Size and shape of the geographical ranges of Andean passerine birds: spatial patterns in environmental resistance and anisotropy.

Aim The geographical distribution of 839 Andean passerine species was analysed using two biogeographical indices: environmental resistance (R₅₀) and anisotropy (A₅₀). R₅₀ quantifies the loss of biotic resemblance occurring from any point in the map to the rest of the continent. A₅₀ 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. Three predictions derived from the climatic-tolerance hypothesis were tested: (1) the latitudinal Rapoport effect predicts an increase in the range-sizes of species, and hence a decrease in the values of R₅₀, towards the south; (2) Janzen’s argument that ‘mountain passes are higher in the tropics’ predicts greater anisotropy in the tropics; (3) because environmental conditions are relatively constant across the tropics, and begin to change at the boundaries, this predicts high spatial turnover – and hence rapid change of R₅₀ values – at the limits of the tropics. Location The Andes, in South America. Methods The geographical ranges of 839 passerine species were drawn on a standardized grid map of the Andean region. The presence–absence (1–0) of each species in each of the 150 cells in the grid map was recorded. These data were used to create equiprobabilistic maps, which then were used to estimate a value of R₅₀ and A₅₀ for each cell in the grid map (sensu Rapoport, 1975, 1979, 1982). Results The spatial patterns in R₅₀ and A₅₀ offer partial support to the original predictions. The tendency for R₅₀ data to decrease with latitude (Rapoport effect) vanishes after controlling for differences in species richness among latitudes compared. In contrast, A₅₀ data clearly offer indirect support to Janzen’s... [ABSTRACT FROM AUTHOR]