Effects of interspecific interaction on habitat utilization of Sigmodon hispidus and Reithrodontomys fulvescens

This study was conducted in order to determine whether Sigmodon hispidus and Reithrodontomys fulvescens have preferred habitats determined by attributes of vegetation structure. The role of interspecific interaction was investigated by observing habitat utilization in areas where one of the codominant rodent species was removed. Vegetation in the mammal plots was quantified by a line intercept method. Raw vegetation measures were converted to relative dominance values which were then entered into a principal components program to categorize and therfore simplify the vegetation data. Those plant species which were consistently located in the same category during all seasons were used to test rodent distribution. The mammal plots were divided on the basis of above and below average abundances of the plant species selected from the principal components analysis. The distributions of Sigmodon hispidus and Reithrodontomys fulvescens in these areas were tested by chi square analyses. Both species were found to prefer areas containing above average amounts of Schizachyrium scoparium and Baccharis hamilifolia. The interspecific interactions between these codominant rodents were determined by observing the habitat utilization in areas were one of the species was removed. It was found that these rodents partition the habitat by different seasonal utilization of these plant species. In areas of species removal, the remaining species tended to expand its habitat utilization to include the plant species normally utilized by the removed species thereby exhibiting competitive release. It is postulated that these plant species are important as cover and food based on the insect faunas associated with these plant species. A facilitative relationship based on decreased densities, survivorship, and reproduction in areas of species removal suggest that neither positive or negative interactions are independently important, but that a combination of both is essential for stability in this rodent system.