The Texas cotton rat, Sigmodon hispidus, was the most commonly trapped associate of the voles between November, 1950, and February, 1952. Although a greater number of individuals of the harvest mouse were taken in a few months, the cotton rat had a greater ecological importance because of its larger size ([Figs 17], [18], [19]). The cotton rat was an especially noteworthy member of the community for two reasons. It has arrived in northern Kansas only recently and its progressive range extension northward and westward has attracted the attention of many mammalogists (Bailey, 1902:107; Cockrum, 1948; 1952:183-187; Rinker, 1942b). Secondly, Sigmodon has long been considered to be almost the ecological equivalent of Microtus and to replace the vole in the southern United States (Calhoun, 1945:251; Svihla, 1929:353). Since the two species are now found together over large parts of Kansas their relationships in the state need careful study.
Fig. 17. Variations in density and mass of three common rodents on House Field. The upper graph shows the sum of the biomass of the three rodents. In the two lower graphs the solid line represents Microtus, the broken line Sigmodon, and the dotted line Reithrodontomys.
Fig. 18. Variations in density and biomass of three common rodents on Quarry Field. For key, see legend of [Fig. 17].
Fig. 19. Changing biomass ratios of three common rodents on House Field and Quarry Field. In late 1951 and early 1952 the cotton rats attained relatively high levels and seemingly caused compensatory decreases in the numbers of voles. The solid line represents Microtus, the broken line Sigmodon, and the dotted line Reithrodontomys.