LIFE HISTORY STRATEGIES IN EXTREME ENVIRONMENTS: COMPARATIVE DEMOGRAPHY OF ARCTIC AND ALPINE PTARMIGAN

Abstract

Arctic and alpine habitats are extreme environments characterized by short breeding seasons, cold temperatures, limited food availability, and potentially high predation rates. Stringent ecological conditions are likely to have important consequences for the evolution of life history traits, but direct empirical tests are few. We compare the demog- raphy of three populations of ptarmigan on an environmental gradient spanning alpine, subalpine, and arctic habitats. Female Willow Ptarmigan (Lagopus lagopus) and White- tailed Ptarmigan (L. leucurus) breeding at subalpine and alpine sites had smaller clutches and lower probabilities of nesting success, fledging success, and renesting than Willow Ptarmigan nesting at a low-elevation arctic site. Annual fecundity, measured as female fledglings per breeding female, did not overlap among the three populations and was ranked: alpine (0.40 6 0.08, mean 6 SE, 95% CI 5 0.26-0.58) , subalpine (1.33 6 0.10, 1.13- 1.54) , arctic (2.04 6 0.18, 1.68-2.39). There was a nonsignificant trend for apparent survival rates (f) of breeding females to vary in the opposite direction: alpine (0.46 6 0.04) . subalpine (0.43 6 0.03) . arctic (0.37 6 0.06). Population growth rates predicted significant declines for the alpine population ( l5 0.65 6 0.07, 95% CI 5 0.52-0.79), but not the subalpine ( l5 1.00 6 0.07, 0.86-1.14) or arctic populations ( l5 1.13 6 0.20, 0.78-1.54). The adjusted estimates of survival necessary to sustain a stationary population indicated that actual variation in female survival was more pronounced than the observed rates: alpine (0.71) . subalpine (0.43) . arctic (0.33). Together, the fecundity and survival values provide evidence that even congeneric populations can exhibit a continuum between high reproductive and survivor life history strategies. Variation in ptarmigan life history traits was consistent with population differences in predation rates on eggs and breeding females, and it was not related to duration of the breeding season, climatic conditions, or food availability. Ptarmigan demography also covaried with body size, but not in the predicted pattern. Overall, the life history strategies of ptarmigan are consistent with our current understanding of the impacts of environmental factors upon life history variation in passerine songbirds.