The geographic ranges of mammalian species in South America: spatial patterns in environmental resistance and anisotropy

We analyse the geographical distribution of South American mammalian specie s using two biogeographic indices: environmental resistance (R-50) and anis otropy (A(50)). R-50 quantifies the loss of biotic resemblance occurring fr om any point in the map to the rest of the continent. A(50) quantifies the extent to which the perimeter: area ratio of the geographical ranges of all species whose distributions overlap at any particular location depart from the perimeter: area ratio of a circle. We test for the latitudinal Rapopor t effect that predicts an increase in the range-sizes of mammalian species, and hence a decrease in the values of R-50, towards the south. We test for the effect of mountains on species' ranges, given that Janzen's argument t hat 'mountain passes are higher in the tropics' implicitly predicts greater anisotropy in the tropics. Continental spatial patterns of variation in R- 50 and A(50) suggest a biogeographic division of South America consistent w ith most classical zoogeographical classifications proposed for the contine nt. Rapid change in mammalian range-sizes and shapes occurs at the limit be tween the Guayano-Brazilian and Andean-Patagonean subregions. R-50 data do not support the latitudinal Rapoport effect: the most widespread species lo cate in the eastern portions of Brazil, the most restricted ones are in ass ociation with the Andes. A(50) data support Janzen's prediction: the effect of mountains on species' distributions is greater in equatorial and centra l regions of the Andes rather than in the south. R-50- and A(50)-contour ma ps reveal that, mainly due to the effects of the lie of the land and likely differences in the history of the fauna, the continent has a biogeographic texture which must have major constraints on local ecological patterns and processes. This stresses the importance of considering the role of biogeog raphic structure in the analyses of geographical gradients in species' dist ributions.