Studies have also explored whether the number of prey animals limit the number of wolves. For over half a century, Isle Royale has been the focus of the longest running predator-prey study on wolves. From 1959 to 1980, the moose and wolf populations of Isle Royale tended to reflect each other. When the moose numbers were high, there was more food for the wolves, meaning better nutrition, higher pup survival rates and an increase in the wolf population. More wolves eventually led to a decline in moose and less food for wolves, meaning fewer wolves survived. As wolf numbers declined, they put less pressure on the moose populations, which in turn helped moose numbers rebound, and the cycle repeats. Yet, even in an isolated ecosystem like Isle Royale, other factors such as deadly viruses, ticks and inbreeding can lead to a species’ decline.
In areas where more than one prey species is available, wolf-prey relations are even more complex. In multi-prey ecosystems, when the primary prey species goes into decline, two things can happen: the predator population may also go into decline, or the predator population may continue to increase by supplementing its diet with alternate prey. Biologists call this “prey switching.”
In the east-central Superior National Forest in northeastern Minnesota, white-tailed deer, moose and beavers are the top menu items for wolves. From 2006 to 2016, the moose population declined by more than half. To determine if wolf numbers also declined, or whether wolves supplemented their diet with an alternate prey, Shannon Barber-Meyer and Dr. L. David Mech compared wolf numbers before and after the moose decline. Their study, which tracked and counted radio-collared wolves, showed that as the moose population declined, the wolf population, instead of decreasing, almost doubled. Wolf scat revealed that wolves supplemented their diets by hunting white-tailed deer. They also continued to prey on moose calves, contributing to the continuing decline of the moose population. Only when the white-tailed deer population declined did the wolf population also begin to decline.
A recent study focused on what the wolves of the Alexander Archipelago and the southeastern mainland of Alaska eat when ungulates became scarce or absent. From 2012 to 2018, researchers from the Alaska Department of Fish and Game and Oregon State University collected 860 wolf scats from twelve study sites. DNA analysis of the wolf scat identified 55 food sources in the wolves’ diet. Although the study confirmed that ungulates represented roughly 65% of the wolves’ diet on a regional level, it also revealed that the kind and proportion of ungulates in their diets varied from one location to another. For instance, on the mainland, wolves’ main prey were moose and mountain goats, while on several of the islands, Sitka black-tailed deer were the main prey. As in other studies, when one of these ungulates went into decline or became scarce, wolves changed the prey that they hunted or scavenged. Surprisingly, the wolves in this study did not switch to just one or two alternate prey species as reported elsewhere, but instead expanded their dietary niche to include a variety of species that included land mammals (beaver, black bear, rodents and others), marine life (mammals and fish) and even birds.
In several other locations ungulates were not the main items on the wolf’s menu. For instance, the wolves who inhabit the area near Gustavus, on the shores of mainland Alaska, had the most varied diet. Here, moose comprised only 28% of the wolves’ diet, while sea mammals (mostly sea otters) comprised 22%. Black bears represented another 11% of their food while seasonally salmon made up 10% of their diet. The researchers suggest that the wolves opt for these other species because they are less dangerous and easier to hunt than moose. The mild coastal climate also makes possible year-round predating and scavenging of marine mammals on the shores near Gustavus.