Biogeographic Relationship of Pocket Gophers (Rodentia: Geomyidae) in Southeastern Colorado

Biogeographic relationships of three species of pocket gophers (Pappogeomys castanops, Geomys bursarius, and Thomomys bottae), were studied at the margins of their ranges in southeastern Colorado. Two species (P. castanops and T. bottae) were found on top of Mesa de Maya, and the extent of isolation of those gophers was assessed by morphometric analysis. Those analyses revealed that the mesa represents at least a partial barrier to gene flow for T. bottae but not for P. castanops. The ranges of T. bottae and P. castanops overlapped on the mesa by 4.8 cm but did not contact one another below the mesa. These species were found to occupy similar soils and vegetation on the mesa, but the average depths of their feeding burrows were significantly different. Pappogeomys castanops might be competitively excluding T. bottae on the mesa, or the location of their zone of contact might be influenced by precipitation. The relationship between T. castanops and G. bursarius also was studied. These species overlapped by nearly 65 km but occupied different soils and vegetation. Their average burrow depths did not differ significantly. Geographic relationships of these species might be the result of ecological specialization and the Dust Bowl of the 1930s rather than competitive exclusion.