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

RUGGIERO, ADRIANA

JOURNAL OF BIOGEOGRAPHY

Blackwell Publishing

Lugar: Oxford; Año: 2001 vol. 28 p. 1281 - 1294

0305-0270

AbstractAim The geographical distribution of 839 Andean passerine species was analysedusing two biogeographical indices: environmental resistance (R50) and anisotropy (A50).R50 quanti®es the loss of biotic resemblance occurring from any point in the map tothe rest of the continent. A50 quanti®es the extent to which the perimeter : area ratioof the geographical ranges of all species whose distributions overlap at any particularlocation depart from the perimeter : area ratio of a circle. Three predictions derivedfrom the climatic-tolerance hypothesis were tested: (1) the latitudinal Rapoport effectpredicts an increase in the range-sizes of species, and hence a decrease in the values ofR50, towards the south; (2) Janzen's argument that `mountain passes are higher in thetropics' predicts greater anisotropy in the tropics; (3) because environmental conditionsare relatively constant across the tropics, and begin to change at the boundaries, thispredicts high spatial turnover ± and hence rapid change of R50 values ± at the limits ofthe tropics.Location The Andes, in South America.Methods The geographical ranges of 839 passerine species were drawn on astandardized grid map of the Andean region. The presence±absence (1±0) of eachspecies in each of the 150 cells in the grid map was recorded. These data were used tocreate equiprobabilistic maps, which then were used to estimate a value of R50 and A50for each cell in the grid map (sensu Rapoport, 1975, 1979, 1982).Results The spatial patterns in R50 and A50 offer partial support to the originalpredictions. 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 prediction of narrowerclimatic tolerances of species around the tropics, even after controlling for differences inspecies richness. The Andean passerine species con®rm that spatial turnover of speciesexists at the southern edge of the tropics, and also at different latitudes within thetropics.Main conclusions The analysis suggests that it is the major structural change in thetopography of mountains ± and not merely latitude ± that ultimately governs the spatialvariation in size and shape of species ranges along the Andes.