Seasonal Changes in the Composition of the Diets of Peary Caribou and Muskoxen on Banks Island

Seasonal changes in the composition of the diets of Peary caribou and muskoxen on Banks Island Nicholas C. Larter & John A. Nagy Caribou and muskoxen are the only ungulate species occupying Arctic tundra environments. We analysed plant fragments found in fresh (< 4 hr old) samples of faecal material to determine the diets of Peary caribou (Rangifer tarandus pearyi) and muskoxen (Ovibos moschatus) on Banks Island, Canada. Willow was a major component of the diets of both animals, dominating the caribou diet during summer and representing substantial proportions of the muskoxen diet during at least seven months of the year. The diet of caribou was more diverse than that of muskoxen and was dominated by sedge, willow (Salix arctica), legume (Astragalus spp., Oxytropis spp.) and Dryas integrifolia. The diet of muskoxen was dominated by sedge and willow. There was substantial overlap (up to 70 %) in the diets of these herbivores with the similarity more pronounced in areas of high muskox density (ca. 1.65 animals/km2). We discuss herbivore diets in relation to foraging behaviour and forage availability. N. C. Larter, Government of the Northwest Territories, Department of Resources, Wildlife & Economic Development, Box 240, Ft. Simpson, NT, Canada X0E 0N0, [email protected]; J. A. Nagy, Government of the Northwest Territories, Department of Resources, Wildlife & Economic Development, Bag Service #1, Inuvik, NT, Canada X0E 0T0. Caribou (Rangifer tarandus) and muskoxen 1978; Thomas & Edmonds 1983; Oakes et al. (Ovibos moschatus) are the only ungulate spe- 1992; Larter & Nagy 1997), Alaska (e.g. Svend- cies successfully occupying Arctic tundra envi- sen 1992; Ihl 1999; Ihl & Klein 2001), and Green- ronments. Caribou and muskoxen have differ- land (Thing 1984; Thing et al. 1987; Klein & Bay ent morphological adaptations which presumably 1990). Many diet st udies have show n willow (Salix enabled them to utilize forage resources with little spp.) to be a noticeable component of the muskox- overlap (Klein 1992). Caribou, with their small en diet regardless of season (e.g. Wilkinson et body size, narrow muzzle, small gut capacity, al. 1976; Thing 1984; Svendsen 1992; Larter & and rapid passage rate (Tyler & Blix 1990; Klein Nagy 1997; Thomas et al. 1999). In early summer, 1992), are representative of a mixed feeder type, willow use was prominent among muskoxen on intermediate between roughage feeders and con- Banks Island (Larter & Nagy 1997). Willow is centrate selectors according to Hofmann’s (1989, also a prominent component of their late winter 2000) classifi cation. Muskoxen, with their large diet especially in years of deeper and harder snow body size, broad muzzle, and large gut capac- (Larter & Nagy 1997; Thomas et al. 1999). ity capable of processing large amounts of low Lichen is an important winter food for barren- quality forage (Klein 1992) are representative ground caribou on the mainland, but in the High of roughage feeders (grazers) according to Hof- Arctic islands, which support low lichen biomass, mann’s (1989, 2000) classifi cation. caribou must use other forages, usually willow A number of studies have reported the diets and graminoids (Reimers et al. 1980; Klein 1992). of muskoxen and caribou in Arctic Canada (e.g. Where food choices are limited or muskox densi- Wilkinson et al. 1976; Parker 1978; Shank et al. ty is high, the two species may compete for food Larter & Nagy 2004: Polar Research 23(2), 131–140 131 (McKendrick 1981). On Banks Island in the Cana- 90 mm (Zoltai et al. 1980). Sachs Harbour (pop- dian High Arctic, lichen biomass is low and the ulation 125) is the only permanent settlement on muskox population represents a substantial pro- the Island. portion of worldwide muskoxen numbers, with Habitat descriptions were adapted from Kevan some of the highest reported densities of muskox, (1974), Wilkinson et al. (1976), and Ferguson 1.5 - 2.0 animals/km2 (Nagy et al. 1996; Larter (1991). There are four major terrestrial habitats: & Nagy 2001a). During the 1990s Peary caribou 1) wet sedge meadow, 2) upland barren, 3) hum- numbers on Banks Island were at historic lows, mock tundra, and 4) stony barren. Muskoxen and 500 - 1000 animals, excluding calves (Nagy et al. caribou forage in all habitats. Wet sedge mead- 1996). Peary caribou rely on willow during the ows (WSM) are generally level hydric and hygric short Arctic summers in order to regain fat prior lowlands characterized by Carex aquatilis, Eri- to winter and there was concern that the availabil- ophorum scheuchzeri, and Dupontia fi sheri. ity of willow during summer may be impacted by Upland barrens (UB) are well drained sites found the increased annual use of willows by herbivores on the upper and middle parts of slopes. Vegeta- (Larter & Nagy 1997; Larter 1999). tion is dominated by Dryas integrifolia and Salix In order to describe, in