ABSTRACT
Environmental factors drive the persistence of natural populations by causing complex, covarying responses in demographic processes (i.e., survival, growth, and reproduction). As most natural populations inhabit seasonal environments, overlooking seasonal differences in this covariation may obscure the mechanisms that buffer or amplify population responses to environmental change. We investigated how environment-driven covariation of seasonal demographic processes affects population dynamics using 40 years of individual-based data from a population of yellow-bellied marmots (Marmota flaviventer). We first used a factor-analytic approach to jointly model demographic processes as a function of a latent variable describing yearly environmental quality. We then parameterized, perturbed, and projected into the future a seasonal population model that explicitly incorporated the covarying demographic responses to the environment. The projections were based on various scenarios of environmental change, including changes in the trend and temporal patterning of environmental quality. We show that, although demographic processes in both the summer and winter season contributed to population fitness, yearly environmental quality exerted the strongest effect on winter demography only. Simultaneous, negative responses to declining average environmental quality of winter survival and mass and reproductive-status change resulted in an increased risk of population quasi-extinction, regardless of summer demography. We emphasize that assessing seasonal demographic effects is key to understanding population persistence in the face of environmental change and should receive much more attention in ecological theory and conservation management.