Understanding The Landscape of Fear
Imagine a small herd of Dall sheep ambling down a scree slope toward a wide glacial-eroded basin. The snow is melting and small sub alpine grasses and sedges are beginning to green up the mountain sides. The sheep begin to forage, keeping their young close by. A wolf appears along a rocky outcropping, scenting the sheep. Just then a pack of five wolves explode down the ridge, surrounding the sheep herd and running off with a yearling lamb. The sheep sprint uphill for safety among the high craggy cliffs.
This common predator-prey interaction has implications for the avoidance strategies of the prey, and pursuit strategies of the predator. Wild animals respond to competition and predation in the environment by adjusting their habits to improve their survival. However, some positive short-term strategies can lead to long-term consequences that in some cases, can lead to local extinction.
Wild animals live in a world full of threats both real and perceived. Their biological responses to and experiences with threats in the environment help inform their daily choices. In the case of the anecdote above, the Dall sheep may accept the risk and return to that basin to forage if that is the most convenient food in the area. The herd may also decide to find new forage habitat farther away. Both situations can be biologically costly, where the first choice risks death and the second choice risks increased herd stress.
There is increasing evidence that wild animals are constantly measuring risk to maximize reward. By taking cues from their environment, animals create a mental map of their surroundings. When a sheep herd experiences a mortality, the sheep will exercise increased vigilance when re-visiting the location of the mortality or leave to find a safer forage location.
It may sound like prey animals like sheep are stuck between a rock and a hard place. If the sheep choose to risk death and feed on high quality forage and survive, the overall herd fitness may improve but individuals may die. Unfortunately, when prey animals become too risk-avoidant, they can trigger the decline of the herd health and eventual extinction.
A recent study on Dall Sheep (Ovis dalli dalli) in Canada suggests a local population of sheep are at risk of extinction due to their severe risk avoidance strategies. To avoid predation by grizzly bears and wolves, the sheep chose to remain in the most rugged and least-quality forage habitats. Ewes chose to raise young on high craggy cliffs while rams selected dense cover with low forage opportunities.
As a previous Hunt Science article found, female ungulates require high quality forage prior to, during, and immediately after giving birth to ensure healthy offspring. In the case of the Dall sheep, the study authors found that by engaging in extreme predator avoidance strategies, the ewes were giving up the nutrients required for raising enough healthy lambs to stabilize the population.
Wildlife populations need to balance predation risk with forage opportunities. In ecosystems with multiple predators, prey animals must respond to multiple attack strategies in varying landscapes. For example, grizzly bears and wolves both prey on Dall sheep lambs, but hunting techniques will vary. Grizzly bears tend to target younger sheep in forested areas, while wolves target sheep in more open landscapes.
Habitats with multiple predators may present an obstacle to prey animals in the form of increased disturbance. Even when predation levels are low, prey animal populations can exhibit excessive levels of vigilance which can result in negative population outcomes.