detail, the seasonal diet arctica. Hummock tundra (HT) is found on mod- composition of Peary caribou and muskoxen on erately steep slopes and is characterized by indi- Banks Island we embarked on a program to col- vidual hummocks which are vegetated primarily lect fresh faecal samples deposited on known by dwarf shrubs (D. integrifolia, S. arctica, with dates during every month of the year. We hypoth- some Cassiope tetragona). Stony barrens (SB) esized that muskoxen would feed on willow to a have a coarse gravelly substrate and are found greater extent during the growing season when in windblown areas, on ridges and on gravel and willow was of higher nutritive quality (Larter sand bars; vegetation is sparse. & Nagy 2001b) and/or during late-winter when The areas of high and low muskox density the availability of sedge (Carex spp., Eriopho- where samples were collected for this study cover rum spp.) would be lowest, due to the combina- 2700 and 18 400 km2, respectively, and have had tion of grazing and snow conditions (Larter & distinctly different densities of muskox since 1985 Nagy 2000, 2001c). We also hypothesized that (Nagy et al. 1996). Similar vegetation types and the diet composition of muskox would be dif- vegetation of similar quality are found through- ferent in areas of different muskox density. This out both areas (Larter & Nagy 2001b). Howev- paper documents the monthly diets Peary cari- er, in the low density area, vegetated habitats are bou, the monthly diets of muskoxen residing in more patchily distributed with SB being more areas of both high and low muskox density, the prominent (Larter & Nagy unpubl. data) and late- percent similarity in diet between the two ani- winter snow conditions tend to be harsher (Larter mals, and relates the monthly changes in diet to & Nagy 2001c). foraging behaviour. A more detailed description of the fl ora of Banks Island can be found in Wilkinson et al. (1976), Porsild & Cody (1980), and Zoltai et al. Study area (1980). Plant taxonomy follows Kartesz (1999). Banks Island is the westernmost island in the Canadian Arctic archipelago and covers approxi- Methods mately 70 000 km2 (Fig. 1). The climate is Arctic maritime along coastal areas where weather sta- Two fi eld camps were established in June 1993 tions are located, tending toward Arctic desert on south central Banks Island (Fig. 1): one camp inland (Zoltai et al. 1980). Winters are long and (Coyote) is located in an area of high muskox cold; mean monthly temperatures are below 0 °C density (1.6 - 1.9 muskox/km2) about 90 km ESE from September through May; mean minimum of Sachs Harbour, and the other (Bernard) in an daily temperatures range from –30 °C to –40 °C area of low muskox density (0.3 - 0.4 muskox/ from December to March. Summers are short and km2) about 130 km ENE of Sachs Harbour (Fig. cool; mean maximum daily temperatures range 1). From June 1993 to May 1998 we made six trips from 5 °C to 10 °C from June through August. to these camps annually in June, July, August, There is little precipitation; the annual mean is November - December, February, and April - May. 132 Seasonal changes in composition of diets of Peary caribou and muskoxen Fig. 1. The study area, Banks Island, Northwest Territories, Canada. The hatched areas indicate where samples were collected from the high muskox density (western) and low muskox density (eastern) areas. The location of the Coyote and Bernard camps is indicated. Larter & Nagy 2004: Polar Research 23(2), 131–140 133 We collected fresh (< 4 hr old) Peary caribou position Analysis Laboratory, Ft. Collins, Colora- faecal samples opportunistically during all these do, for analysis. Diet composition was determined fi eld trips. Additional faecal samples were col- by analysing plant fragments (Sparkes & Malech- lected during fi eld trips in March and May 1993 ek 1968) according to Hansen et al. (1976). The and January 2001, from animals killed by hunt- microhistological technique has inherent limits, ers (since 1990), and from animals taken during a such as an inability to separate some species, and collection in winter 1993–94. In total 298 samples a limited percent of identifi able fragments in the were analysed from all months, range of 10 to 49 slides (Johnson et al. 1983; Barker 1986). How- samples/month. We present monthly diet compo- ever, we deemed this method suitable for this sition pooled across sex and age classes and years study, since we were interested in the proportion- with the individual caribou as the sample unit. al dietary contribution of different forage classes, We assume inter-annual variation was minimal. not of individual species. Moreover, this method Previous analyses (Larter & Nagy 1997) indi- had been used in previous work on Banks Island cated that between-group variation was greater (Oakes et al. 1992; Larter & Nagy 1997). The diet than within-group variation in the diet of muskox.

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