Response of the Fulvous Harvest Mouse (Reithrodontomys fulvescens) to Artificial Habitat Patchiness

Abstract

Responses of fulvous harvest mice (Reithrodontomys fulvescens) to habitat patchiness were investigated by experimentally creating 0.2-ha patches with or without a shrub layer. Results based on 555 captures of 230 individual harvest mice indicated that significantly more were captured in patches with shrubs. There were no differences in survival, movement or body mass between the patch types, but there was an excess of reproductive males in patches with shrubs removed on one of the experimental grids. Harvest mice may have avoided the patches with shrubs removed because of a lack of vertical structure which facilitated avoidance of cotton rats inhabiting the same patches, or because of a decrease in insect prey. INTRODUCTION The fulvous harvest mouse (Reithrodontomys fulvescens) is a codominant rodent on the Texas coastal prairie with the hispid cotton rat (Sigmodon hispidus) Uoule and Cameron, 1975). Relatively few population studies have been conducted on this species since Packard's (1968) initial ecological study in E Texas. Several recent studies on the Texas coastal prairie used species removal techniques to ascertain the role of interspecific interactions on resource utilization by R. fulvescens and S. hispidus Uoule and Jameson, 1972; Cameron, 1977a; Joule and Cameron, 1980; Cameron and Kincaid, 1982; Kincaid and Cameron, 1982a). Other aspects of the biology of the fulvous harvest mouse were reviewed by Spencer and Cameron (1982). Reithrodontomys fulvescens was more of a habitat generalist when contrasted with the narrow requirements of the other common coastal prairie rodents, Sigmodon hispidus and the rice rat (Oryzomys palustris) (Kincaid et al., 1983). Reithrodontomys habitat, unlike that of Sigmodon or Oryzomys, was characterized by a positive association with Baccharis halimifolia (sea myrtle), a common shrub on the coastal prairie. Little bluestem grass (Schizachyrium scoparium) was found in association with Baccharis in areas heavily used by R. fulvescens. A related study found that Reithrodontomys were trapped more frequently in Baccharis (by traps set in bushes) in the presence of cotton rats than in areas where cotton rats had been removed; in the latter areas, harvest mice used the ground more frequently (Cameron and Kincaid, 1982). These findings suggested that shrubs are an important component of the habitat of the fulvous harvest mouse. The object of this study was to determine the response of fulvous harvest mice to a patchy habitat created by removing shrubs. In particular, we tested the following hypothesis: that Reithrodontomys density, survival, movement and reproduction differed in areas where shrubs were removed because of decreased cover. In addition, we present new information on the demography of Reithrodontomysfulvescens. METHODS This study was conducted from February 1979 through October 1980 at the University of Houston Coastal Center, 56 km SE of Houston, Texas. To study population dynamics of harvest mice in relation to habitat patchiness, demographic monitoring was conducted on two 1.6-ha fields (Grids 1 and 2). These fields were separated by more than 100 m of low-lying area and were bounded on the N by a mowed fence line and on the S by a water-filled drainage ditch. They were dominated by Schizachyrium scoparium (little bluestem grass), Tripsacum dactyloides (eastern gamagrass) and Spartina spartinae