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by Elizabeth Harper
Updated by Jess Edberg, Information Services Director - International Wolf Center
The wolf is a carnivore, an animal suited
for catching, killing and eating other creatures. Wolves prey primarily
on large, hoofed mammals called ungulates. In Minnesota, the white-tailed
deer is the wolf's primary prey, with moose, beaver, snowshoe hare
and other small mammals also being taken. Elsewhere, wolves prey
on caribou, musk-oxen, bison, Dall sheep, elk, and mountain goats.
All of these ungulates have adaptations for
defense against wolves, including a great sense of smell, good hearing,
agility, speed, and sharp hooves. As these prey are so well adapted to protecting
themselves, wolves feed upon vulnerable individuals, such as weak,
sick, old, or young animals, or healthy animals hindered by deep
snow. By killing the inferior animals, wolves help increase the
health of their prey population a tiny bit at a time. When inferior
animals are removed, the prey population is kept at a lower level
and there is more food for the healthy animals to eat. Such "culling"
also ensures that the animals which reproduce most often are healthy
and well suited for their environment. Over many generations, this
selection helps the prey become better adapted for survival.
Wolves require at least 3.7 pounds of meat
per day for minimum maintenance. Reproducing and growing wolves may need 2-3
times this much. It has been estimated that wolves consume around 10 pounds
of meat per day, on average. However, wolves don't actually eat
everyday. Instead, they live a feast or famine lifestyle; they may
go several days without a meal and then gorge on over 20 pounds
of meat when a kill is made.
In Minnesota, each wolf in eats an average of 15-20
adult-sized deer or their equivalent per year to meet their nutritional
requirements,. Based on this average, and the estimate of 3,020 wolves in
Minnesota, wolves kill the equivalent of about 45,300 to 60,400 adult-sized
deer per year. In comparison, Minnesota hunters take around 52,500 deer per year
in wolf range (over 250,000 for the entire state) and several thousand are
killed during collisions with vehicles.
Wolf predation on ungulates varies seasonally.
It is highest during mid to late winter, when animals are suffering from
poor nutrition and the snow is deep, making them easier to kill. It is
also quite high in early summer when prey animals have their young, as wolves
prey heavily on vulnerable young.
 The
question of whether wolf predation is additive (the number of animals
killed are in addition to those which would die otherwise) or compensatory
(animals wolves kill would die anyway) is a complicated one, as
wolf predation effects vary with the prey species, time of year,
area, and system. It is quite probable that wolf predation is both
additive and compensatory, and the real question is how much of
it is additive.
For example, wolf predation on deer is moderated
by the severity of the winters. In a severe winter, wolves may kill
healthy deer which would have survived the winter had they not had
been made vulnerable by the deep snow. This would be an example
of wolf predation as an additive factor. Conversely, in a mild winter,
when the snow levels are low, healthy deer easily escape wolves.
Therefore, the deer captured are primarily sick or weak. This would
be an example of compensatory mortality, as most of these deer probably
would not have survived the winter. This is why it is rare to find
a starving deer in Minnesota wolf range.
Reciprocally, prey populations may limit wolf
numbers. When considering the examples above, the potential for
prey numbers or conditions to regulate wolf numbers is observable.
In a mild winter, deer will be healthier and wolves may not be able
to catch enough animals to feed themselves. This may cause a decrease
in the wolf population. It is also possible that several severe
winters in a row would decrease deer populations and wolves may
not be able to kill enough food to eat, so again wolf numbers would
decrease.
Another factor complicating our ability to
determine the precise effect of wolf predation, is that it is difficult
to tease out the effects wolves have on their prey populations in
areas where there are many different predators. For example, in
Yellowstone National Park, in addition to wolves, there are grizzly
bears, coyotes, mountain lions, bobcats, lynx, wolverines, and black
bears which all prey on Yellowstone ungulates.
In summary, we cannot generalize about what
kind of effect wolves have on their prey populations, because their
effect is dependent on so many factors. It is possible to get an
indication of wolf and prey population trends in a small area or
system, but generalizing from one to the other is not always valid.
Mech, L.D. 1970. The Wolf: The Ecology and Behavior of an Endangered
Species.
New York: Natural History Press, Doubleday Publishing Company.
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