State-dependent mortality, not behavior, fragments population distribution of a long-lived mammal after ecological disturbance

ContextNatural selection favors species with strong fidelity to seasonal ranges where resources are predictable across space and time. Extreme disturbance events may negate the fitness benefits of faithfulness-with consequences for population distributions.ObjectivesWe hypothesized that extreme events fragment population distributions through two mechanisms: (1) reductions in fidelity or (2) elevated mortality. We tested the relative contributions of these mechanisms to population dynamics of mule deer (Odocoileus hemionus)-a long-lived mammal-with long-term, individual-based information before and after disturbance occurred.MethodsWe evaluated our hypotheses in response to disturbance during winter using a unique dataset of the movement and fate of adult females from a migratory population of mule deer over 8 years in western Wyoming, USA. First, we calculated fidelity of individuals between progressive winters and identified vacant space between population-level ranges to represent gaps in the population distribution. We then assessed: (1) how internal state and disturbance conditions affected fidelity, (2) how internal state, disturbance conditions, and fidelity affected survival, and (3) how survival and fidelity affected creation of gaps in population distribution.ResultsDisturbance weakened fidelity, but fidelity did not affect survival. Nutritional condition and age affected survival. Weakened fidelity did not change population distribution; rather, nutritional condition underpinned population dynamics, meaning that behavior alone may not prevent the creation of gaps in distribution following extreme disturbances.ConclusionsExtreme events may render behavioral plasticity incapable of mitigating mortality risk, and the environmental conditions that animals experience during the months, seasons, or even years before an event may regulate population-level organization in its aftermath.