Fauna Series No. 5

Fauna of the National Parks — No. 5 The Wolves of Mount McKinley

CHAPTER THREE:
DALL SHEEP (continued)

Some Wolf-Sheep Relationships

EFFECT OF ARTIFICIAL INTRUSIONS ON WOLF PREDATION

In a national park the objective is to preserve a piece of primitive nature where natural interrelation ships may prevail. The more complete the biotic unit and the larger the area the greater is the opportunity to achieve that ideal. But unfortunately few national parks are large enough to be uninfluenced by artificial activities taking place both within and outside their boundaries.

Figure 47: At Polychrome Pass the high way traverses the heart of excellent winter range for mountain sheep. This affords wolves ready access to the range, and an opportunity to surprise sheep at sharp curves on the road. [October 22, 1939.]

In Mount McKinley National Park the automobile highway passes along the sheep hills for some 70 miles. This road gives the wolves a special advantage in that they have an easy trail to different parts of the sheep range. In several places the road winds among the cliffs used by sheep in winter, saving wolves a laborious climb. Because of the many sharp turns in the road the wolves have an opportunity to surprise the sheep at close range. These advantages to the wolves are not especially great because the road affects vitally only a small part of the sheep range. This illustrates the need for carefully examining all proposals for park developments.

Another activity which intrudes on the natural conditions within the park is the trapping of wolves on lands outside the park. Trapping adjacent to the park boundaries, and a certain amount of control within the park, annually has eliminated some of the park wolves. The effect on the wolf-sheep relationships is not well known, but probably such trapping does not take the annual increase during most years or greatly alter the wolf population. The status of the wolf within the park probably is, at the present time, similar to the status of the wolf in Alaska as a whole.

Still another artificial intrusion is the shooting of caribou adjacent to the park. Although this hunting does not now appear to be important it apparently has been extensive in the past. The reduction or curbing of the caribou herds by shooting might increase the wolf predation on the sheep by leaving a smaller caribou population.

The effect of the artificial intrusions will have to be carefully evaluated in considering the conservation of the wildlife in the park.

DO CARIBOU DIVERT PREDATION FROM SHEEP?

Frequently there is serious direct competition for food between ungulates. Although the food habits of sheep and caribou overlap broadly, their ranges, especially in winter overlap very little. Generally the caribou feed on the lower hills and rolling terrain where lichens are more abundant, and the sheep are up on the wind-swept ridges.

The caribou is an exceptionally good example of a buffer species. This is especially noticeable in the spring. When the caribou calves make their appearance they furnish to the wolves a large and easily obtainable food supply. At this time wolf predation on sheep practically ceases.

The caribou also serve as a buffer in winter. There are some suggestive correlations between wolf predation on sheep and the presence or absence of caribou in the vicinity of the sheep hills in winter. In a year that caribou wintered among the sheep hills the predation on sheep appeared to be less than when caribou were absent. In the winter of 1938—39 between 1,500 and 2,000 caribou wintered between Teklanika and Savage Rivers and others wintered along the north border of the park. That year there was a high survival of yearlings among the sheep. In the winter of 1939—40 there were practically no caribou in the sheep areas. The survival of sheep yearlings that winter was about half of what it had been the preceding winter (assuming that lamb crops were about equal). In the absence of the caribou the wolves turned to the sheep. However, crusted snow conditions also favored the wolf in his sheep hunting in the winter of 1939—40, so the two winters are not strictly comparable. The 1940 lamb crop was so small that one can not draw many conclusions from the figures. There was a heavy winter mortality among the few lambs present even though caribou were present to act as a buffer. Wolf pressure on individual lambs may have been higher than usual because there were so few lambs.

Although the caribou are very definitely a buffer species at times, there is still a question whether or not they increase or decrease the wolf predation on the sheep over a period of years. It is likely that because of the presence of the caribou, the sheep range supports a larger wolf population. True, when caribou are present they probably bear the brunt of the predation. But when caribou winter elsewhere, the wolf population, whose size is probably adjusted to their presence, is, so to speak, left stranded, and dependent on sheep for sustenance. The majority of the wolves in the park do not move off with the caribou as some persons have assumed. Wolves are accustomed to obtaining their food in the one territory. Thus it appears that the caribou herds, because of their periodic absences from the sheep ranges, may influence predation in such a way as, in the long run, to lower the size of the sheep population. But because of the small amount of information on this point our conclusions must be tentative. There is no doubt, however, that the caribou at times is a highly important buffer species for the sheep.

