In the course of live-trapping operations several species of small mammals other than Microtus ochrogaster were taken in the traps. Also, from time to time, direct observations of certain mammals were made and various types of sign of larger mammals were noted. These records gave a picture of the mammalian community of which the voles were a part. The three associated species which were most commonly trapped were Sigmodon hispidus, Reithrodontomys megalotis and Peromyscus leucopus. These three species have been commonly found associated with Microtus in this part of the country (Fisher, 1945:435; Jameson, 1947:137). 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. Both this study and the literature (Black, 1937:197; Calhoun, loc. cit.; Meyer and Meyer, 1944:108; Phillips, 1936:678; Rinker, 1942a:377; Strecker, 1929:216-218; Svihla, 1929:352-353) showed that, in general, the habitat needs of Microtus and Sigmodon were similar. Studies on the Natural History Reservation, both in connection with my problem and otherwise, suggested, however, that Sigmodon occurred in only the more productive habitat types used by voles, where the vegetation was relatively high and rank. On the Reservation the cotton rat was found mostly in the lower meadows; they were more moist and had a more luxuriant vegetation than the higher fields. Although a few cotton rats were taken in Quarry Field and still fewer in Reithro Field, the population of those hilltop areas did not approach, at any time, the levels reached on House Field, which produced a more luxuriant cover. Only when the levels of population were exceptionally high did the cotton rats spread into less productive habitats. At all times, there were areas on the Reservation used by Microtus which could not support a population of Sigmodon. The cotton rats reacted differently to the floods of July, 1951, than did the voles. Although the population of the cotton rat decreased slightly immediately after the wet period, this decrease was Several instances of cotton rats eating voles, caught in the same live-trap, were noted. There is reason to believe that young voles, unable to leave the nest, are subject to predation by cotton rats. This would accentuate any competitive advantage gained otherwise by the cotton rats. The population of Sigmodon retained its high level, relative to Microtus, until February, 1952. In March only one individual was captured and after that none was trapped until August, 1952, when a single subadult male was captured. Early in March, 1952, before the These limited data seem to indicate competition between Sigmodon and Microtus in Kansas. Extremely wet conditions seem to give Sigmodon a competitive advantage whereas Microtus is better able to survive dry summers and severe winters. However, these relationships need further clarification by an intensive study of the life history of Sigmodon in Kansas (especially the more arid western part), including its coactions with the communities it has invaded successfully recently. The harvest mouse (Reithrodontomys megalotis) also was a common inhabitant of the study plots, but this small rodent seemed not to be a serious competitor of the voles, as its food consists almost entirely of seeds (Cockrum, op. cit.:165) not usually used by voles. In this study, at least, no conflict over space was apparent. Harvest mice frequently were taken in the runways of voles and even in the same trap with voles. Reithro Field, the part of the Reservation having the heaviest population of the harvest mouse, differed from the habitats that were better for voles in being higher, drier and less densely covered with vegetation. However, during the summer of 1951 when the voles were most abundant, Reithro Field supported a large population of voles. Estimates of population of the harvest mouse were of doubtful validity in summer because it was readily trapped only in winter and early spring. Many individuals marked in late spring were not trapped again until late autumn although presumably they remained on the area. This seasonal variation in trapping success seemed to be a matter of acceptance and refusal of bait (Fitch, 1954:45). The presence of the wood mouse (Peromyscus leucopus) on the study plots indicated an overlapping of habitats. Both House and Quarry Fields were on the ecotone between forest and meadow and a mixture of mammals from both types of habitat occurred. No sign of the homes of the wood mouse was found on the study plots, and on the larger trap line, operated by Fitch, wood mice were captured only near the edge of the woods. Only six deer mice (Peromyscus maniculatus) were taken on the study plots. This small number probably provided an inaccurate index of the association of the deer mouse and the prairie vole, because samples from snap-traps and the data of other workers on the Reservation showed a more common occurrence of the two species together. The deer mice seemed to prefer a sparser vegetation and did not approach so closely to the forest edge as did the voles. It may have been, in part, the presence of P. leucopus in the ecotonal region which made it unsuitable for P. maniculatus. Other mammals noted on the study areas were the following: Didelphis marsupialis, Blarina brevicauda, Scalopus aquaticus, Canis familiaris, Canis latrans, Procyon lotor, Felis domesticus, Sylvilagus floridanus, Microtus pinetorum, Mus musculus and Zapus hudsonius. |