Ecology of Rodents in Indian Cove (Mojave Desert), Joshua Tree National Monument, California

Rodent populations were studied during a 29-month period of crisis and recovery of food resources. Four species were abundant or common. Some of the elements of their biological success were observed: (1) Ecological stratification, e.g., Dipodomys merriami and Perognathus longimembris are both nocturnal herbivores, but the latter species is seasonally dormant; Onychomys torridus is a nocturnal carnivore, and Ammospermophilus leucurus is a diurnal herbivore. (2) P. longimembris was not captured during most of the fall and winter, apparently being dormant during this time. Dormancy of this small pocket mouse (8.5 g) probably accounts for its greater persistence; 30% of marked individuals survived from one spring to the next, as compared with 12–19% for D. merriami (37.5 g) which is active all year. (3) D. merriami is able to maintain a population density exceeding those of other nocturnal rodents, except P. longimembris, when this latter species is active. Densities of D. merriami ranged from 0.45 to 3.72 per hectare. Reproduction of D. merriami is correlated with winter rains and subsequent development of vegetation. Body weight increases during the winter months to a peak in early spring; reproductive activity follows the same pattern. Male kangaroo rats come into reproductive condition earlier than females, which may insure impregnation of early active females. Population densities of D. merriami vary with production of vegetation, with a considerable lag. The maximum density was observed about one year after the start of a drought period; low densities persisted for over one year after the vegetation had recovered. Home ranges of D. merriami tended to vary inversely with population density. Frequencies of capture were greatest when there was the most environmental stress. Activity tended to be greatest the first part of trapping nights. P. longimembris had greater home range and frequency of capture than D. merriami.