Life-history and demographic variation in an alpine specialist at the latitudinal extremes of the range

Alpine environments are unique systems to examine variation in life-history strategies because temperature and seasonality are similar across broad latitudinal gradients. We studied the life-history strategies, demography and population growth of white-tailed ptarmigan Lagopus leucura, an alpine specialist, at the latitudinal extremes of the range in the Yukon (YK, studied from 2004 to 2008) and Colorado (CO, 1987-1996). The two populations were separated by 2,400 km of latitude, and the Yukon site was approximately 2,000 m lower in elevation than the Colorado site. Yukon females bred on average 9 days earlier than those in Colorado, but the latter study was conducted 15 years earlier and breeding dates may have advanced over this period. The length of the breeding season was similar between the two populations, and females had comparable probabilities of re-nesting after failure. The two populations differed in how they allocated effort to the first clutch as Yukon females laid larger clutches (7.1 vs. 5.9 eggs) but smaller eggs (18.8 vs. 20.5 g) than those in Colorado. Demographic rates also differed; nest survival was higher in the Yukon (0.40) than in Colorado (0.24), and the resultant annual fecundity was nearly twice as high in the Yukon (3.92 vs. 1.77 chicks/female). In contrast, annual adult survival was higher in Colorado although the confidence intervals overlapped (females: YK = 0.35, CO = 0.44; males: YK = 0.48, CO = 0.59). Estimates of annual population growth (lambda) indicated both populations were declining, especially in Colorado (lambda (YK) = 0.83, lambda (CO) = 0.66), and thus, dispersal movements are likely key to long-term persistence in both cases. Our findings suggest that breeding-season temperature and seasonality affect measures related to timing of reproduction, but not the costs and benefits of clutch and egg size.