Experimental Species Removal: Demographic Responses by Sigmodon hispidus and Reithrodontomys fulvescens

Abstract

Experimental species removal was conducted for three years to assess the demographic consequences of interspecific interactions between cotton rats ( Sigmodon hispidus ) and fulvous harvest mice ( Reithrodontomys fulvescens ). Six 1.6-hectare (ha) plots were monitored monthly by live-trapping; two control, two Sigmodon -only, and two Reithrodontomys -only plots were maintained by appropriate species removal. Frequent trapping and habitat discontinuities between plots minimized the effect of interplot movement and maintained the experimental perturbation. Seasonal vegetation analyses were conducted on each field. Density, percent reproduction, and expectation of further life but not survival were significantly lower for both Sigmodon and Reithrodontomys on experimental plots (codominant species removed). Lower density for both species and lower reproduction for Reithrodontomys were significantly correlated to lower abundance of preferred habitat on these plots. Lower expectation of further life reflected increased mobility ( AvD p ) of Sigmodon on experimental plots, thereby contributing to lower densities and decreased reproduction through lower mate contact. Cotton rat mean individual body weight was significantly lower and male harvest mouse mean body weight significantly higher on experimental plots; lower cotton rat body weight reflected immigration onto experimental plots in response to a density gradient. Sigmodon sex ratio was skewed toward males on control plots, whereas Reithrodontomys sex ratio was skewed toward males on both plots; differential movement or trappability by either sex may be responsible for this result. The significantly lower proportion of adults and young adults on Sigmodon experimental plots was due to emigration; there was no between-plot age class difference for Reithrodontomys . Most demographic differences between control and experimental plots are explained by differential movement, survival, or abundance of preferred habitat. However, increased mean body weight of male Reithrodontomys and earlier appearance of Reithrodontomys juveniles on experimental plots remain unexplained. These results suggest interspecific interactions and/or behavioral interactions may be of secondary importance in determining the demographic structure of rodents in this system.