remain unknown. At both global and continental scales, range size is largest near the equator and decreases with increasing latitude, the opposite of Rapoport's rule. However, range size did increase latitudinally above 25-30 degrees N in both the Neararctic and Palearctic suggesting that the pattern would be more accurately considered a local effect than a general rule. Larger range sizes at low latitudes may occur because more land area in the tropics provides much suitable habitat for ectotherms and there are few large scale physical barriers to dispersal. Rapoport patterns result from the occurrence of a small number of wide-ranging cold tolerant species that have reinvaded northern latitudes after Pleistocene glaciation. Patterns of the longitudinal and latitudinal extents of species ranges and their positions illustrate the importance of climate, mountain ranges, deserts, and coastlines, as barriers potentially affecting range size.