Variability and correlations among vital rates and their influence on population growth in mule and black-tailed deer

Abstract

To reverse observed range-wide population declines, managers of mule and black- tailed deer (Odocoileus hemionus) require information on the vital rates and life stages that are most influential to population growth for which to target management actions. We conducted a range-wide literature review and used hierarchical models to provide biological descriptions of mule and black-tailed deer vital rates, their variability, and how they correlate with one another. We then used matrix models and life-stage simulation analysis to determine the individual vital rates that contributed most to lambda, i.e., annual population growth rate. Adult female survival was the vital rate with the greatest ability to predict lambda and explained 62% of the variation in population growth. While annual juvenile survival explained 44% of the variation in lambda, summer and winter juvenile survival by themselves were far less explanatory than adult female survival. Winter fawn:doe ratios, a metric often collected by management agencies, only explained 10% of the variation in lambda. When adult female survival was 0.84, our simulations estimated an equal probability that a population would increase versus decrease, and correspondingly, the estimate of lambda was 1.0 with a 95% credible interval of 0.88-1.14. Simulations suggested that populations with adult survival rates less than 70% would always decline, but as survival increased beyond this value lambda increased linearly and never plateaued. In contrast, the probability of observing a stable lambda plateaued when annual juvenile survival reached approximately 0.5. The mean lambda calculated from all simulated values within the observed range of vital rate values across the species geographical distribution was 0.975, and in 61% of the simulations lambda was < 1. After 20 years, we estimated that this distribution of lambda values would cause populations to decrease in 92% of instances with a mean decrease of 44%. Our results align with the observed declines in mule deer populations throughout their range over recent decades and suggest that these trends will continue until management can improve survival of adult females.