The effect of experimental manipulation of sympatric populations of three rodent species during a declining population phase

Three abundant rodents, the hispid cotton rat (Sigmodon hispidus), the fulvous harvest mouse (Reithrodontomys fulvescens), and the eastern rice rat (Oryzomys palustris) are sympatric in southest Texas. From January 1970 to May 1970 six presumably homogeneous areas, three experimental and three control, were live-trapped. Mark-release-recapture and selective removal techniques were used to monitor species compositions within the areas. Estimates of the number of individuals per species, body weights, and sex ratios were obtained at two week intervals throughout the study. Linear regression analysis was used to describe the detectable weight changes under various conditions. When S. hispidus was experimentally removed no change in the mean weights of R. fulvescens and O. palustris was evident. When R. fulvescens and O. palustris were removed S. hispidus females exhibited a significant increase in mean weight. Selective removal of a species appears to have fixed effect on the total number of animals per unit area over a short period of time. A significant change in sex ratio of Sigmodon, from a greater proportion of males to females, was recorded in the areas where Reithrodontomys and Oryzomys were removed. Although the number of captures per species declined, comparisons of the different experimental manipulations could be made throughout the study.