Fauna Series No. 5

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

CHAPTER THREE:
DALL SHEEP (continued)

Food Habits

The general pattern of the food habits of the sheep was established by observing their feeding, examining the vegetation where they had fed, examining stomach contents. A detailed study of the summer food habits was not made so that no doubt many minor food items are not here recorded. It was of interest to find that, although the main food consisted of grasses and sedges, considerable browse was eaten.

Figure 24: Igloo Creek is in the foreground; the far mountains are on the other side of Teklanika River. Sheep frequently descend to the creek bottom to feed on the willows. The spruce in the middle distance is at timber line on Igloo Creek. [September 1939.]

WINTER FOOD HABITS

Much of the information on the winter food habits was secured from the analysis of the stomach contents of sheep which had died on the range. The stomach contents of 75 sheep were analyzed. Of these, 54 had died in winter when no green grass was available, and 21 had died during the fall and spring, when a small amount of green grass was to be obtained. Since the only essential difference between the two series is the presence of some green grass in one of them, they have been combined in Table 3, page 76. Seventeen of the stomachs were from rams, 26 from ewes, 2 from lambs, 5 from yearlings, and 25 were undetermined.

Since most of the sheep from which these stomachs came were old or diseased it is possible that the contents are not entirely representative of the entire population. Healthy sheep might wander down in the bottoms more to feed on willow or sage, and weak animals probably would not move about a great deal in search of variety. However, the series of stomachs probably gives a fairly good picture of the winter food habits.

TABLE 3—Results of food analyses of 75 Dall sheep stomachs collected in Mount McKinley National Park, from animals that succumbed in autumn, winter, and early spring.

An annotated list of the foods known to be eaten by the sheep in winter follows:

Grasses and Sedges.—The principal winter food of the mountain sheep consists of grasses and sedges. Every stomach contained some grass, and the average amount present was 81.5 percent.

in the middle of September, when the ground was covered with 14 inches of snow, and again in October after a heavy snowfall, I noted, in following trails, that the sheep were feeding extensively on the seed heads of Calamagrostis canadensis, C. langsdorfi, Festuca rubra, Agropyron latiglume, and Trisetum spicatum. Heads of other grasses are perhaps also eaten. Such feed must be very nourishing. Considerable amounts of dry grasses were eaten by early October.

There is a good growth of grass and sedge on the winter range, and since the wind generally blows the snow off the exposed ridges, usually these foods are readily available, On the high ridges species of Poa and Festuca form much of the grass cover. Along with these highly palatable grasses there is a much-utilized short sedge (Carex hepburnii) which grows in solid stands.

Figure 25: The ewe and ram are on one of the exposed grassy ridges which are grazed closely in winter. [Polychrome Pass, May 17, 1939.]

Willow.—Willows (Salix spp.), both the tall and dwarf types, are eaten at all seasons. They were present in 63 stomachs and the average percentage in these was 10.2. The largest percentage found in any stomach was 75. The species of willow are not listed separately except in the case of one of the dwarf willows (Salix reticulata) which was noted in two stomachs. Some willows may be somewhat more palatable than others but several species among both the tall and dwarf varieties are highly relished.

Dwarf willows are widely distributed over the ridges, and the tall willows grow along the streams and in swales between the spur ridges, sometimes far up the slopes where the sheep can feed upon them without too much danger from wolves. Often in October and November sheep descended to creek bottoms to feed on willows. They also fed extensively on those growing in the swales between the ridges. Here in the fall they waded through 14 inches of snow in search of willow leaves that were still green. Dried leaves were also eaten at this time. Along with the leaves, twigs up to a quarter of an inch in diameter were eaten. Generally the leaves are picked off the sides of the twigs, sometimes down the twig a foot or two. Because willow is eaten so much during the early winter, I expected a higher representation in the winter stomachs, but as the snow deepens in the swales, willow probably is not so readily available.

