Experimental species removal: I. Comparison of dispersal strategies of Sigmodon hispidus and Reithrodontomys fulvescens. II. Analysis of Sigmodon hispidus and Reithrodontomys fulvescens interactions

Sigmodon hispidus (the hispid cotton rat) and Reithrodontomys fulvescens (the fulvous harvest mouse) are the codominant rodent species on the Texas coastal prairie. Eight isolated 1.6 ha replicated plots were used to study dispersal strategies and the interspecific interactions of these rodents. Each replicate was live-trapped monthly for nineteen months. For two replicates all captured individuals were analyzed as to species, sex, body weight, and time and site of capture, and then marked and released. These replicates contained the control (resident) populations. In two replicates all Reithrodontomys were removed, in two replicates all Sigmodon were removed, and in two others both species were removed. Monthly dispersal of individuals into plots where that species was previously removed was measured as to numbers per month, body weight, and sex. These data were tested against the population structure of control populations. The number of dispersers of both sexes of Sigmodon and Reithrodontomys were significantly correlated with monthly change in resident density. Sex ratio comparisons between dispersers and residents were found to be nonsignificant for both species. The rank order of size (age) classes of dispersers was significantly correlated to their presence in the control plots for male and female Sigmodon and for male Reithrodontomys. Significantly higher proportions of juvenile Reithrodontomys females were found to disperse. The rank-order template dispersal strategy found during this study has the advantage of introducing a stabilized age structure into newly acquired habitat space for these species. Sigmodon reflected higher densities and population biomass in the plots where Reithrodontomys was also present. Sex ratios of Sigmodon reflected a higher percentage of males in the replicates containing sympatric populations of Reithrodontomys. Male and female Sigmodon experienced different selection pressure when the species composition was disturbed. These differences for Sigmodon were significant over the duration of the study. Sigmodon and Reithrodontomys reflected asynchronous breeding patterns in the control replicates. In the replicates where Reithrodontomys was removed, Sigmodon extended its breeding period. The higher densities and biomasses of Sigmodon and Reithrodontomys in the presence of each other imply the important relationships between the two species are facilitative, rather than competitive. Reithrodontomys reflected the same changes as Sigmodon during periods of increased density. Possible mechanisms of facilitation between the rodents include: (a) buffering of severe predation pressure on either species by the presence of the other, (b) diminishment of intraspecific competition affected by a codominant, mixed-species mosaic, and (c) 'active' facilitation of one species by another species.