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Abiotic, bottom-up, and top-down influences on recruitment of Rocky Mountain elk in Oregon: A retrospective analysis


  • Associate Editor: Gary C. White


Understanding the relative effects of the many factors that may influence recruitment of ungulates is fundamental to managing their populations. Over the last 4 decades, average recruitment in some populations of elk (Cervus elaphus) in Oregon, USA declined from >50 to <20 juveniles per 100 females, and several competing hypotheses address these declines. We developed a priori models and constructed covariates spanning 1977–2005 from hunter-killed elk, elk population estimates, cougar harvest, and weather statistics to evaluate abiotic, bottom-up, and top-down factors that may explain annual variation and long-term trends of pregnancy, juveniles-at-heel in late autumn, and recruitment of juvenile elk in spring. In models of pregnancy status, August precipitation, age, and cougar index had positive effects, whereas previous year (t − 1) winter severity or winter precipitation(t−1) and elk density had negative effects. In models of juvenile-at-heel in late autumn, August precipitation, August precipitation(t−1), cougar index × elk density(t−1), and age had positive effects, whereas cougar index, elk density(t−1), and winter precipitation(t−1) had negative effects. Juvenile recruitment was best explained by positive effects of August precipitation(t−1), lactation rate, and cougar index × elk density(t−1) and negative effects of cougar index and elk density(t−1). Winter severity, precipitation, and temperature were not significant in explaining variation in elk recruitment. Annual variation in pregnancy, juvenile-at-heel, and recruitment was most influenced by August precipitation, whereas long-term trends in recruitment were most influenced by cougar densities with relatively weak effects of elk density. These results provide insight into causes of year-to-year and long-term trends of elk recruitment and provide a basis for more rigorous evaluation of factors affecting recruitment of elk. © 2012 The Wildlife Society.