A new article coming up in Nature by David Storch and Petr Keil and Walter Jetz calculates continental scale SAR and EAR relationships for amphibians, birds and mammals. They find that, after correcting for mean range size of the taxa, there are nearly universal relationships across taxa and continents.
While the SARs are clearly not power-law shaped, with increasing slope on the log-log scale (see figure below), the EARs seem to follow approximately a power law with exponent one, i.e. they are linear, which is quite interesting (ok, careful with power laws and log-log plots, but let’s be generous for the moment).
Figure 2 from Storch et al.: Universality of SARs (left) and EARs (right) after rescaling. The lower row shows the effect of alternative sampling designs (used with permission, copyright for reuse see publisher).
In interpreting these values, one should note the range and scale of the areas considered in this study: all this is for large areas over approximately 10^4.5 until 10^6.5 km^2. When you think about squares, we are looking at squares with a width of around 200 to 2000 km, so a lot of things are going on below that scale.
Storch also compare the data to SAR and EAR calculated from alternative stochastic simulations for range size and range placing, from which they conclude that the empirical pattern is consistent with
… more or less independently located contiguous ranges, with the mean range size of species being the only biologically relevant variable affecting the exact properties of the patterns.