Montane Mammalian Biogeography in New Mexico

The validity of vicariance in explaining disjunctions in distributions of montane species is substantiated by numerical analyses of the montane mammalian fauna of New Mexico. Regression analyses identify latitude, elevation of the highest peak, and forest area as significant, positive correlates of montane species number. Distance clustering techniques indicate the close correspondence between these variables plus longitude and the specific compositions of montane faunas. Because both species number and species composition are well-described using these data, a vicariant explanation of Pleistocene biogeography in the Southern Rocky Mountains of New Mexico is supported. The geography of montane biotas in the Southwest is complex. Mountain ranges are delimited sharply by associations of grassland and desert at lower elevations such that the distributions of montane species are disjunct. A vicariant explanation, whereby disjunctions are produced by the appearance of barriers fragmenting ancestral distributions, has been proposed by Brown (1971) and Metcalf (1977) to describe the current distributions of montane mammals and mollusks, respectively. This model of disjunctions contrasts with dispersal explanations, which postulate the crossing of pre-existing barriers. Under the vicariant explanation, montane faunas dispersed as assemblages among mountain ranges during glacial periods when climatic conditions favored displacements of montane vegetation to lower elevations. With the retreat of glaciers, climates became effectively more xeric with the result that montane faunas once again were isolated from one another. If vicariance is applicable in this case, well-defined distributional patterns should exist which are independent of distance from faunal sources and barriers to presentday dispersal. The purpose of this paper is to investigate the validity of vicariance with regard to montane biogeography in the Southwest using numerical techniques on distributional data, a methodology discussed by Platnick and Nelson (1978). The analyses employed