Fauna Series No. 7

Fauna of the National Parks — No. 7 The Wolves of Isle Royale

DYNAMICS OF WOLF-MOOSE COACTION

Control of the Moose Population

Although the wolves are killing many moose and the herd seems stable, is it accurate to say they are controlling the moose population? It might be argued that they are not, for Isle Royale supports one of the highest year-round moose densities reported (page 98), and the yearling-total population ratio compares favorably with figures from wolf-free areas. This agrees with work by Cowan (1947) in the Rocky Mountain national parks of Canada. He found that yearlings composed 22 percent of 178 moose observations in wolf-inhabited areas and 23 percent of 187 observations in wolf-free areas. There also was no apparent difference in survival rates of young elk, deer, or sheep in the two areas. Evidently wolf predation compensates to a greater or lesser degree for other types of calf mortality. The important question is whether wolves merely substitute for other mortality factors or whether they kill more animals than other factors would.

The history of the Isle Royale moose herd affords an answer. Before wolves became established, the herd increased to an estimated 1,000 to 3,000 animals in the 1930's, decreased drastically a few years later, and built up again in the late 1940's (page 22). The limiting factor was food supply. Signs of severe over-browsing are still evident. In fact one species, Canada yew (Taxus canadensis), has been suppressed greatly on Isle Royale, whereas it grows luxuriantly on nearby Passage Island, which is uninhabited by moose. Today there appears to be sufficient browse, because much of the second-growth birch, aspen, and willow in the 1936 burn is growing beyond the reach of moose, and new stands of balsam fir and aspen a few feet high have become evident in other areas for the first time in decades. Apparently the wolves are maintaining the moose population below the level at which food would restrict it. If the wolves were exterminated, a significant increase in moose numbers probably would be noticed within a few years; when the population overtook its food supply, another die-off would occur, and the cycle would repeat itself. Malnutrition, disease, and parasitism probably would be the mortality factors, and these tend to cause catastrophic losses instead of the low, steady mortality which characterizes predation.

Apparently the Isle Royale wolf and moose populations have reached a state of dynamic equilibrium. Each is relatively stable, so any substantial fluctuation in one probably would be absorbed by the other until another equilibrium is reached. For example, wolves must travel long distances and test many animals before dispatching one. If some extraordinary factor suddenly reduced the moose population by half, the wolves probably would have such difficulty killing enough animals that inferior individuals might not be allowed to share what prey is taken. Conversely, if the moose population increased significantly, wolves would find easier hunting and might eat only preferred parts of their prey, as the wolves did in Minnesota when deer were more plentiful (Stenlund, 1955). Increased predation then might reduce the herd to a level that again rendered hunting more difficult.

Figure 104——Pyramid of biomass— based on biomass consumed only.

Probably a close predator-prey equilibrium would most likely occur in such a situation as the Isle Royale ecosystem, where populations are discrete and the wolf depends on only one prey species. Undoubtedly the low prey-predator ratio, 30 moose per wolf, also is important. In Mount McKinley National Park, where Murie (1944) concluded that wolves controlled the Dall sheep, there is an estimated 25 to 37 sheep per wolf (calculated from Murie). However, in areas where wolves do not control prey populations, the ratio is much larger. Figures from Cowan (1947) show that there are 300 to 400 head of big game per wolf in the Rocky Mountain national parks of Canada. Cowan concluded (p. 172) the following about predation in the area:

Under the existing circumstances the predators present, coyote, wolf, fox, lynx, wolverine, mountain lion, grizzly, and black bear, together are not taking the annual net increment to the game herds, nor even removing the cull group, a large part of which becomes carrion following death from disease, parasitism, or malnutrition.

In wolf-inhabited areas of Minnesota, there are about 153 deer per wolf (calculated from Stenlund, 1955), and Stenlund estimated that wolves were killing about 16 percent of the herd, much less than the annual turnover.

In British Columbia, Hatter (1950a) found that wolves could not control the irrupting moose population. Arnold (1954) reported that in Michigan, where deer greatly outnumber wolves, the wolves were not controlling the herd. In Alaska, Klein and Olsen (1960), found that deer-inhabited areas free from wolves are characterized by stable or slowly increasing populations exceeding carrying capacity; heavy winter mortality; and severely deteriorated range, whereas in wolf-inhabited areas, range and deer appeared to be in fair to good condition, with light winter mortality from starvation. The authors emphasized that factors other than the wolf may be involved.

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Last Modified: Thurs, Jul 4 2002 10:00:00 pm PDT
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