NATIONAL PARK SERVICE Fauna of the National Parks of the United States No. 3 Birds and Mammals of Mount McKinley National Park

FLUCTUATIONS IN ANIMAL POPULATIONS

UNDER natural conditions in a primitive area such as Mount McKinley National Park there is a constant shifting of animal populations. The so-called "balance of nature" is fluctuating rather than static. For this reason the visitor who goes to the park should not be surprised if he finds individual birds or mammals of certain species more abundant or, as it may be, less numerous than previously reported by other observers.

For example, it is a matter of common knowledge that in the far north there are regular cyclic fluctuations of abundance in meadow mice, lemmings, and especially in the varying bares. In the ease of the bares, data over a long period of years indicate that peaks of abundance occur about every 10 or 11 years with intervening periods of decreasing and increasing numbers. Following the increase in rabbits and mice, which have short periodic fluctuations, there is normally a gradual increase in predatory birds, such as owls, and mammals, such as Canada lynx. With the decrease or disappearance of rabbits, probably due to disease, the owls are forced to starve or move to other localities where food is available. Canada lynx are likewise reduced by failure of food supply.

In Mount McKinley National Park weather conditions may also be a contributing factor to such fluctuations of animal populations. As has been said, during our first season of field work an early spring and a warm dry summer were encountered. These conditions are believed to have contributed to the presence and breeding of such species as the Kennicott's willow warbler (Acanthopneuste borealis kennicotti) and the surfbird (Aphriza virgata), for in 1932 both of these species were absent from localities where previously they had been found breeding and had been seen regularly. As a matter of fact, in 1932, it would have been difficult for certain ground-nesting birds, such as the surfbird, to have nested and successfully reared broods of young, because of heavy, late spring snowstorms (fig. 7). On June 14, 1932, 4 to 6 inches of wet snow fell and lay for several days in a thick freezing blanket over the surfbird's and pipit's habitat. in 1926, no robins were found nesting on the ground under overhanging protecting banks, whereas in the same area, after the winter of heavy snows and prolonged cold weather, such ground nestings of robins were not unusual, especially in the latter part of the breeding season following the destruction by severe storms of unprotected nests in open willows.

Figure 7.—The winter of 1931-32 was severe and the spring late; snow fell to a depth of 4 to 6 inches on June 14. Many species of birds, including the surfbird, were absent from localities where in 1926 we had found them nesting.
Photograph taken May 29, 1932, Savage River. W. L. D. No. 2607.

The cyclic fluctuations in numbers of varying hares have been marked in McKinley Park. In the summer of 1926 these hares were abundant over all the wooded portions of the park that were visited. In certain favorable spruce woods, hares were estimated to average 3 or 4 to the acre, but over most wooded areas they averaged about 1 to the acre. The peak of abundance was apparently reached that season. By spring, the hares were so numerous that all young aspens and willows had been gnawed off and even a spruce tree that had blown over during a storm had been denuded of all foliage (fig. 8). With the abundance of these bares, hawk owls were coincidently abundant; however, repeated search both during the day and the night over these same areas in 1932, failed to reveal a single hare or hawk owl.

Figure 8.—This wind-thrown spruce was quickly denuded of foliage, and even of bark, by numerous hungry Mackenzie varying hares.
Photograph taken May 22, 1926, Savage River. M. V. Z. No. 5004.

Charles Sheldon (1930, p. 120) states that mice of several species (Microtus, Evotomys and Lemmus) were incredibly abundant in 1907 in the Toklat region in McKinley National Park, so that "the surface was almost everywhere covered with mice . . . traps set on the [river] bars would in less than half an hour contain a mouse, which, unless secured immediately, was eaten by others." Sheldon found that in 1908 mice were scarce and rabbits were at the periodical minimum—most of them having died. Dr. C. Hart Merriam states (Sheldon, 1930, p. 329): "In the case of the rabbit, the decrease is due to disease; in the case of the lynx, to starvation augmented by defective reproduction."

In 1926, we found meadow mice of three species and red-backed mice widespread and abundant in the McKinley district. On June 11 on the Savage River, at 2,800 feet elevation, meadow mice were so abundant in the open meadows bordering the river bars that many short-billed gulls were observed each morning and evening stalking and capturing half-grown meadow mice which are not very suspicious and which run about freely during the early and late hours of the day. By remaining motionless the gulls deftly picked up the mice as they sped along their runways. Careful counts of used burrows checked by actual sample trappings showed that in these favored localities there were at least 50 mice per acre present over considerable areas.

As late as May 30, 1932, when I revisited this area, I found that most of it was still covered with a solid blanket of drifted snow from 4 tot 6 feet thick (fig. 9). In late June when this meadow became free of snow I went over it again thoroughly but failed to find a single fresh mouse burrow or other "sign." In other localities in McKinley Park where we had found mice numerous in 1926, a thorough search in May and June 1932, showed that mice were either exceedingly scarce or entirely absent.

Figure 9.—This meadow, which was teeming with Toklat River voles on may 30, 1926, was covered with 6 feet of snow when revisited on may 30, 1932, and no voles were present.
Photograph taken May 30, 1932, Savage River. W. L. D. No. 2767.

Estimates of numbers of birds and mammals are based upon recorded daily counts of individuals seen. Since most of the country is open tundra, visibility is good and an accurate count is therefore possible. In the case of the larger mammals such as caribou, grizzlies, red fox, and Dall sheep, certain routes covered in 1926 were repeated in 1932, care being taken to go over the identical routes and to count the mammals. As the same observer covered identical areas on nearly the same date of the month, it is believed that quite comparable counts were made. For some of the smaller mammals, systematic trapping was employed. In 1926, certain areas were selected that were typical shrew and meadow-mouse habitats. Plots were laid out containing, by observation, average populations, and check plots nearby were also selected. On the first plots it was attempted, by intensive trapping, to capture all of the mammals in the particular area. This number trapped on a given area of known size was used as an index to estimate the number of individuals per acre. In the case of ground squirrels, pikas, and marmots, it was possible, with binoculars, to make a fairly accurate count of the population on a given area. In 1932 these same plots were again visited and checked by trapping or by counting as had been done 6 years previous.