Horsetail.—Although horsetail (Equisetum arvense) was found in only four stomachs, it is nevertheless highly relished. The species is found in swales and wet spots, and sometimes in good stands quite high up the slopes. Because of its occurrence In swales it is probably not readily available in winter because of snow. At East Fork at the base of the ridges there is an extensive stand which had been closely cropped in the spring of 1941. There were many sheep droppings on the area. In the fall after the snow appeared several places were noted where sheep had pawed for it through the snow. Two ewes killed by wolves on October 5, 1939, had been feeding in a swale on this plant when surprised. Their stomachs contained almost 100 percent Equisetum.

Figure 26: Ewes feeding in about 14 inches of snow. They pawed readily down to grass but fed mostly on the leaves and twigs of willows and grass heads protruding above the snow. [Igloo Creek, September 19, 1939.]

Sage.—There are at least three species of sage which are highly palatable. Artemisia arctica and A. frigida occur scatteringly and were nowhere found to be abundant.

These sages are probably eaten in winter although none was recognized in the stomach contents. In late fall they were found much eaten. The tall herbaceous sage, Artemisia hookeriana, is highly palatable at all times. It is most abundant along streams and reaches far up the slopes in the draws between ridges. It often grows in dense clumps. In September and October, when the leaves and stems were dry, the sheep frequently fed on this herbaceous sage in the creek bottoms. Tracks at Big Creek showed that a band had crossed the creek and fed on a sage patch near the mouth of a ravine. In the high draws at East Fork this sage was uniformly closely browsed by spring. It was found in 20 of the stomachs and averaged 5.4 percent of the contents. The greatest amount of it found in a stomach was 35 percent of the contents.

Cranberry.—Cranberry (Vaccinium vitis-idaea minus) is an abundant evergreen widely distributed over the hills. Although eaten in only small quantities, it apparently is frequently sampled. It was found in 30 stomachs. Usually only a trace was noted, but in one stomach it made up 70 percent of the contents. In October, when a variety of food was available, the sheep were several times observed feeding on cranberry.

Blueberry.—Blueberry (Vaccinium uliginosum) was found in eight stomachs, making up 60, 40, and 15 percent of the contents of three of them, and 1 percent of the contents in each of five others. The berries, leaves, and twigs were eaten, but the berries were especially sought. The stomach of a lamb which was killed on October 4, 1939, contained 40 percent blueberries. On that date a band had been feeding extensively on blueberries. On October 7 and 12 it was again noted that sheep had fed heavily on them. Some of the berries had been picked up from the ground. The deterioration of the berries as the season advances, and their reduced availability because of snow, probably is the explanation for the relatively few stomachs containing blueberry in the winter.

Crowberry.—Crowberry (Empetrum nigrum) was present in 14 stomachs, making up 30 percent of the contents of one stomach, but on the average comprising 2.7 percent of the contents. The twigs and leaves of this evergreen were found in the stomachs, but very likely the berries were also eaten, It was found browsed in early October. Perhaps it would be eaten more extensively if it were more abundant on high ridges and grew taller. Crowberry is most plentiful on the lower slopes.

Figure 27: A swale, between lateral ridges of Igloo Mountain, where sheep often fed in the fall on the willow which protruded through the snow. [September 19, 1939.]

Mountain Avens.—Mountain avens (Dryas spp.) was found in 19 stomachs averaging 0.4 percent of the contents. There are about five species of dryas in the park. Some grow mainly on the old river bars where they form extensive mats, while others are found on the ridges. The principal cover of many slopes is dryas, consequently these slopes are poor winter range for sheep. Because it is much eaten in summer it has sometimes been referred to as an important winter food. However, in winter it becomes dry and brittle and loses much of its palatability.

Loco.—Loco (Oxytropis) was found in seven stomachs. Some species were highly relished in the fall while the leaves were still green. The remains found in the stomach contents consisted of the basal part of the plant nipped off at the root. Dry leaves may have been present in other stomachs and escaped detection.

Hedysarum.—Both Hedysarum americanum and H. mackenzi are highly relished in the fall but none was found in the stomachs. The seed pods were often noted eaten in September.

Dwarf Birch.—Dwarf Arctic birch (Betula nana) was found in only two stomachs, The leaves and fine twigs of dwarf birch were noted eaten in late September. It is not readily available on the high ridges.

