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

Abstract

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.

Bottom-up and top-down influences on pregnancy rates and recruitment of northern Yellowstone elk

“…We used hunter reports to determine lactation status for elk harvested during the Gardiner Late Elk Hunt as an index of early‐season calf survival (Johnson et al ). We included data only from elk harvested in December or January because overall lactation rates normally decline through mid to late winter, confounding our estimate of early‐season calf survival.…”

“…Forage conditions during summer may influence female body condition, the probability of pregnancy, and subsequent calf survival (Cook et al , Johnson et al , Middleton et al ). We predicted yearling pregnancy rates may be particularly sensitive to the effects of environmental variations and forage conditions (Gaillard et al , Eberhardt ).…”

“…However, despite its low elasticity, the high variability in juvenile survival makes juvenile survival rate highly influential in ungulate population dynamics (Gaillard et al , Raithel et al ). Juvenile survival may be affected by an array of abiotic, bottom‐up, and top‐down factors (Barber‐Meyer et al , Johnson et al ), and is expected to be the first vital rate to decrease as resources decline because of density dependence or other environmental factors (Eberhardt , ; Gaillard et al ; Bonenfant et al ).…”

“…Density‐dependent reductions in per‐capita nutritional resources would be expected to reduce body fat and consequently pregnancy rates in ungulates, as seen in experimental work (Kie and White , Stewart et al ); however, density‐dependent effects on pregnancy rates in free‐ranging ungulate populations are perhaps more rare. Direct effects of predation on neonates may eliminate the energetic costs of lactation, thus increasing female body condition and probability of pregnancy during the following year (Cook et al , Johnson et al ). Indirectly, predation risk effects may result in behavioral shifts that carry costs to reproduction, although these effects are debated in elk—wolf systems (Creel et al ; White et al ; Middleton et al , ).…”

Influences of supplemental feeding on winter elk calf:cow ratios in the southern Greater Yellowstone Ecosystem

“…Elk productivity has been linked with abiotic factors such as summer rainfall and winter severity (Wang et al , Garrott et al , Griffin et al , Johnson et al ) and supplemental feeding may reduce some of these environmental effects (Smith and Anderson 1998). However, feeding studies on free‐ranging elk and red deer ( C. elaphus ) indicate little effect on fecundity and mixed results on survival (Smith , Putman and Staines 2004).…”