Climate variation drives dynamics and productivity of a subalpine breeding bird community

Abstract

Effects of global climate change on local climate and dynamics of animal populations can be extreme at high latitudes and high elevation. Here, we investigated climate variation and its effects on the diversity and dynamics of a subalpine bird community in California's Sierra Nevada over 32 years (1978-2009). For the first 16 years, we also assessed the effects of climate on fledging timing, fledging success, and fledgling numbers. We considered 4 climate variables: snowmelt timing, spring mean temperature, summer minimum temperature, and summer precipitation. Climate metrics were highly variable among years and exhibited evidence of temporal pattern. Cooler years with late-melting snowpack were less common in later years, and summer temperature increased, while summer precipitation tended to decrease, over the study period. Years of early snowmelt were associated with higher species richness, more breeding territories, population increases, earlier fledging, and fewer fledglings/successful breeding territory. After accounting for snowmelt timing, spring temperature was positively associated with population change and, although variable among species, negatively associated with fledging date and productivity metrics. Summers with higher minimum temperatures were associated with higher fledging success and (although variable among species) more fledglings. Three species showed evidence of population trend: Yellow-rumped Warbler (Setophaga coronata; increasing), Clark's Nutcracker (Nucifraga columbiana; declining), and Chipping Sparrow (Spizella passerina; declining). We suggest that enhanced diversity and population growth in mild springs were driven primarily by the recruitment of young first-time breeders, which may also help explain lower productivity in those years. Spring and summer temperatures may have also affected productivity through effects on predator activity or on dynamics of moisture, vegetation, and food availability. Finally, summer precipitation was largely accumulated through summer storms, which were observed to cause occasional deaths to eggs and nestlings. Our results highlight the value of long-term single-site multispecies data in understanding population and community dynamics at an environmental extreme with high annual variability. ©