Date of Award:

5-2024

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Wildland Resources

Committee Chair(s)

Daniel R. MacNulty (Committee Co-Chair), Tal Avgar (Committee Co-Chair)

Committee

Daniel R. MacNulty

Committee

Tal Avgar

Committee

Kezia Manlove

Committee

James Powell

Committee

David Stoner

Abstract

Predators can have important ecological effects through killing and eating their prey, the so-called consumptive effect, but predators can also have a nonconsumptive effect (NCE) on their prey – this happens when the risk of predation itself causes prey to alter their behaviors or other traits and these alterations ultimately reduce prey survival, reproduction, or population size. While scientists understand the consumptive effects of predators well, we are still unsure whether NCEs are important in free-living systems. In this dissertation, I sought to better understand the potential NCEs of predators (wolves and cougars) on elk in northern Yellowstone National Park by leveraging data collected over 4 decades by the National Park Service. I first investigated the importance of elk density in shaping elk use of risky places. Increasing density means more mouths to feed, but it also means safety in numbers; as a result, increasing population density shifts the balance of the food-safety tradeoff common in ecosystems from safety at low density to food at high density. I then investigated the importance of elk age in determining how they respond to predators. Wolves hunt by singling out the weakest members of a herd, whereas cougars hunt by ambushing an unaware individual; therefore, wolf predation risk increases more sharply with age. I found that elk respond to this by taking fewer risks with wolves as they age. On the other hand, they take more risks with cougars as they age, a result of trying to maximize reproduction (by increasing their food intake) in their remaining years. Finally, I measured the potential NCE of wolves and cougars on elk birth rates, while controlling for the effects of elk density and age. I found evidence that an NCE likely occurs, but the overall effect was too weak to explain large changes to elk population size over the decades. This information improves our understanding of how predators affect prey populations, which is particularly important given large carnivore recovery and reintroductions around the world.

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Creative Commons Attribution-Noncommercial 4.0 License
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