How can we determine the effect of predation on big game animals?

One method that carries a lot of weight in many people's mind is to look at mortality sources in a prey population and if that mortality is high, then predation must be having an effect.

For example, a study east of San Antonio radio collared 60 fawns in two areas over a 3 year period. These results implicate predators pretty strongly. Authors explain this predation as a result of drought that reduced forage and cover, making the fawns weak and susceptible to predators. Can you be sure those fawns killed by predators would have lived if predators had not been present? Could those fawns have been part of a "doomed surplus" that year because of the drought conditions? There is no way of knowing for sure. All we know is that in some years, predators can kill many fawns. This does not tell us much about whether predators can control prey populations.


In a longer term study done on the Welder Wildlife Refuge, coyotes and bobcats were removed from a 361 ha enclosure and populations of deer were monitored for 7 years both in and outside the enclosure.

Deer populations and fawn:doe ratios increased for 3-4 years, then the habitat began to suffer, fawn:doe ratios and the population densities declined, and deer densities returned to previous levels.

So we have conflicting views of how predation influences populations.

The idea is that nature is very complicated and we are only beginning to understand how these systems operate.


Four sets of factors can influence whether predators can control prey populations:

I. Factors influencing the ratio of predators:prey.

Probability of predator controlling prey increases as ratio of predator to prey increases.

A variety of research suggests that a ratio of 1 wolf:200 caribou is stable. Above this, caribou populations decline. Below this, caribou populations increase. This ratio for moose is estimated to be 1 wolf:20 moose.

A. Numerical response of predators to prey abundance.

More prey ==> more predators. Maintain predator:prey ratio and help stabilize prey populations.

So, if deer populations increase, do coyote and bobcat populations also increase? If not, then the ratio changes and predation impacts should diminish.

B. Prey diversity.

Alternative prey means that predators may not show a numerical response. Can maintain their numbers even when one prey species begins to decline. Means an increased ratio of predator:prey. Ex) Coyotes can live on small mammals, so even if deer populations decline, coyotes can remain abundant and continue to heavily depredate fawns.

C. r value and longevity of prey and predator.

Predators usually have lower r and are longer lived than prey. Predators cannot increase as fast as prey, so ratio of predator:prey decreases when prey increases. Reverse is true when prey populations decrease.

Ex) Cougars have lower reproductive rates than deer. Deer populations can increase more quickly than cougar numbers, thus continually lowering the predator:prey ratio until something stops the increase in deer numbers. Cougars, on average, live longer than deer. If deer populations drop, because of weather or some other temporary conditions, then the ratio of predator:prey will increase until some of the cougars also die.

D. Predator swamping by prey.

One strategy susceptible prey can use to reduce predation rates is to concentrate prey in time in space so that the available predators are unable to eat much of it. This is the strategy of caribou who have their calves on the open tundra. It is no problem for a wolf to catch and eat a calf. However, because most cows have their calves about the same time (within a few days), there are far more calves than the wolves could possibly eat. This causes a low predator:prey ratio. If calving was spread out over several weeks, then predators would have a much more favorable ratio, could eat calves as they became available, and could potentially shut down reproduction.

E. Geographic concentrations of predators.

There maybe areas where predators tend to concentrate because of abundant food sources. Garbage dumps are one example where there may be an unusually high density of bears or coyotes. Prey populations would be expected to be low in these areas because of the high density of predators.

F. Intrinsic regulation of predator abundance.
Territoriality may limit number of predators in an area and thus limit the numerical response. Cougars and wolves are territorial. This places an upper limit on the number of individuals that can live in an area. Thus, even if prey increased, the number predators may not increase.

II. Factors influencing vulnerability of prey.

A. Habitat quality.

Habitat that has poor escape cover, poor juxtaposition of escape cover relative to other resources, or poor visibility of predators, leaves prey more vulnerable to predators. Food or water far removed from cover could greatly increase predation on deer. Feeders are essentially a resource far enough from cover that the deer must expose themselves for a human predator to be successful. Inadequate cliffs for escape cover of bighorn sheep would increase predator effectiveness and could cause sheep populations to decline. May be density dependent if high prey populations degrade good areas, forcing animals to use poor areas.

B. Decreased animal quality at high prey densities.

Quality may decrease because of poor nutrition, responses to stress, or intraspecific fighting and injury. Will affect subordinate animals most.

C. Dispersal of subordinate prey animals into poor habitats.

As prey density increases, more subordinate animals forced into poor habitat and are taken by predators.

D. Predator evasion strategy of prey.

Evasion strategy reduces predation, but may not be affective against introduced predators or in areas where habitat is not appropriate (e.g no cliffs for bighorn sheep). Mule deer are more susceptible to harvest than white-tails because mule deer evolved in open habitat where distance was effective defense against its natural predators. Mule deer will often run 100 m and stop. While this worked against cougars, bears, and wolves, it is not effective against firearms. This behavior trait has been implicated for the eradication of mule deer from northern Mexico.

III. Changes in predator behavior. Called Functional Response of predators.

A. Concentration of predation.

Repeated success causes predators to concentrate effort on prey species. Greater prey abundance => predator concentrates effort on prey species. This behavior is shown by black and brown bears who spend significant amounts of time in spring searching for moose or elk calves. They have found calves in certain areas or habitats in the past and return to those areas in subsequent years.

B. Predator learning.

Experience causes predators to improve attack methods, increasing their rate of predation. Higher prey densities increase predator experience and results in more efficient predation. The bears mentioned above not only spend more time in areas where elk and moose calves are likely to be born, but modify their behavior so that they look like a hunting dog, searching back and forth through the grass trying to flush calves.

C. Predator group facilitation.

A numerical response of predators to prey abundance may result in larger group sizes and more successful hunting than small groups. Wolves may be more efficient when hunting in larger packs. This is especially true when hunting large animals like elk or moose. For these large packs to form, however, there must be a high enough density of elk or moose to support the wolf packs.

D. Concentration of predator effort.

An increase in alternative prey (buffer species), may attract predator effort away from particular prey species.

ex) lynx predation on caribou calves decreases when snowshoe hares (buffer species) are abundant.

IV. Density-independent processes can affect the above classes of density-dependent factors. Weather or habitat change due to flooding, fire, logging, biotic succession, etc.. may alter effects of factors discussed above.

A dry year in south Texas results in less vegetation cover and increased effectiveness of coyote and bobcat predation on deer fawns. Snow conditions in the north may be such that wolves can run on the surface while deer fall through with each step. In such a situation, a deer quickly tires and the effectiveness of wolves greatly increases. These snow conditions are usually temporary so that prey populations are not reduced greatly.

(source Hewitt, Texas A&M)