from three sites near our trapping sites: one where we were trapping (Red Rocks Canyon), one about 24 km from the trapping site, and one 48 km from the trapping site; available data were comparable (Figure 2-21). As shown in Figure 2-22, there was an interaction effect between precipitation and temperature. Low rodent population abundances were associated with high precipitation during cold periods and low precipitation during warm periods (Calisher et al., 2005a). Cold, wet fall/winter conditions and hot, dry spring/summer conditions were associated with negative effects on populations of most species, including deer mice. For example, the cold and wet fall of 1997 coincided with an El Niño/Southern Oscillation event with high winter precipitation and abrupt declines in relative abundance of rodents.

Of importance to rodent populations is not only the amount of precipitation that occurs, but when it occurs. An abundance of precipitation in September or October, for example, is not the same as a uniformly distributed precipitation occurring in spring. Precipitation affects vegetation and insects, both of which are dietary resources for deer mice, so that the timing of precipitation during the growing season is important. When it occurs outside the growing season it may not contribute to resource availability, but it may negatively impact deer mouse survivorship. Our data also indicate that the timing of rainfall is an important determinant of breeding success and recruitment of juveniles into the population. For example, although juvenile recruitment likely peaked in the autumn (Figure 2-23), the only period during our study when juvenile recruitment appeared to

FIGURE 2-21 Quarterly precipitation as recorded at three weather stations in or near the Pinon Canyon Maneuver Site, southeastern Colorado, 1995-2000.

SOURCE: Reprinted with permission from Calisher et al. (2005a).