Do Wolves Control their Own Numbers?

Do Wolves Control their Own Numbers?

B y L . David Mech

Editor’s note: This story was first published in the winter 2021 issue of International Wolf magazine. Not a member? Join today to receive the next issue of International Wolf plus more great benefits!

When assessing changes in wolf numbers, comparing wolf numbers in various areas or studying what controls wolf numbers, understanding wolf density is key.

In January 2021, gray wolves were removed from the federal Endangered Species List (delisted)  except for the Mexican gray wolf. Management of the gray wolf then reverted to individual states. In Montana, Idaho, Wyoming and parts of Washington, Oregon and Utah, wolves were delisted several years earlier, and some of these states soon opened regulated hunting and trapping seasons on the wolves. Managing wolves by opening a season on them, in the same way states manage deer, elk, bears and other wildlife, made many members of the public question why states felt they had to begin controlling wolves.

“Don’t wolves control their own numbers?” these folks asked. Scientists had long pondered that question, and much research has been devoted to the subject. After all, wolf packs are territorial, and the most common natural cause of death is wolves fighting and killing each other while defending those territories. But the question of whether wolves control their own numbers is a trick question, and trying to answer it is not easy.

The trick is that, to start with, one must distinguish between two important concepts regarding wolf numbers: wolf density and wolf population. Wolf density is the number of wolves in an area of a given size—for example, wolves per 1,000 miles². This measure is not one the public sees very often. Most often, media mention the number of wolves in a state or region, like “2,500 wolves in Minnesota.” That’s a perfectly useful measure for some uses, but not for others.

When assessing changes in wolf numbers, comparing wolf numbers in various areas or studying what controls wolf numbers, understanding wolf density is key. That is because wolf populations can fluctuate in three ways: (1) by increasing or decreasing within a certain area—i.e., a change in density; (2) by occupying a larger or smaller area—i.e., a change in distribution, or (3) by changing in density and distribution.

Most wolf populations that have increased during the last few decades, both in North America and Europe, have done so mainly by spreading to new areas, usually after building up in density. Learning what determines changes in wolf density is key to answering the question of whether wolves control or limit their own numbers.

As long ago as 1967, Canadian biologist Doug Pimlott proposed that wolf numbers were self-limiting. I tended to agree with him in my 1970 book, The Wolf: Ecology and Behavior of an Endangered Species, which is still in print. That conclusion was based on the finding that in many areas where wolves were legally protected, their numbers increased but only to a certain density—about one per 10 square miles, even without humans controlling them.

As years went by, however, more data about wolf populations and their prey accumulated, and the question of whether wolves controlled their own numbers drew more attention. Wisconsin biologist Lloyd Keith decided to test another possibility. Instead of wolves limiting themselves, he wondered if wolves were, like many animals, limited by their food supply. In 1983, he examined the relationship between the number of wolves in an area (wolf density) and the accumulated weight (biomass) of their main prey population in that same area.

By that time, wolf biologists had published studies of seven wolf populations and the numbers of their prey. Keith ran a test of the relationship between the wolf densities in each of those seven studies and the weights of their available prey for each of the same areas. He found a correlation:  the greater the amount of prey, the more wolves per area.

A perfect relationship, or correlation of 100%, between those two measures would mean that the amount of prey completely explained the number of wolves. In other words, the number of wolves in a given area was essentially determined, controlled or limited, by the amount of their available food. The correlation Keith found was 64% based on those seven studies. Thus, his findings showed that the available amount of food explained 64% of the differences in the number of wolves per area The remaining amount of difference could be explained by sampling error in the studies, or by an inadequate sample of wolf and prey systems.

After many more published studies of wolf populations and prey numbers produced similar results, one of Keith’s students, Todd Fuller, in 1989 again checked the relationship and found it was even stronger then, based on 25 studies. The new correlation was 72%. In 2003, Fuller teamed up with other authors to examine the relationship based on adding data from seven more studies. Later, other biologists closely examined each of the 32 studies that Fuller and his colleagues included in their latest analysis and realized that some of those 32 wolf and prey populations were subject to human hunting and trapping; other studies involved the eastern wolf, which was considered by some biologists to be a separate species from the gray wolf. Purifying the sample by deleting those studies from the analysis left 26 studies that were still valid to examine for testing the relationship.