Behavioral adaptations are complex and variable and show an evolutionary relationship to landscapes. In large mammals, behavior that evolved over thousands of years underlies trophic cascades mechanisms. Elk originated in Asia, on high grassland steppes. They colonized North America about 10,000 years ago, crossing the Bering land bridge. Lacking competition from other elk species in North America, they spread widely across many habitat types, from Pacific Northwest rain forests to sagebrush deserts. Long-legged cursors, elk run with their heads up and a straight-legged gait (as opposed to bounding). They escape predators via rapid and sustained flight, an adaptation found in ungulates from open plains with low flight impediments. On landscapes with both open and closed habitat structure, they may use a combined strategy of hiding in forest cover to lower predator encounter rates and seeking open terrain, such as grasslands, where predation risk may be reduced.
Recent studies have examined factors that can render prey more vulnerable, such as differences in ungulate grouping behavior and terrain features. In Banff National Park, landscape ecologist Mark Hebblewhite found predation risk lowest in small groups of elk, with groups larger than twenty-five having the highest probability of being preyed upon by wolves, possibly because they are more likely to contain weak or sick individuals and are easier for predators to detect.
In Yellowstone, ecologist William Ripple developed his predation risk hypothesis while sitting on a high terrace in the Lamar Valley, where he noticed patchy willow growth. Out in the open, willows were browsed down, but where there were visual or terrain obstacles, willows flourished. He and a student, Joshua Halofsky, proceeded to measure elk behavior and found heightened vigilance in areas with escape impediments. Elk spent more time with their heads up, scanning for predators, in these areas, behaving more skittishly than when they were assured of a clear escape route.
According to Douglas Smith, leader of the Yellowstone Gray Wolf Restoration Project, the concept of predation risk eludes easy definition. For example, an area where wolves take down prey after a long chase may not necessarily be the site of greatest predation risk. That may actually be the site where prey first encounter predators. Additionally, Smith notes that most wolf kills occur between dusk and dawn. Because elk and other ungulates have poor vision, obstacles to their viewshed may not play a significant role in the dynamics of predation risk. 30 Presence of other predator species complicates matters. Wildlife biologist Kyran Kunkel found that avoidance of one species, such as the cougar, which hunts by stealth, makes prey more vulnerable to another, such as the wolf, which runs down its prey.
Elk have a sophisticated response to predation risk that includes gathering in larger groups in open areas. Landscape ecologist Matthew Kauffman and colleagues found that open areas enable wolves to detect prey more easily and thus present greater predation risk. Wildlife ecologists Stewart Liley and Scott Creel found that elk adjust their vigilance in response to the size of their group and the type of immediate threat they face from wolf presence, with environmental variables such as obstacles having a secondary influence on vigilance.
Some researchers recommend that trophic cascades studies incorporate radio-collar data to measure behavioral predation risk (i.e., wolf presence). According to Smith, the complexity of these interactions merits deeper investigation.
I ended up putting in 150 miles of track transects in the Crown of the Continent Ecosystem and doing 700 focal animal observations. In doing this work I found compelling evidence of a trophic cascade. Where wolf density was high, elk avoided areas with debris and other escape impediments. Most carcasses and the greatest amount of wolf sign, such as tracks and scat, occurred in thick forests, debris, ravines, and riverbanks, which I had characterized as high predation risk sites. My focal animal observations suggested that the more wolves there are in a landscape, the more wary elk become. This response may be triggering cascading effects in this ecosystem, enabling aspens to grow above browse height. Indeed, the ecology of fear may be behind the changes at my home, where shrubs and trees have reclaimed the meadow after wolves returned and deer, to stay alive, have had to act more like deer and less like livestock. These pervasive effects influence even small, nonkeystone predators, as we shall see.