The following species were found in the stomachs in small quantities: Saxifraga tricuspidata, Ledum groenlandicum, Loiseleuria procumbens, Potentilla fruticosa, Rhododendron lapponicum, Alnus fruticosa, Arctostaphylus uva-ursi, Aster sp., Cladonia sp., leaf lichen sp., and moss.

The following species were found eaten in late fall: Heracleum lanatum. (seed heads and leaves), Delphinium sp. (when dry), Shepherdia canadensis (mainly berries), and Mertensia sp.

SUMMER FOOD HABITS

In summer, as in winter, the principal food of the sheep Consists of grasses, sedges, and willows. However, at this season much Dryas is also eaten, and a variety of other species are eaten in small amounts. Frequently sheep were seen feeding on the leaves of willow. For example, on June 11, 1940, 36 sheep fed steadily on two species of willow for 40 minutes, and on June 20, 1940, a band fed for a half hour on willows. On some of the higher slopes where the shrubby willows are absent there is an abundance of dwarf willows only a few inches high which are grazed extensively. Some species of Oxytropis and Hedysarum are much sought. The leaves and flowers of herbaceous cinquefoil, Aconitum sp., Campanula lasiocarpa, Arnica, and Conioselinum. were eaten.

The contents of one stomach, that of a 2-year-old ewe that died on June 8 as a result of a snag injury, were available for analysis. The following determinations were made: green grass, 85 percent; willow, 14 percent; Oxytropis sp., trace; Pyrola sp., trace; and Salix reticulata, trace.

MINERAL LICKS

Sheep were often seen at licks, and well-defined trails led to some of them. A lick on Ewe Creek was much used, and two in Teklanika Canyon, similar to the preceding, were used extensively. On East Fork the sheep visited a lick on the side of the slope where water seeped out. The mud in this lick was black, while in some of the others the material was gray. On Double Mountain a lick was used by both sheep and caribou. On Toklat River two licks were also found in use.

Above the Ewe Creek lick many sheep droppings were found which were made up entirely of clay, so it is evident that considerable quantities of the earth are eaten at times.

Dixon (1938, p. 223) had samples of the Ewe Creek lick analyzed by Dr. G. L. Foster of the Division of Biochemistry, University of California. He reported that calcium and iron phosphate were the two minerals present in the lick which would be soluble in digestive fluids. Insoluble substances, chiefly magnesia and silicates, were also present.

Carrying Capacity of the Winter Range

After observing so many overgrazed big-game ranges in the States it was a pleasant experience to find the sheep range in Mount McKinley National Park not overutilized. By spring the grasses on the more accessible ledges are closely grazed but they are not harmed even in these places where grazing is concentrated. Willows are not now overutilized, although in years past, when sheep were more abundant, patches on the ridges were killed off by overbrowsing.

There are now from 1,000 to 1,500 sheep using the winter range which comprises an area of between 200 and 250 square miles. But of course not all of this area is a source of food. Many cliffs and rock slides support little growth; the vegetation on many slopes, ravines, and stream bottoms is deeply covered with snow; and extensive areas have a cover which is mainly Dryas, thus furnishing little winter grazing. Hence, without an extensive survey of cover types, it would be difficult to determine the amount of range actually available to the sheep in winter, and this amount would fluctuate from year to year because of varying snow conditions.

Figure 28: Cathedral Mountain in foreground; south extension of Double Mountain on center sky line. Dryas predominates on so many slopes that Cathedral Mountain is a rather poor winter range for Dall sheep. [September 1939.]

The carrying capacity of the winter range is several times the number of sheep now present in the park. As will be discussed later in detail, it is the wolf, rather than the food supply, that appears to be the chief factor limiting the size of the sheep population. The wolf reduces the size of the sheep range by confining sheep to the more rugged country where they are less vulnerable to wolf attack than elsewhere.

In the rugged country, the wolf preys on the sheep sufficiently to keep them below the carrying capacity of the range as determined by food supply. Thus the present sheep numbers apparently are dependent on the extent of cliff protection and the degree of wolf pressure.

Continued >>>

Last Modified: Thurs, Dec 20 2001 10:00:00 pm PDT
http://www.nps.gov/history/history/online_books/fauna5/fauna3d.htm