Rainfall and fluctuating plant populations in relation to distributions and numbers of desert rodents in southern Nevada.

Abstract

Fluctuations in rainfall and size of desert rodent and plant populations for each of five consecutive years were documented on 68 sites in Mojave, Great Basin, and transition desert communities of central-southern Nevada. Post-reproduction rodent densities in the summer usually followed the seasonal patterns of winter annual success the previous spring, in turn directly related to the success of germination the preceding autumn.In the "Dipodomys merriami environment", at the lower elevations with relatively low rainfall and high temperatures of the Mojave Desert, D. merriami and D. deserti numbers (indicating success or failure of reproduction) closely followed the seasonal and site patterns of winter annual success. In the "Dipodomys microps environment", at the middle and higher elevations with the relatively high rainfall and lower temperatures of the transition and Great Basin deserts, D. microps numbers followed winter annual success patterns in years of high or low rainfall in the months preceding reproduction, but there was reproduction in years of intermediate rainfall whether or not there were winter annuals in the environment. Reproductive patterns of D. merriami in the "D. microps environment" followed those of D. microps. Perognathus longimembris, the ubiquitous heteromyid of the region, also exhibited both reproductive patterns, depending on whether it was in the "D. merriami" or the "D. microps environment". The Mojave Desert P. formosus, edaphically restricted in the region, followed the patterns of the "D. merriami environment" and its numbers were highly correlated with winter annual success.Herbaceous perennials, capable of large germinations of plants behaving as biennials or winter annuals, are shown to have large effects on D. merriami numbers. Size of populations of Eurasian annual species of Bromus and Salsola were not correlatable with rodent reproduction patterns.From all patterns in all environments, it is inferred that environmental conditions affecting water balance in the rodents play the key role in initiation of reproduction. Environmental moisture levels during the winter season are usually under the control of rainfall of late autumn-early winter, coincidentally the period of greatest predictability of heavy rains. The herbaceous plants, present in the winter months most years, are available sources of water (and vitamins) at the time of initiation of reproductive activity. For D. microps, identified with the mesic environments of the region, the herbaceous plants may or may not be necessary sources of water for reproduction, depending on the seasonal rainfall regime. For D. merriami, herbaceous plants appear to serve as a water reservoir required for reproduction in its usual environments. The same relationships evidently obtain for the other heteromyids, and the cricetids as a group.