The species area relationship (SAR) has a long tradition of study in ecology, and is important for comparisons of diversity, upscaling, and conservation. Hence, if the SAR has a universal shape, that could be of great utility. Such universality has recently been demonstrated at continental extents, but the universality of the SAR at other spatial extents remains less clear. Here we systematically survey the shape of the SAR at regional (~1000 km) extents across the world. Using range maps of all known amphibians, birds, and mammals, we empirically characterize the SAR in thousands of 1000 km circular regions distributed globally. We assess whether the SAR in these regions follows universal behavior similar to that observed at continental extents, and develop a theory based on geometric probability to understand the behavior of the SAR at regional extents.
Results/Conclusions
In contrast to the SAR at continental extents, we find that at regional extents the SAR exhibits highly non-universal behavior. Empirically, we show that this non-universality is associated with the variation in the perimeter and area of species ranges within the geographic region that the SAR is measured. Our theory, based on geometric probability, correctly accounts for much of this variability, and suggests that although the SAR at regional extents does not exhibit a universal shape, the shape is nonetheless predictable based on simple characteristics of species ranges. While universality of the SAR would facilitate applications of the SAR for diversity comparisons and conservation, our results suggest that at regional scales, these applications may be complicated by non-universality and that specific knowledge of the perimeter and area of species ranges would be necessary.