The potential for a carnivore to indirectly affect the demography of plants depends largely on the extent that the carnivore influences herbivore foraging behavior and (or) population density. The hypothesis that wolves are ecosystem engineers that have suppressed elk herbivory and triggered widespread vegetative recovery of woody deciduous plants (e.g., willow, aspen, and cottonwood) in northern Yellowstone National Park assumes that wolves are the dominant force modifying elk behavior and density. However, the science underlying this cornerstone assumption is far from settled. Here, we provide a critical overview of what is currently known about the influence of wolves on the distribution and abundance of northern Yellowstone elk. We summarize previous research together with the latest findings from our ongoing, long-term study of wolf-elk interactions in northern Yellowstone.
Wolves more often fail then succeed when attacking elk, and this fact, together with its underlying causes, sets the stage for understanding why the predatory effects of wolves are not necessarily as strong as often portrayed. Wolves overcome their limited hunting ability by targeting the young (< 1 year-old), the old (>10 years-old), and the physically compromised. These elk generally represent a small fraction of the total elk population, which means the majority of elk - including reproductively-important prime-age females (2-10 years-old) - face a minimal threat from wolves. This helps explain why elk interrupt their foraging only when wolves are an immediate threat, and why elk continue to select for nutritionally-important but risky habitats despite the presence of wolves. On the other hand, increasing wolf predation rate, strong preference for elk calves, aging female elk, and strong female elk winter range fidelity explains how wolf predation likely contributed to a decline in elk abundance inside the park and a skew in elk winter distribution towards areas outside the park. Complicating the case for a wolf-caused, density-mediated trophic cascade is the role of other large predators (bears and cougars), including humans that hunt elk (and wolves) outside the park.