Can wolves and prey exist in equilibrium?
"The prey are always ahead of the predators. If predators were ahead of the prey in an evolutionary sense, they wouldn't hunt twelve times for every kill, and they wouldn't tend to kill fawns and old individuals, but would instead kill a cross-section of the prey population. The typical prime member of the prey population is invulnerable to the wolf. The wolf is essentially a glorious scavenger. They just don't wait for the prey to die." Mech cited in Steinhart, 1995
Pack size is a function of the biomass of available prey (Zimen, 1981). It is evident that prey populations determine predator density, just as critically as predators determine prey density. They define each other and depend on each other to maintain a stable balance in nature. In North America, the wolf is a predator primarily of large ungulates. At high ungulate prey densities, wolf territories become smaller, and wolf densities increase. At low ungulate prey densities, wolves become nutritionally stressed, more nomadic, less territorial, and more solitary (Lariviere, 1999). A solid prey base is essential for the successful reintroduction of wolves. In addition, the reintroduction of this top predator is going to have a significant affect on the prey populations.
An individual wolf is capable of consuming up to 44 lbs. of meat in 24 hours, and is also capable of surviving without food for long periods of time (Zimen, 1981). Studies on Isle Royale found that a wolf pack could consume 900 lbs. of moose in 2 days (Zimen, 1981). Terrestrial carnivores can limit herbivorous mammal populations, and there is evidence that ungulates decline following range increase in predators or the reintroduction of predators (Estes, 1996). Concerns about the effect of a large predator base on ungulate herds have motivated the removal of the wolf from YNP in 1914-1926 (Weaver, 1978 cited in Ripple, 2000).
The main prey base for wolves are ungulates-elk, deer, bison, bighorn sheep, caribou, moose, and pronghorn, among others (Schullery, 1996). Wolves will also feed on smaller prey such as ground squirrels, marmots, beavers, snowshoe hares, and other smaller creatures (Mech et al., 1998). These predators work together as an entire pack when hunting large prey. The phases of hunting involve; locating and spotting of prey, stalking and creeping up, confrontation, predator and prey being aware of each other, quick spurt towards prey, a chase if the prey takes flight, and finally the kill (Zimen, 1981). Wolves have a short and swift attack and they work together to corner the prey and need each other in order to kill these species which are often much bigger and stronger than an individual wolf. Wolves are also very economical with their efforts. They quickly give up if the struggle to kill is too great, the prey too strong to pursue and attack, or even on the occasion that the prey reciprocates the challenge, as is sometimes the case with large moose. Wolves use a "hit or miss" method when hunting (Zimen, 1981). Many different studies have reached the same conclusion that wolves kill the most vulnerable prey, being the very young or the very old, and the sick or the weak. Zimen (1981) concludes that wolves kill this more vulnerable prey not by choice, but because these are the only prey they are able to kill. After observing wolf hunts in Isle Royale, Mech concluded that wolves approach about twelve deer or moose for every one that they actually catch (Steinhart, 1995).
This evidence of prey selection leads to conclusions that wolf predation has a healthy effect on prey populations, because it weeds out the weak members and the strong are able to escape attacks and exist in an overall more healthy, strong population. The resulting age structure of the prey populations shifts in favor of reproducing age groups, so reproduction rates may increase (Zimen, 1981). Prey populations have weak self-limitation capabilities (Tanner, 1975), so wolf predation evens out fluctuations in prey populations and contributes to stability in the system. Wolves regulate populations of white sheep in Alaska, moose on Isle Royale, and white-tailed deer in Ontario and Minnesota (Tanner, 1975). Biologists are being convinced more and more that predators serve a useful function as they kept prey populations from destroying their own food resources and removed genetic mistakes from the population, the weak and diseased, thereby keeping the species strong (Steinhart, 1995).
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