Microhabitat Utilization and the Effect of Species Removal on a Population of Peromyscus maniculatus and Reithrodontomys megalotis

Abstract

The extent of competition between deer mice, Peromyscus maniculatus, and western harvest mice, Reithrodontomys megalotis, was assessed by a species removal experiment conducted on a relict grassland in Ellis County, Kansas. Vegetation was sampled to determine microhabitats. There was no indication of ecological release for either species when mouse densities were manipulated. Vegetational analysis revealed significant differences in microhabitats utilized by the two species. The cricetine rodent component of the Mixed Prairie in Ellis County, Kansas, consists primarily of two species, deer mice (Peromyscus maniculatus) and western harvest mice (Reithrodontomys megalotis). Both are found in all communities of the mixed grass prairie. P. maniculatus is most abundant in big bluestem-little bluestem communities (Fleharty, 1972; Hansen and Fleharty, 1974; Martin, 1960) whereas R. megalotis is captured most often in lush, weedy habitats and avoids open little bluestem communities (Fleharty, 1972; Martin, 1960). Diets of both species consist of seeds and insects (Brown, 1946). Research conducted in a remnant prairie in Kansas in the late 1960's (Fleharty, 1972) revealed asynchronous fluctuations in population densities of deer mice and western harvest mice. Densities of Peromyscus were greatest in autumn and winter and least in spring and summer, whereas densities of Reithrodontomys were greatest in winter and spring and least in autumn. These asynchronous fluctuations and the fact that both species rely heavily on seeds as a food source suggested the existence ofinterspecific competition. Interspecific competition also has been suggested between the related species Peromyscus gossipinus and Reithrodontomys humilis (Packard, 1968). The purpose of this study was to determine if competition influences fluctuations in density and utilization of microhabitats in areas where deer mice and western harvest mice coexist. This content downloaded from 157.55.39.180 on Mon, 25 Apr 2016 07:05:58 UTC All use subject to http://about.jstor.org/terms VOLUME 91, NUMBERS 3-4 133 MATERIALS AND METHODS The study was conducted on a 14.2-ha remnant grassland located in the southeastern quarter of section 1, T1 4S, R19W, Ellis County, Kansas. This area was described by Martin (1960) and Brock (1968). Six 75 x 75-m plots were established in February 1985. The plots were isolated from surrounding habitats and one another by 15-m mowed strips (Cameron, 1977) except that four plots (3 through 6) were not mowed on one side because adjacent pastures were heavily grazed. Each plot contained 36 trap stations arranged in grid fashion, spaced at 15-m intervals. Trapping was initiated in March 1985 and was conducted monthly through February 1986. A Sherman live trap (7.6 x 8.9 x 22.9 cm) baited with a mixture of rolled oats, raisins, and peanut butter was placed at each station. Traps were set for five consecutive nights during the new moon phase in each census period. Sex was determined and toes were clipped for each captured animal to facilitate future identification. Plots were divided into three treatments with two replications. Deer mice, P. maniculatus, were removed from two plots (2 and 4), harvest mice, R. megalotis, were removed from two plots (3 and 6), and two plots were used as controls (1 and 5). The decision as to which plot would receive which treatment was based on preliminary trapping. The number of rodents occurring on each grid was estimated for each month using the minimum number known to be alive method (Krebs, 1966). Microhabitats were characterized by sampling three vegetational variables: plant height at each trap station, mean litter depth at each trap station, and species composition of plants around each trap. Plant height was sampled in June and September with a forage disk (Sharrow, 1984). Four litter-depth samples were measured at right angles 20 cm from each trap and averaged to give mean litter depth. Species composition of plants was obtained using the modified step-point procedure (Owensby, 1973). Eight points, 45 cm from the trap, were sampled in a circular fashion at each station to determine plant frequencies. Trap sites were classified into associations based on plant frequencies using principal component analysis (Ray, 1982) as described by Jeffers (1978) and Greig-Smith (1983). Only plants with frequencies of 20 percent or more were included in principal component analysis. Product moment correlation coefficients (Sokal and Rohlf, 1973) were used to determine to what extent P. maniculatus and R. megalotis were associated with plant species, litter depth, and plant height. Seasonal correlation between mice species was also calculated.