Subsections:
Winter Habitat Use of Muskoxen: Spatial Scales of Resource Selection
Summary
References

During the snow season, which lasts up to 9 months in the Arctic Refuge, muskoxen remain in small areas, restricted by the availability of forage and by strategies needed to conserve energy (Reynolds 1998b). Human disturbance or destruction of their habitat could displace muskoxen from these limited wintering areas.

To determine what kinds of sites are used by muskoxen in late-winter and why these sites are selected, we set the following research objectives: 1) determine selection of vegetation types based on use and availability, and 2) compare snow depth and hardness, vegetation biomass, and environmental variables at feeding and non-feeding sites. Our study sites were located in the eastern half of the Arctic Refuge coastal plain and 1002 Area, between the Jago River and the Kongakut River.

Fieldwork was conducted in March, April, and July 1989-1990 at 44 late-winter foraging sites used by muskoxen. These sites included feeding zones (with feeding microsites or craters), non-used adjacent zones (contiguous to the feeding zone), and non-used nonadjacent zones (100 m beyond the adjacent zone). At each foraging site, a randomly-oriented transect was laid across the site, passing through the center of the feeding zone and defining unused zones.

Foraging sites were located from observations and tracks of muskox groups in March and April. Fecal pellets were collected for diet selection analysis. Winter foraging sites were relocated the following July when vegetation and environmental characteristics along transects were measured (Wilson 1992). Snow conditions, environmental characteristics, forage cover, and non-vegetated cover were included in an analysis of variables related to habitat selection.

In late winter, muskoxen fed most commonly in moist sedge tundra (37%) and tussock sedge tundra (37%) and used these types in proportion to availability. Dryas terrace (9%), riparian grass forb gravel bars (7%), wet sedge (5%), partially vegetated tundra (2%), and shrub tundra (2%) were selected less frequently than their availability. Muskoxen were not observed feeding in riparian shrub, Dryas ridge, barren ground, or water (Wilson 1992).

Total vegetation cover was greater in feeding zones than in unused adjacent and non-adjacent zones. Cover of evergreen shrubs, sedges, and dead vegetation was greater in feeding zones. Non-vegetative cover was greater in adjacent and nonadjacent zones (Wilson 1992).

Diet selection based on fecal analysis of winter pellets (corrected for digestibility) indicated a high use of sedges (39.1%) and mosses (24.6%) (Wilson 1992). Sedges and grasses were selected (use > availability); and horsetails, lichens, willows, and other shrubs were avoided (use < availability). Although selection for grasses was high, grasses did not make up a large proportion of the diet or the available habitat.

Analysis of rumen samples indicated that sedges (31%), grasses (19%), mosses (15%), and forbs (13%) comprised most of the diet. The proportion of willows was 8% in rumen samples. In other studies, riparian willows were used by muskoxen in late winter (O’Brien 1998, Robus 1991). During our study, however, snow limited the use of most riparian shrub communities. Willows were browsed in areas where they protruded through the snow (Wilson 1992).

Snow depth was shallower and softer in feeding zones than in nonadjacent zones, and shallower in feeding zones than in adjacent zones (Fig. 7.7). Snow depth and hardness were less in microsites than in unused portions of feeding sites. Snow depth was the single variable most influential in discriminating between used and non-used areas. Muskoxen also appeared to avoid walking through areas of soft deep snow. Most feeding zones were near some type of topographic relief that had been subjected to wind scarring. Snow depth was shallower in feeding zones of tussock sedge tundra and moist sedge tundra, suggesting that within vegetation types, muskoxen chose feeding zones based on snow depth alone. No differences were detected between feeding zones and adjacent and nonadjacent zones in wetness, slope, micro-relief, or aspect (Wilson 1992).

Figure 7.7. Snow depth (cm) in muskox feeding zones (areas containing feeding sites), adjacent zones (unused areas adjacent to and surrounding feeding zones) and nonadjacent zones (unused areas 100 meters beyond adjacent zones) in late winter 1989 (n = 20) and 1990 (n = 24) on the coastal plain of the Arctic National Wildlife Refuge, Alaska, USA. (from Wilson 1992)

Snow depth in feeding zones was deeper in 1989 than in 1990 (34.5 cm versus 23.0 cm, respectively), and total vegetation cover was greater in 1990 than in 1989 (Wilson 1992). In 1990, forb cover was greater in feeding zones compared with a greater cover of sedges and non-vegetative material in 1989 feeding zones. Muskoxen did not select for areas of high total vegetative cover in 1989, indicating that detection of vegetation characteristics may require shallow snow cover. In 1989, mean crater depths (29.7 cm) and mean feeding zone depths (34.5 cm) approached or exceeded the maximum snow depths in feeding areas (>30 cm) observed in other muskox studies (Rapota 1984, Smith 1984).

