The dynamics of caribou and muskoxen foraging in arctic ecosystems

Abstract

Forage system characteristics Numbers of caribou (Rangifer tarandus) and muskoxen (Ovibos moschatus) in the Northwest Territories, Canada have changed rapidly in the last few decades (e.g. Gun et al. 1991). Our ability to respond to questions raised by those rapid changes is, however, compromised by our ignorance of the dynamics of the relationship between arctic ungulates and their forage which in turn inhibits our understanding of the population dynamics. The arctic ecosystem is relatively simple in terms of species diversity and herbivore numbers fluctuate against a background of unpredictably variable weather: a simple and changeable system is more easily stripped down to its essential components to describe their interactions. Arctic ecosystems have three dominant attributes which are the salient points in my conceptual model proposed here to unravel the population dynamics of arctic herbivores, (a) Plant productivity is restricted to brief annual pulses; (b) the weather is highly unpredictable and (c) most of the biomass is below ground locked into a slow cycle of decomposition. The nutrient input in tundra ecosystems is low and mostly slowly cycled through soil organic matter at the characteristic cool ambient temperatures. Plants depend heavily on the internal recycling of nutrients from storage tissue below ground. The tie-up of plant nutrients in detritus opens an accelerating role for herbivory in nutrient recycling animal fecal material breaks down and releases nutrients to the plants more rapidly than the overall detritus pool (Dowding et al. 1981). Muskoxen which forage in herds, graze intensively and deposit large amounts of dung in situ, wil l have a larger role in nutrient recycling than caribou within the plant communities where they forage. Caribou forage in smaller groups and graze extensively while walking. Arctic plants have a pulse of growth restricted by temperatures to a brief season. The onset of plant growth varies by weeks (Svoboda 1977, Muc 1977). Most plants are conservative and complete their growing season within a fixed period despite annual variations in the duration of the potential growing season. Flowering varies between years and is depressed by as much as 80% following a late cool season but productivity of green plant biomass in meadows is less responsive to summer temperature (Muc 1977, Svoboda 1977).