Predator-Prey Interactions

An Indicator for Conservation Biology?

Aaron Wirsing, Assistant Professor of Wildlife Science, is interested in the ecological role played by predators — how they affect prey population mortality and how they act as agents of change in prey behavior. He is also interested in the value of animal behavior as an indicator for conservation.

The College welcomed Wirsing to the faculty in September 2008; he holds a master's degree in wildlife resources from the University of Idaho and a PhD in biological sciences from Simon Fraser University. His wide interests in predator-prey dynamics span the terrestrial and marine realms.

Wirsing is currently leading a long-term assessment of the indirect effects that tiger sharks have on a seagrass community in an Australian coastal marine ecosystem — How do the sharks affect the rest of the bay's residents? He says, "Marine ecoystems around the world are rapidly being changed by humans. Seagrass beds are declining and populations of sharks, turtles, and sea cows have been devastated. We need to learn how a natural seagrass ecosystem functions and what role sharks play in marine environments so that we can predict how human uses of the oceans will change them and how we can begin to restore already damaged systems."

The research is confirming that prey often modify their behavior in response to predation threat, changing the way that they exploit resources and ultimately influencing the whole ecosystem. But anti-predator behavior is hard to predict and appears to be highly contingent, so much of Wirsing's research explores the sources of this contingency — whether and how much these modified behaviors in particular cases hinge on predator hunting modes, prey escape tactics, and the landscape in which the predator-prey interaction occurs.

Wirsing is also exploring how the habitat preferences of snowshoe hares in the northwestern U.S. can inform lynx management and recovery efforts. Snowshoe hare populations in the Northwest tend to be low-density and show little or no cyclic activity. Some of Wirsing's previous work suggests that this absence of cycles is owed to a combination of habitat fragmentation and intense predation by a suite of generalists. His lab is testing this explanation at sites in Idaho and Washington. As part of this effort, he is also exploring factors affecting hare numbers in the region, including the impact of forest management decisions, at a variety of spatial scales.

Wirsing was the featured speaker in the Winter 2009 “Sustaining our Northwest World” lecture series, sponsored by the College and the UW Alumni Association, in which he used his tiger shark research and recent studies from our national parks to make the case for carnivore conservation in the Pacific Northwest. 

Discussing the possibility of wolves returning to Washington’s North Cascades, he contrasts the situation there with the example of Yellowstone National Park. “In 1995, wolves were reintroduced to Yellowstone, an event that brought about monumental changes to that ecosystem. The changes included an increase in tree growth, depressed elk numbers, coyote displacement, and an increase in scavengers. But unlike Yellowstone, researchers in Washington state are able to analyze the ecosystem early before wolves have been fully restored.  “Here,” he says, “there is already a predator on the prowl — the mountain lion. If wolves return, there will be two top predators hunting the North Cascades woods.”

The effect that a single predator can have on an ecosystem is quite broad, ranging from changes in behavior patterns of prey, changes in vegetation, and even an increase in competition between predators. What makes Washington’s carnivore conservation case so interesting is that wolves and mountain lions each have unique hunting characteristics that, when acting in concert,  may have surprising effects on the North Cascades ecosystem. “They don’t hunt in the same way. If they did, they could be interchangeable,” explained Wirsing. “Mountain lions are stalking predators, so they like to ambush and attack from above from steep slopes. Wolves are coursing predators, so they like running and they hunt in more open areas. In other words, the danger they pose is different spatially. Thus, prey like mule deer, for example, can’t just avoid one dangerous area.  Instead,  they have two spatially distinct predators to cope with and may have to pick their poison by occupying areas where at least one predator lurks.  The way in which mule deer negotiate this problem must be ascertained before we can fully piece together the effects of wolves on mule deer and ultimately, the North Cascades ecosystem.”

Top predators, when present, can exert positive indirect effects on plants.  Tree growth is often suppressed in areas that lack top predators because of unbridled herbivory and, as a result, they become artificially open after a while.  Says Wirsing, “If this is happening in the North Cascades, then only with the return of wolves will we expect to see normal tree sapling growth.”

As his research continues, Wirsing said that he feels a growing appreciation for the role of big predators. He says, "Studies of animal behavior have great potential to inform the practice of biological conservation, yet the relationship between these two fields is tenuous. The development of behavioral indicators that can be feasibly collected and used will help shape meaningful conservation recommendations. “

Aaron Wirsing with tiger shark at Shark Bay, Australia research site.
Assistant Professor Aaron Wirsing joined the Forest Resources faculty in September 2008.