Forage senescence and disease influence elk pregnancy across the Greater Yellowstone Ecosystem
Owen R. Bidder,
Thomas Connor,
Juan M. Morales,
Gregory J. M. Rickbeil,
Jerod A. Merkle,
Rebecca K. Fuda,
Jared D. Rogerson,
Brandon M. Scurlock,
William H. Edwards,
Eric K. Cole,
Douglas E. McWhirter,
Alyson B. Courtemanch,
Sarah Dewey,
Matthew J. Kauffman,
Daniel R. MacNulty,
Johan T. duToit,
Daniel R. Stahler,
Arthur D. Middleton
Affiliations
Owen R. Bidder
Department of Environmental Science, Policy, and Management University of California, Berkeley Berkeley California USA
Thomas Connor
Department of Environmental Science, Policy, and Management University of California, Berkeley Berkeley California USA
Juan M. Morales
School of Biodiversity, One Health, and Veterinary Medicine University of Glasgow Glasgow Scotland
Gregory J. M. Rickbeil
Department of Environmental Science, Policy, and Management University of California, Berkeley Berkeley California USA
Jerod A. Merkle
Department of Zoology and Physiology University of Wyoming Laramie Wyoming USA
Rebecca K. Fuda
Wyoming Game and Fish Department Pinedale Wyoming USA
Jared D. Rogerson
Wyoming Game and Fish Department Pinedale Wyoming USA
Brandon M. Scurlock
Wyoming Game and Fish Department Pinedale Wyoming USA
William H. Edwards
Wyoming Game and Fish Wildlife Health Laboratory Laramie Wyoming USA
Eric K. Cole
National Elk Refuge U.S. Fish and Wildlife Service Jackson Wyoming USA
Douglas E. McWhirter
Wyoming Game and Fish Department Jackson Wyoming USA
Alyson B. Courtemanch
Wyoming Game and Fish Department Jackson Wyoming USA
Sarah Dewey
National Park Service Grand Teton National Park Moose Wyoming USA
Matthew J. Kauffman
U.S. Geological Survey, Wyoming Cooperative Fish and Wildlife Research Unit, Department of Zoology and Physiology University of Wyoming Laramie Wyoming USA
Daniel R. MacNulty
Department of Wildland Resources Utah State University Logan Utah USA
Johan T. duToit
Department of Wildland Resources Utah State University Logan Utah USA
Daniel R. Stahler
Yellowstone Center for Resources National Park Service Yellowstone National Park Wyoming USA
Arthur D. Middleton
Department of Environmental Science, Policy, and Management University of California, Berkeley Berkeley California USA
Abstract For various temperate ungulate species, recent research has highlighted the potential for spring vegetation phenology (“green‐up”) to influence individual condition, with purported benefits to population productivity. However, few studies have been able to measure the benefit on vital rates directly, and fewer still have investigated the comparative influence of other phenological periods on ungulate vital rates. In this study, we tracked phenological changes throughout the duration of the growing season and examined how their timing affected the probability of pregnancy in an ungulate population. We did this for elk (Cervus canadensis) across the Greater Yellowstone Ecosystem (GYE) by sampling 1106 adult females in winter at 25 sites over a 13‐year period and assessing sources of variation in pregnancy using a Bayesian hierarchical model. Pregnancy rates were generally high across the GYE (82.4%), and the primary influences on probability of pregnancy were the timing of vegetation senescence (“brown‐down”) in autumn and exposure to the reproductive disease brucellosis. Earlier forage brown‐down in fall negatively influenced the probability of pregnancy of elk aged 6–9 years by an estimated 17.2% within the range (ca. 32 days) of observed brown‐down end dates. While summer habitat quality has been inferred to influence elk pregnancy previously, our findings specify the key influence of foraging conditions later in the seasonal cycle, immediately before the breeding season. The reproductive disease brucellosis was also an important factor, reducing the probability of pregnancy by 12.4% in elk in the 6‐ to 9‐year age class. Because pregnancy was tested before most disease‐induced abortions occur, the apparent mechanism for this effect is a prolonged reduction in fertility beyond the period of initial exposure in which fetal mortality is typically expected. Our results prompt greater scrutiny of the combined effects of late‐season phenology and disease on reproductive rates and population productivity in temperate ungulates.
