Northern Geographic Limits of Four Desert Rodents of the Genera Peromyscus, Perognathus, Dipodomys, and Onychomys in the Rio Grande Valley

Abstract

Four species of desert rodents were studied in the Rio Grande Valley of New Mexico in an effort to detect the factor or factors excluding each one from areas north of their ranges. Their dis- tributions were analyzed in relation to climate, vegetation, soil texture, and biotic competition. The cactus mouse (Peromyscus eremicus) and rock pocket mouse (Perognathus intermedius) seem to be limited by temperature. Each is saxicolous and inhabits rocky outcrops as far north as certain isotherms for average annual maximal temperature, these being 71 F for P. eremicus and 69 F for P. intermedius. Occasionally the former inhabits nonrocky sites, but the lack of mesquite (Prosopis juliflora) seems to prevent this animal from occupying such areas as far north as the 71 F isotherm. Both Merriam's kangaroo rat (Dipodomys merriami) and the south- ern grasshopper mouse (Onychomys torridus) may be limited by intra- generic, interspecific competition. Each species has an affinity for a habitat characterized by compact soils and certain kinds of vegetation. Areas of such habitat become increasingly less abundant from the southern to the northern parts of the ranges of the two species. In these northern areas, Ord's kangaroo rat (Dipodomys *ordii) excludes Merriam's kangaroo rat from most choice sites, and the northern grass- hopper mouse ( Onychomys leucogaster) excludes the southern grass- hopper mouse from sites with suitable habitat.