So how do these relationships work? Let's say you are a white-tailed deer foraging on the Leopold reserve. There are no large predators in the forest that could threaten you. So you feed steadily on shrubs and grasses, looking up only to interact with others of your kind or search for food. Now let's say you are a white-tailed deer foraging in the North Fork. You take a bite and look up, sacrificing food for safety, highly vigilant. You are living in a landscape of fear, where your ability to survive depends on your ability to detect and escape predators as well as obtain food. The resulting stealth and fear dynamics—and the relationship between top predators and their prey—have profound ecological implications.
Risky Business: Predation and Resource Selection
Predator-prey interactions have two components: predators killing prey and predators scaring prey. While the lethal effects of predation are well documented, nonlethal effects may have equally strong consequences. Joel Berger tested this by tossing snowballs imbued with predator scents, such as wolf urine and grizzly bear feces, at ungulates. In addition to pungent snowballs, he experimented with tape recordings of predator sounds (lion roars and wolf howls) and neutral sounds (water and monkeys). Where wolves had been absent for decades, such as in Rocky Mountain National Park, elk responded to the snowballs or predator sounds with some curiosity, but none became alarmed or ran. In Denali National Park and Preserve, where wolves had been present for many years, ungulates responded by becoming hypervigilant. Berger and colleagues continued this work on a circumpolar scale, working in Greenland and Siberia, where predators had long been present, and finding similar results with moose and caribou (Rangifer tarandus). Beyond individual responses, Berger wanted to know how prey animals acquire knowledge, how fear is transmitted, and how current behavior can help unravel the ambiguity of past extinctions and contribute to future conservation. Ultimately his work will help shed light on how predators shape prey behavior and landscapes.
Research about predation risk has the potential to inform human choices about which landscapes can be allowed to harbor dangerous animals. Berger and colleagues found that in Wyoming moose increased vigilance behavior in the presence of grizzly bears, keeping their heads up longer and staying on the move to avoid predation. This reduced browsing on willows, enabling the willows to flourish, thus improving habitat for songbirds and increasing biodiversity. Awareness of these landscape-scale effects can be used to make management decisions about grizzly bears, perhaps allowing them to expand their ranges.
Predation is the main driver of fear in prey because it can lead to death. Fear of predation involves a response to predation risk, whereby prey react to predator presence—or even to the mere threat of it. Fear causes the adrenal glands to secrete adrenaline, a short-acting substance that prepares the muscles and brain for flight. It also produces cortisol as part of an animal's long-term response to chronic stress. An elk uses all its senses to evaluate the threat of predation. Its ability to assess and control its risk of being preyed upon strongly influences habitat selection and feeding decisions. Prey animals establish an optimal baseline level of vigilance in the absence of direct evidence of predator presence. Individuals who successfully balance the benefits of risk avoidance against energy costs (missed opportunities to eat) have a greater chance of survival.
This response is not limited to large mammals. Working with animals at the opposite end of the size spectrum, Oswald Schmitz found a behavioral trophic cascade consisting of spiders and grasshoppers. The top predator he studied, the nursery web spider Pisaurina mira, preferentially preys on the grasshopper Melanoplus femurrubrum. In the absence of spiders, grasshoppers selected a diet composed almost entirely of grass rather than forbs (flowering plants that are not grasses). In this famed experiment Schmitz glued spiders' mouths shut to render them unable to prey on grasshoppers. In the presence of spiders with glued mouths, grasshoppers nevertheless reduced their feeding time and preferentially ate forbs, which provide greater cover and safety from predation. This shift resulted in a trophic cascade.