Effects of Drought and Prolonged Winter on Townsend’s Ground Squirrel Demography in Shrubsteppe Habitats

Abstract

During a mark–recapture study of Townsend's ground squirrels (Spermophilus townsendii) on 20 sites in the Snake River Birds of Prey National Conservation Area, Idaho, in 1991 through 1994, 4407 animals were marked in 17639 capture events. This study of differences in population dynamics of Townsend's ground squirrels among habitats spanned a drought near the extreme of the 130‐yr record, followed by prolonged winter conditions. Townsend's ground squirrels have a short active season (≈4 mo) in which to reproduce and store fat for overwintering. Their food consists largely of succulent grasses and forbs in this dry shrubsteppe and grassland habitat. The drought in the latter half of the 1992 active season produced early drying of Sandberg's bluegrass (Poa secunda) and was associated with low adult and juvenile body masses prior to immergence into estivation/hibernation. The following prolonged winter was associated with late emergence of females in 1993. Early‐season body masses of adults were low in 1993 relative to 1992, whereas percentage of body fat in males was relatively high. These weather patterns in spring 1992 and winter 1993 also resulted in reduced adult persistence through the ≈7‐mo inactive period, especially for adult females, and near‐zero persistence of >1200 juveniles. Consequently, densities of Townsend's ground squirrels across the 20 livetrap sites declined. The demographic effects of drought and prolonged winter lasted at least through the subsequent breeding season. Adult females that survived these weather extremes produced fewer emergent young per female than did adult females prior to the event. Prior to the drought/prolonged winter, yearling female body masses were higher than, or indistinguishable from, those of adults. Females produced in 1993 had lower body masses as yearlings than did adult females. Demographic response to the drought and prolonged winter varied with habitat; ground squirrels in sagebrush habitat showed less decline in persistence and density and produced more young per female during the next active season following the drought (1993) than did ground squirrels in grassland habitat, where densities had been significantly higher prior to the drought and prolonged winter. Studies involving habitat comparisons of animal demography should always be placed in the context of long‐term weather patterns, because habitat quality rankings based on density, reproduction, and survival may differ with environmental conditions. Physiological effects of environmental “crunches” on consumers may persist beyond the period of influence on food resources, reducing reproductive success and growth rates of future offXspring.

Relationship between habitat characteristics and densities of southern Idaho ground squirrels

“…High quality, stable food sources are associated with greater burrow densities of southern Idaho ground squirrels. These findings are similar to those reported in other studies examining the relationship between vegetation and other species of ground squirrels (Yensen et al ; Van Horne et al , ; Bennett ; Steenhof et al ). Although most of the vegetation at our study sites was comprised of invasive annual species, we found greater burrow densities in plots with more native shrubsteppe vegetation (mostly perennial plants).…”

A darkling beetle population in West Texas during the 1997–1998 El Niño

“…Our study also highlights that differential survivorship associated with body size can affect sex ratios in populations with sexual size dimorphism. Climatic extremes do not always affect sex ratios of populations significantly (Smith, 1996;Brown & Brown, 1998), but, like our study, sometimes they do (Moran & Hurd, 1994;Asakura 1995;Van Horne et al 1997), reinforcing the notion that sex ratios can evolve in variable environments through natural selection independent of maternal strategies of sex allocation. Short-term survival through transient, acute stress can be critical in shaping populations for many subsequent generations.…”

Socially mediated effects of climate change decrease survival of hibernating Alpine marmots

“…Results on dominant adult marmots were in agreement with previous studies on other mammalian herbivores (Van Horne et al . ; Garel et al . ).…”