The Impact of Climatic Extremes on Cotton Rat (Sigmodon hispidus) Populations

Abstract

The impact of weather extremes was examined for cotton rat (Sigmodon hispidu-s) populations occupying an Andropogon-dominated old field and a fescue (Festuca spp.) pasture on the Piedmont of Georgia. Cotton rat abundance was greatly reduced in spring 1977 relative to the springs of 1975 and 1976. Record winter cold followed by prolonged drought in spring and summer 1977 was associated with reduced Sigmodon reproduction and an absence of the normal autumn population buildups. The old-field Sigmodon population remained depressed through March 1979, whereas the pasture population became locally extinct after September 1977. Weather-induced habitat modification also occurred in conjunction with the Sigmodon collapses. The old field experienced some Andropogon mortality and a retrogressive shift to an earlier successional stage as Solidago, Lespedeza and several other species became established in 1977. Vegetation cover in the old field became patchy as these plant species colonized the area. In contrast, plant species composition remained unchanged in the pasture despite the weather conditions. The reduction in cotton rat abundance in the pasture was associated with a sharp decline in vegetation biomass and cover through 1978. INTRODUCTION The direct effects of local climatic conditions on Sigmodon populations have been documented (Dunaway and Kaye, 1961; Fleharty et al., 1972; Glass and Slade, 1980; Sauer, 1985). In particular, the effects of cold weather on reproduction (Goertz, 1964) and nest-building (Dawson and Lang, 1973) have been examined. Exposure to long periods of subfreezing temperatures can kill cotton rats (Goertz, 1964; Dunaway and Kaye, 1961) or weaken their resistance to further climatic stresses (Sauer, 1985). Severe local climatic conditions may also reduce the available food and cover and thus exert indirect effects on Sigmodon populations. The possible impact of these indirect effects have remained largely unstudied. The Piedmont region of Georgia experienced one of the most severe winters on record during 1976-1977 (U.S. Dep. Commerce, 1973-1982), which included prolonged periods of subfreezing temperatures. The severe winter was followed by a particularly dry spring and below average rainfall for 11 months (U.S. Dep. Commerce, 19731982). The direct effects of these conditions on wild animal populations were probably severe (Robbins et al., 1986). However, in some areas the indirect effects resulting from habitat modification appeared to be even more critical. The extreme weather conditions seemed to interrupt natural successional processes and alter vegetation structure. The purpose of the present study was to determine the changes which occurred in two Sigmodon populations during this period of extreme climatic conditions. We were also interested in assessing the nature and extent of habitat modification that resulted and the possible effect of any changes on Sigmodon populations. MATERIALS AND METHODS The study was conducted on Georgia Power Company's Plant Bowen near Cartersville, Georgia. This 800-ha reservation occurs at the transition from Piedmont to Ridge 'Present address: Department of Natural Resources, Environmental Protection Division, 205 Butler Street, S.E., Suite 1154, Atlanta, Ga. 30334.