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In southern California the Dulzura kangaroo rat (Dipodomys simulans) occurs in shrub-dominated habitats adjacent to forb- or grass-dominated habitats, which support the endangered Stephens's kangaroo rat (D. stephensi). We monitored a number of populations of both species from 1996 through 2000, spanning the 1997- 1998 El Nino Southern Oscillation (ENSO) event. Although populations of D. stephensi were not significantly impacted by this climatic event, D. simulans declined from 10-24 individuals/ha to about 7-12 individuals/ha over a 6- to 8-month period; numbers subsequently continued to decline to 1-2 individuals/ha and failed to recover in the approximately 2.5 years of post-ENSO monitoring. We noticed no signs of sickness or illness among these animals, although adult body masses of D. simulans (but not D. stephensi) declined through the ENSO event, suggesting that food resources for the former species may have been compromised. Alternatively, populations of D. simulans may have been unusually high before the ENSO event, such that the onset of rains merely hastened a predictable winter die-off. Finally, other rodents could have robbed seeds from the food caches of Dulzura kangaroo rats, influencing the ability of the latter to survive the ENSO event. Although this explanation remains speculative, cache pilferage has been demonstrated among other heteromyids, and a recent theoretical study suggests this may be an important mechanism of species coexistence. Further investigations of cache pilferage could enhance our understanding of the ecology and evolution of these diverse small mammal assemblages.

Stephens' kangaroo rat (Dipodomys stephensi) is considered to be threatened by the state of California (California Department of Fish and Game 1972, 2000) and endangered by the federal government (Kramer 1988). The proximate factor leading to this status was urban expansion in western Riverside County, California. The more distal factor is that this species has an anomalously small geographic range (ca. 28,000 ha) for its body size (Bleich 1977). Currently, this species is managed on a small number of protected reserves in western Riverside County, and there is much interest in understanding how habitat manipulation might influence population parameters. Dipodomys stephensi occurs in sparsely vegetated annual grasslands and adjacent shrubby habitats (Bleich 1977, O'Farrell 1990, Price et al. 1991), although it appears to generally favor forb-dominated grassland habitat; at least 2 studies (Price et al. 1991, D. A. Kelt and E. S. Konno unpublished data) reported that this species was never observed in adjacent coastal sage-scrub habitat. Reduction of shrub cover has been documented

Reanalysis of a recently published data set on the responses of 2 species of kangaroo rats to the 1997-1998 El Nino Southern Oscillation (ENSO) event revealed an error during data manipulation, and underscored the perils associated with electronic data storage, manipulation, and analyses. Renewed analyses revealed that Dipodomys simulans was not catastrophically impacted by the ENSO event. Both species were negatively impacted by 2 separate rainy periods, and both species appeared to recover rapidly. The rate of demographic recovery may be a function of habitat preferences. Dipodomys stephensi occurs in valley bottoms and recovered numerically only after cessation of rains, whereas D. simulans, which occurs on better-drained habitat in adjacent hill slopes, appeared to initiate demographic recovery half way through the rainy period associated with the 1998 ENSO. In contrast, however, D. simulans was not observed to recover for about 5 months after the rainy period in 2000.