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

Abstract

To reverse range-wide population declines, managers of black-tailed and mule deer (Odocoileus hemionus) require information on the vital rates and life stages most influential to population growth to target effective management actions. We extracted black-tailed and mule deer vital rates from a range-wide literature review and used hierarchical models to summarize 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 annual population growth rate (i.e., lambda). Annual adult female survival explained the greatest amount of variation (62%) in lambda. Annual juvenile survival explained 44% of the variation in lambda, whereas summer or winter juvenile survival by themselves were far less informative. Winter fawn:doe ratios, a metric often collected by management agencies, explained only 10% of the variation in lambda. Given an adult female survival of 0.84, our simulations estimated a lambda of 1.0 (95% credible interval = 0.88–1.14), indicating equal probability that a population would increase or decrease. Simulations further indicated that given adult survival rates <70%, the population would always decline, but as survival increased lambda increased linearly. In contrast, estimates of lambda plateaued when annual juvenile survival reached approximately 0.5, indicating higher survival rates yielded diminishing returns to population stability. Using simulated values within the observed range of vital rate values across the species' geographical distribution, the mean lambda 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 observed declines in mule deer populations throughout their range over recent decades and indicate that these trends will continue until management can improve survival of adult females.