HISTORICAL EVIDENCE THAT WOLVES ARE CURBING SHEEP NUMBERS

During the period of wolf scarcity beginning sometime between 1908 and 1916 the sheep prospered, in fact built up to what apparently was an overpopulation about 1928. Unusually severe snow conditions in 1932 reduced the population which already had been depleted by the hard winter of 1929. According to one observer, not more than 1,500 sheep remained. This population was not far from that present on the range in 1941, 9 years later. We do not know definitely the number of sheep in 1932 after the die-off but it probably was not less than at present. During this 9-year period, so far as we know, a rather uniform wolf population existed and it is known that wolves preyed extensively on sheep. The fact that sheep increased rapidly in the absence of wolves and have not increased during their presence strongly indicates that the wolves have been the factor preventing the sheep from increasing. I am fully aware how frequently "obvious conclusions" are wrong, especially in prey-predator relationships. However, I found no other factor which seemed sufficiently operative to hold the sheep numbers in check. It seems therefore, that the wolves are the controlling influence. Granting that the wolves have held the sheep in check for several years, that does not necessarily mean that the sheep will not increase or decrease in the future in the presence of the same wolf population. The presence and numbers of the caribou is an uncertain factor which influences wolf predation on sheep so the yearly movements of the caribou and their status must be considered. Furthermore, the abundance or scarcity of snowshoe hares may have an important bearing on relationships. Other factors, of course, may change to alter the relationship, so one cannot predict with certainty the future relationships and population trends.

The historical data corroborate the evidence obtained in the field study that wolves are the chief factor limiting the sheep population at the present time.

EFFECT OF THE WOLF ON SHEEP NUMBERS

At the time that the sheep population was at its maximum many of the sheep lived away from the traditional rocky habitat and occupied low hills and even high banks along the rivers. The use of this gentle terrain was probably necessary because of food shortages in the rugged hills. In the absence of the wolf they were able to survive in the areas lacking protective cliffs. But when the wolf appeared and became plentiful the sheep in these places became so vulnerable that no doubt the strong as well as the weak could be captured. In the presence of the wolf, therefore, the range of the sheep was gradually constricted, and they were confined to more rugged habitats where they could cope with the wolf on better terms. in confining the sheep to the cliffs, the wolf is an important factor in molding their habits, and through the past ages probably has done much to develop and preserve the sheep as we know them today.

The relationship between wolves and sheep probably differs according to the types of terrain. Where the cliffs are exceptionally rugged and extensive a sheep population might be largely immune from wolf attack until it becomes so large that it over flows into less rugged surrounding country. Where cliffs are only moderately rugged or not very extensive, the sheep might remain rather low in numbers, perhaps at a point where the law of diminishing returns makes it unprofitable for the wolf to hunt them; in some unfavorable cliffs perhaps all the sheep would be destroyed. Therefore we would expect to find considerable variation in the abundance of sheep, in the presence of wolves, in different sheep areas, due primarily to the character of the country.

Since the wolves appear to hold the sheep in check it is of interest to discuss how predation, which apparently is mainly upon the weak, operates to control the population.

The predation on the old-age animals and those weakened by disease curbs the numbers to some extent. A few of the old ewes, although weaker than the average and doomed to die in a year or two from old age, might still bring forth a healthy lamb if not eliminated by the wolf. The same is probably true for some of the sheep suffering from malformed dentition as a result of disease. Predation on the healthy sheep, captured by chance or otherwise eliminates a few members of the population and their potential offspring. But this predation does not appear to be as important as that on the yearlings.

From the skull study it could not be determined how much the wolves increased the death rate among the yearlings. Comparative studies in periods of wolf scarcity would be enlightening in this regard. But it was evident that the losses among the yearlings in the winters of 1939—40 and 1940—41 were so great that, combined with all other losses, they resulted in a reduction in the total sheep population. It appears to me that the large losses were due mainly to heavy wolf predation. The lamb and yearling losses in mountain sheep herds in Wyoming, in the absence of any serious predator, are sometimes fully as great as in Mount McKinley National Park. But in Wyoming it was evident that the young were diseased for they coughed considerably in late summer, and many died during the summer and fall. This disease is probably correlated with poor range conditions. In Mount McKinley National Park the young sheep appeared healthy at the beginning of winter. No coughing or other evidence of disease was noted excepting actinomycosis which would probably affect only a nominal number of animals. On the other hand, the wolf was known to prey upon the sheep extensively at times. From the total observations made, the conclusion seemed evident that large losses among the yearlings are caused by wolf predation at times, and that these losses are probably most important in holding the sheep numbers in check.

THE SHEEP AND THE WOLVES MAY NOW BE IN EQUILIBRIUM

It is unfortunate that we do not have accurate information on sheep numbers since the die-off of 1932 so that we would know more about the trend of the population. One ranger thought that the low was reached about 1936, and that in 1939 there was a definite increase over the past few years. These thoughts were based on general observations. In 1940 and again in 1941 there were decreases. My impression is that the sheep numbers have not varied greatly since 1932. If this is actually the case then it is probable that the sheep and the wolves have reached an equilibrium, with, of course, some ups and downs. If we were assured that there would not now be any striking reduction in sheep numbers, the wolf-sheep relationship might be considered a satisfactory one, especially to anyone who has seen the ungulate over-populations and the over-grazed big game ranges in the States. Artificial intrusions may give the wolf an undue advantage over the sheep, hence it is desirable to continue observations in order that the status of the sheep may at all times be known and proper steps taken when needed to assure their perpetuation under as natural conditions as possible.

Last Modified: Thurs, Dec 20 2001 10:00:00 pm PDT
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