Fauna Series No. 7

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

HISTORY OF ISLE ROYALE MAMMALS

Moose Irruption

Authors writing about the history of moose and caribou in the Lake Superior area (Hickie, n.d.; Swanson et al., 1945; de Vos and Peterson, 1951; Peterson, 1955) agreed that as the caribou population decreased from 1890 to 1910, moose, which had been scarce, became more common. By 1912 moose were "very common" in Lake County, Minn. (Johnson, 1922). Fires and logging probably benefited the moose at the caribou's expense.

Adams (1909) did not list moose as present on Isle Royale in 1905. He did mention an observation of some maples which had been broken down and stripped of leaves and bark and whose small branches had been eaten. He attributed this to caribou, but Murie (1934:10) wrote that it was probably ". . . the work of moose, for this type of feeding agrees exactly with the feeding habits of the moose and is not characteristic of the caribou." Hickie (n.d.) also believed that moose reached Isle Royale about 1905.

The popular theory is that the animals immigrated during the winter of 1912—13 when ice bridged the island with Canada. However, in 1915 the population size was estimated at 200 (Hickie, n.d.). Since moose are not herding animals, whenever they did reach the island, they probably did so in groups of one, two, or three. It seems unreasonable that there arrived enough separate groups to increase to any number near 200 in 2 years. Moreover, moose hesitate to cross even small stretches of ice, for it is difficult for them to maintain their footing there. Since moose are excellent swimmers and have been seen swimming in Lake Superior several miles from shore (Hickie, n.d.), it appears more likely that they reached Isle Royale by swimming from Canada. Indeed, P. M. Baudino of Calumet, Mich., told me that in the early 1930's in late June he observed a bull moose about half-way between Amygdaloid Island (part of Isle Royale) and Sibley Peninsula, swimming toward Canada.

Figure 15—Cow and calf swimming between islands in mid-July.

If the first moose which arrived on Isle Royale swam from Canada, they probably arrived in the early 1900's when the moose population increased substantially along the north shore of Lake Superior. By 1915, moose were well established on the island. Conditions apparently were ideal, for the herd increased to an extremely high density, as is shown in table 4. Most of the estimates presented are subjective and show only trends, but it is interesting that the figures (until 1930 when the peak was reached) fit the theoretical sigmoid curve expected when any species invades new favorable habitat.

TABLE 4.—ESTIMATES OF ISLE ROYALE MOOSE

Adolph Murie spent the summer of 1929 and spring of 1930 studying moose on Isle Royale. He (1934) found that all the winter browse species and several of the summer foods were overbrowsed and predicted that disease and starvation soon would cause an extensive die-off. According to Hickie (1936) this began in 1933. In the spring of 1934, approximately 40 dead moose were found on about 10 percent of the island; the few carcasses autopsied were emaciated. Hickie spent the winter of 1934—35 investigating the situation, and established that the browse was all but gone. Don R. Coburn, game pathologist, examined 24 carcasses, finding "little but malnutrition as the cause of death." In 1936, the population was estimated to be down to 400—500 animals. From 1934 to 1937, the Michigan Conservation Department live-trapped 71 moose and released them on the Michigan mainland. The starving animals were easy to lure into the traps (Hickie, n.d.).

Besides the harm to several species caused by overbrowsing, great damage to the balsam had been inflicted by the spruce budworm since 1929. In 1936 fire destroyed browse on more than a quarter of the island. Aldous and Krefting (1946) believed that the lowest moose population existed between 1935 and 1937.

A few years after the fire, browse was recovering in the burn, and the moose herd began increasing. In 1945 Aldous took an aerial sampling of the population and estimated that 510 moose were present (Aldous and Krefting, 1946). During the same study, an intensive browse survey led the authors to believe that Isle Royale's carrying capacity for moose had been reached.

Another aerial sampling, in 1947, produced an estimate of 600 moose (Krefting, 1951). A browse study in 1948 showed that browse was deteriorating, and another die-off was predicted (Krefting, 1951). Krefting believes that this occurred from 1948 to 1950. Several carcasses were found during these winters. The herd is estimated to have decreased from about 800 in 1948 to 500 in 1950 (Krefting, 1951).

During a month of browse investigation in the park during early 1953, Cole (1953) judged the moose food supply to be adequate. In 1956, he found that some of the browse was escaping, and he, too, believed that a marked moose reduction had started about 1949 (Cole, 1956). In early 1957, Cole attempted a complete aerial count of the moose. He observed 242 animals and estimated from tracks the presence of another 48 (Cole, 1957). (During the present study, this census technique was found to have serious limitations.)

Figure 16 —Washington Harbor. Winter headquarters for personnel involved in wolf study is located at head of harbor.

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