Regulation of wolf populations

natural and anthropogenic

Wolves have been persecuted by humans almost everywhere, to the point that they are extinct over much of their former geographic range. Although they have been reintroduced to a few areas, they are still subject to human trapping and shooting (both legal and illegal) in most parts of their present range and humans are the single most important factor determining wolf numbers in most areas.

Even in the absence of human interference (or at least where the human impact is light), adult wolf densities vary enormously. Some of the variation is geographic, depending on prey type and availability. Some of the variation is temporal, again depending partly on prey availability, but also on other factors, such as disease, which can reduce wolf numbers markedly. (At least one disease apparently originated with domestic dogs, and others are carried by dogs, so even this factor is partially related to a human impact.)

Most wolf researchers agree that, in general, wolf densities are correlated with the food supply. A summary of 31 intensive studies showed a significant linear correlation between average wolf numbers and average biomass of ungulates in an area (i.e., numbers and sizes of moose, deer, caribou, etc.), accounting for 64% of the variation in wolf density (Fuller et al. 2003). Many individual studies arrived at the same conclusion.

In any single year, however, a close relationship between wolf density and ungulate prey biomass may not be found, in part because of time lags between changes in prey biomass and the wolf population response. Furthermore, estimating prey availability as simple biomass of ungulates can only be a rough indication of food availability, because various other factors affect the vulnerability of ungulates to wolf predation and need to be considered; e.g., distribution (deer density can be lower near the centers of wolf pack territories, where hunting pressure is higher), season (snow depth in winter reduces ungulate mobility), and prey health (ungulates are subject to some debilitating diseases). In addition, the availability of alternate prey such as beavers and hares would affect the closeness of the relationship.

In addition, human impacts on both wolves and ungulate prey affect the relationship between wolf numbers and their food supply, producing variation around the linear correlation: Where wolf numbers have been markedly reduced by humans, or where wolves have only recently been protected, the ratio of wolf numbers to prey tends to be low; that is the wolf population is below the limit set by its prey base. When wolf numbers are low relative to the food supply, reproductive rates and success can be high. Litter size tends to be larger and pup survival is better when food is plentiful. Therefore, a depleted wolf population can restore its former numbers in just a few years, and a newly established or protected wolf population can soon reach the limit of its food supply. Immigration from neighboring regions can add to the rapid recovery.

Because of the ability of depleted wolf populations to recover rapidly when food is abundant, programs of wolf control have to kill many wolves in order to reduce the population size. For example, in the Tanana Flats, AK, a seven-year control program reduced a population of 239 wolves to about 143 individuals, a reduction of 96 wolves. To do so, program killed 337 wolves. That amounts to roughly 3.5 wolves killed for every single wolf reduced from the population.

On the other hand, where wolves are not exploited by humans, or where ungulate prey is heavily harvested by humans, the ratio of wolf numbers to prey tends to be high; that is, the wolf population is near the limit set by the prey base. If the ungulate population is severely depleted by humans, the total number of wolves it can support must be small.

Particularly when wolf density is high relative to the food supply, the relationship between wolf density and food supply is mediated, in part, by social factors. Then reproduction may be inhibited, more individuals may disperse from their home packs, and lethal strife among wolves may increase.

Obtaining good data on wolf populations requires intensive field work. All studies of wolf population regulation depend on accurate census data from field surveys in the area of concern. Because wolf population sizes vary geographically and temporally, it is not sufficient just to assume that information from one time or place can be applied to another. Reliance of the reports of trappers and shooters for estimates of wolf population size is totally inadequate.

Selected references:

  • Fuller, T. K., L. D. Mech, and J. F. Cochrane. 2003. Wolf population dynamics. Pp. 161-191 in Wolves (eds. L. D. Mech and L. Boitani). University of Chicago Press, Chicago.
  • Hayes, R. D. and A. S. Harestad. 2000. Demography of a recovering wolf population in the Yukon. Can. J. Zool. 78: 36-48.
  • Van Ballenberghe, V., A. W. Erickson, and D. Byman. 1975. Ecology of the timber wolf in northeastern Minnesota. Wildlife Monographs 43: 1-44.
  • Van Ballenberghe, V. and L. D. Mech. 1975. Weights, growth, and survival of timber wolf pups in Minnesota. J. Mammal. 56: 44-63.
  • Vucetich, J. A. and R. O Peterson. 2004. The influence of prey consumption and demographic stochasticity on population growth rate of Isle Royale wolves Canis lupus. Oikos 107: 309-320.
  • Wydeven, A. P., T. R. Van Deelen, and E. J. Heske (eds). 2009. Recovery of gray wolves in the Great Lakes region of the United States. Springer, NY.