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

Abstract

Declines in calf recruitment in some Rocky Mountain elk populations have raised concerns that recovering carnivore populations may be limiting elk population growth. To help resolve continuing questions about the influence of wolves (Canis lupus) reintroduced during 1995–1997 on elk calf production and survival, we conducted retrospective modeling of factors such as weather, elk density, and recovering carnivore populations potentially affecting northern Yellowstone elk pregnancy and calf survival using a long-term data set collected during 1985–2008. We found elk density negatively affected yearling pregnancy rate but had little effect on adult pregnancy rate. Adult pregnancy rate declined as individuals aged and increased following summers with higher precipitation but did not vary across a wide range of carnivore densities. Fall lactation rates, an indicator of early-season calf survival, increased with elk age and decreased as the ratio of grizzly bear (Ursus arctos horribilis) per elk increased. Over the 23 years of this study, the index of calf recruitment decreased as the wolf/elk ratio increased, decreased following severe winters, and increased as elk density decreased. Estimates of the wolf/elk ratio effect on calf recruitment were negative and confidence intervals did not overlap 0, whereas estimates of the wolf/elk ratio effect on elk pregnancy and early calf survival were positive and confidence intervals did overlap 0. Although a myriad of factors affect each stage leading to calf recruitment, and some of these factors such as weather are out of management control, management actions aimed at reducing wolf densities may result in increasing calf survival and elk population growth rates. However, the desire to manage wolf numbers to achieve a balance with ungulate population objectives must be balanced with other social considerations such as the desire to maintain wolf populations for tourism and wildlife-watching opportunities. © 2014 The Wildlife Society.