Partially vegetated tundra and Dryas terraces had the shallowest snow; the deepest snow occurred in shrub tundra and moist sedge tundra. Gravel bars with riparian forbs and grass had the greatest total cover of vegetation; moist sedge tundra had the least. Muskoxen selected feeding zones with shallower snow and greater vegetation cover compared with what was available (Wilson 1992).

Winter habitat for muskoxen is limited in quantity because animals must select foraging areas with shallow soft snow and a high cover of vegetation. Areas with little vegetation or deep hard-packed snow were not used. In this study, feeding zones were primarily along narrow bands of windblown vegetated bluffs adjacent to creeks, rivers, and the coastline, reflecting the importance of terrain features to habitat selection (Nellemann and Reynolds 1997).

Snow depth was one of the most important variables distinguishing used and unused area in this study of muskox habitat (Wilson 1992). Snow depth influences the availability of forage and can limit accessibility to some forage types (Evans et al. 1989). Snow depth affects energy budgets. Digging through snow to find forage is energetically costly for ungulates (Fancy 1986). The time lost while digging craters also reduces the daily rate of forage intake (Fleischman 1988).

In high snow years, when some habitats are not available and muskoxen spend more energy moving and foraging, muskoxen may be energetically constrained, resulting in lower survival and less successful reproduction. As winter progresses and snow accumulates, or if deep snow falls early in the winter, muskoxen may be forced to select foraging areas with deep snow or low plant biomass.

If muskoxen are limited in their accumulation of body reserves during summer, effects of a severe winter or overuse of winter range will have greater impacts on reproductive success and survival. If, in addition, animals are disturbed by human activities and cannot optimally use available habitats, the effects of a severe winter likely will be magnified.

Activities associated with the extraction of petroleum resources on the coastal plain of the Arctic Refuge have the potential to displace muskoxen into areas of deeper snow where forage availability is low and energetic costs to procure food are high. Displacement from, or permanent loss of, limited winter habitat could affect reproductive success and survival of muskoxen on the coastal plain of the Arctic Refuge.

To minimize potential effects of petroleum exploration and development on muskoxen in the Arctic Refuge, areas occupied by muskoxen in winter should be avoided; and areas of potential winter habitat should not be selected as sites for permanent facilities.

Muskoxen are year-round residents of the 1002 Area on the coastal plain of Arctic National Wildlife Refuge. Numbers of muskoxen in the Refuge have declined over time with <300 currently living on the coastal plain including <250 in the 1002 Area. Calf production has also declined over time.

Severe winters (deep snow and prolong snow seasons) and increasing rates of predation are important factors in the dynamics of this population. Muskoxen have expanded their range east and west of the Arctic Refuge coastal plain and emigration has contributed to declining numbers.

Most calves are born in April and May, several weeks before green forage is available. To survive the long months of winter and to maintain body reserves needed for successful reproduction, muskoxen conserve energy in winter by reducing activity and movements. In winter, muskoxen feed on dried sedges and other low quality forage in areas of low snow. Windblown ridges adjacent to rivers are frequently used in winter. During the short weeks of summer, when green forage is available, muskoxen increase their movements and activity and feed on a variety of high quality forage to regain body weight before the next winter. River corridors and nearby uplands are often used by muskoxen in summer.

Muskoxen in the Arctic Refuge are vulnerable to disturbance from activities associated with petroleum exploration and extraction because of their year-round residency, their small population numbers and their need to conserve energy for the 9 months of winter if they are to successfully reproduce.

Disturbances that displace muskoxen from preferred winter habitats into areas of deeper snow or that increase their activity and movements could significantly increase their energetic costs in winter. Female muskoxen that are required to expend greater energy to survive the winter will have fewer reserves for pregnancy and lactation and may not reproduce successfully. Muskoxen frequently use habitats along or adjacent to rivers. These locations may be sites for gravel and water extraction and winter road construction if petroleum development is permitted in the Arctic Refuge.

Avoidance by industry of areas used by muskoxen and the location of permanent facilities away from river corridors, flood plains, and adjacent uplands could reduce the probability of disturbance and displacement of muskoxen. Status and distribution of muskoxen in and near the 1002 Area should be monitored to document future trends.

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| Home | Section 1 - Introduction | Section 2 - Land Cover | Section 3 - Porcupine Caribou Herd |
| Section 4 - Central Arctic Caribou Herd | Section 5 - Forage Quantity and Quality | Section 6 - Predators |
| Section 7 - Muskoxen | Section 8 - Polar Bears | Section 9 - Snow Geese | Acknowledgements |