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

摘要

Aim The geographical distribution of 839 Andean passerine species was analysed using two biogeographical indices: environmental resistance (R-50) and anisotropy (A(50)). R-50 quantifies the loss of biotic resemblance occurring from 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. 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 R-50, 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 R-50 values - at the limits of the tropics.