Dynamics of Pocket Gopher Distribution on the Edwards Plateau of Texas

Abstract

Two species of pocket gophers are allopatric on the Edwards Plateau of central Texas but remains of both species have been found together in late Pleisto- cene and early Recent cave faunas. Geomys bursarius texensis occupies a small range in Mason and McCulloch, and perhaps Gillespie, Counties. Thomomys bottae confinalis is found in a small area in Kimble County and a broader, separated area in Edwards and Sutton counties. Thomomys b. confinalis no longer occurs at the type locality in Kerr County. No geographic variation was found in Thomomys popula- tions from the eastern Edwards Plateau, and isolation of the populations may have been relatively recent. Populations of Thomomys seem to be expanding at this time. The present small range of Geomys b. texensis may be only a remnant of its late Pleistocene range. Study of late Pleistocene small mammal assemblages from the southern Great Plains has disclosed many instances of species that lived sympatrically then but today are allopatric (Hibbard, 1970). Over most of North America, the present geographic ranges of the smooth-toothed pocket gophers (Thomomys) and the plains pocket gopher (Geomys bursarius) are separated (Hall and Kelson, 1959). The two species have been found together in late Pleistocene fossil deposits in Kansas (Schultz, 1969), Oklahoma (Stephens, 1960) and northern Texas (Dalquest, 1965). In Texas the present geographic ranges of Thomomys bottae and Geomys bursarius are separate but approach each other closely (Fig. 1) in the central part of the state. Remains from cave deposits in this same region show that the two species were sympatric from over 10,000 years ago to some 4,000 years ago, and probably later. The present allopatry is apparently a rela- tively recent development. We have attempted to cast light on the changes in distribution of these two species by mapping the distri- bution of pocket gophers of the eastern part of the Edwards Plateau, studying geographic variation in the populations involved, and attempting to find evidence of their past distribution. PRESENT DISTRIBUTION. Vernon Bailey collected a single speci- men of smooth-toothed pocket gopher 35 mi. E of Rock Springs,