Fear as an ecosystem engineer

This post contributed by Cristina Eisenberg, conservation biologist at Oregon State University

Over the past three years I have conducted thirteen hundred focal animal observations on elk in the northern and southern Rocky Mountains. This involves patiently watching one animal at a time for up to twenty minutes and recording its wariness–that is, the amount of time it spends with its head down feeding versus head up, scanning for predators. Prey group size and a host of environmental factors can influence vigilance behavior.

My research questions have to do with whether the vigilance of ungulates—such as elk, deer and other hooved animals— varies based on wolf population dynamics or other environmental factors that can influence predation risk. For example, would lone wolves passing through an area occasionally, but not denning there (as is the case with a returning wolf population in the Southern Rocky Mountains) have the same effect as several well-established packs using an area? Do terrain features such as downed wood, which may make it more difficult for an elk to escape a wolf, increase elk wariness? And could fear-based behavior vary by season, age and sex of the animals observed, herd size or human management of wolves? Termed the ecology of fear by ecologist Joel Brown, these predator-driven dynamics can have far-reaching effects on ecosystems via trophic cascades.

Trophic cascades are the direct and indirect effects of an apex, or top, predator in a food web. In 1974 in the Aleutian archipelago, Jim Estes and his colleagues found that removing sea otters releases their primary prey, sea urchins, from predation. As sea urchins explode in number, they consume vegetation unsustainably, thereby reducing habitat for other species such as fish.

The presence of a predator, such as the wolf, affects prey foraging behavior as prey try to balance the need to detect predators with meeting their  nutritional needs. These behavioral effects have been observed between spiders and their grasshopper prey by Oswald Schmitz and colleagues, as with sea urchins in terrestrial systems: Intensive browsing can lead to herbivores literally eating themselves out of house and home and, consequently, to a loss of biodiversity and ecosystem destabilization. Lacking apex predators to keep ungulates in check, ecosystems can support fewer species, such as birds and butterflies , because the plants that create habitats for these species have been over-browsed.

Some predators and their prey naturally fluctuate in population size; this cycling can leave noticeable marks on the landscape. However, scientists are finding that these interactions are complex beyond the typical ebb and flow of predator and prey numbers. Assessing ungulates and large carnivores in the northern hemisphere, conservation biologist Joel Berger suggested that fear of predation is a learned behavior. In addition, John Laundré found that, subsequent to wolf recolonization in the mid-1990s in Yellowstone National Park, elk became more vigilant. But in 2009 Scott Creel and David Christianson learned that elk can become accustomed to wolf presence, possibly losing their fear of predators over time.

In Yellowstone trophic cascades researchers are investigating the effects of predation risk on aspen growth. In 2003 Ripple and Beschtaformulated their terrain fear factor, a model predicting that the growth of young trees and shrubs would be greatest at sites in which there was a low likelihood of elk detecting wolves and ones where there were limited escape routes for the elk. Since then they have documented how wolf presence may indirectly enable aspen and other woody species to grow above the height an elk can browse: two and a half meters.

Meanwhile, based on a decade of data collected from wolf kill sites in parks (with which they created a predation risk map), Matt Kauffman and his co-researchers found that elk do not avoid foraging in high predation risk areas. Kauffman and colleagues excluded elk from several areas of varying predation risk and found that the impacts of elk browsing on aspen are not diminished in sites where elk have a higher risk of predation by wolves.

A leading issue in this debate is that different researchers define predation risk differently. Mark Hebblewhite deconstructed wolf predation risk into several components, including risk of detection and risk of death. These components include the place where wolves first detect an elk, the landscape where the chase takes places and the kill site. According to Yellowstone Gray Wolf Restoration Project leader Doug Smith, what then constitutes the area of highest predation risk? Is it the place where wolves first detect their prey? Or is it where wolves take down their prey?

While it will take years to parse out these intricate ecological dynamics, many scientists agree that across a variety of landscapes, fear can be a powerful ecosystem engineer.

Cristina Eisenberg is completing her doctorate in Forestry and Wildlife at Oregon State University. She also is the author of The Wolf’s Tooth: Keystone Predators, Trophic Cascades, and Biodiversity, published by Island Press in 2010.

Photo Credit: Cristina Eisenberg, taken in the field during a focal animal observation