EFFECTS OF REDUCED INTERSPECIFIC INTERACTIONS ON POPULATION DYNAMICS IN MERRIAM'S KANGAROO RAT, DIPODOMYS MERRIAMI.

Abstract

Nocturnal rodents were censused every two weeks from January 1975 until September 1976 on two 1.69 ha (4.13 ac) live-trap grids. Grids were located about 48 kilometers (30 miles) south of Tucson AZ in a cresosote bush-cactus-mesquite-grassland ecotone. All nocturnal rodents, except Dipodomys merriami, were removed from one of the grids beginning in May 1975 and continuing biweekly until September 1976. Effects on the population biology of D. merriami were subsequently analyzed. Density, home range, weight changes, production of young, sex ratio and minimum residence (time between first and last capture) of D. merriami were analyzed on each grid. Effects of removal on D. merriami were minimal. Density on the removal grid increased immediately after removal began; however, this effect decreased with time, as numbers of d. merriami decreased on both grids. Total heteromyid density on the control grid also decreased during the experiment. Removal caused no significant effect on home range. A similar, consistently inverse relationship between home range and density occurred on both grids. Mean weight for both reproductively active and inactive males and females was not significantly different following removal. The number of juveniles increased slightly after removal began, but production of young on both grids was similar, and low. The two populations exhibited different sex ratios for four months after removal began, with males being caught more frequently on the removal grid. Minimum residence times were similar on both grids. It appears that the relatively increased availability of reources had only a temporary effect on D. merriami. In the longer perspective, D. merriami seems to have exhibited what is probably an evolved response to increased resources; i.e., long-term changes in population parameters occur slowly, and only when improved conditions persist for relatively long periods of time.