Foraging Ecology and Population Dynamics of Collared Pikas in Southwestern Yukon

THE FORAGING DECISIONS MADE BY HERBIVORES influence population dynamics through their effects on energy gain, energy expenditure, and ultimately, survival (McNamara and Houston, 1997). In turn, foraging by herbivores may influence the amount of vegetation available for the future. Therefore, herbivores and vegetation often are coupled in a strong reciprocal relationship: the abundance of one affects the abundance of the other through time (e.g., Caughley, 1976). This interaction is particularly important for herbivores living in seasonal environments where food abundance and quality vary dramatically between growing and winter seasons. Herbivores must adapt their foraging behaviour to contend with these changes and survive until the following growing season (Owen-Smith, 2002). In addition to seasonal effects, daily foraging decisions are constrained by a number of other factors, which may be classified as either internal (such as energetic or nutritional needs), or external (such as predation risk or interspecific interactions). These constraints limit the availability of food items to the herbivore and explain why most species exhibit some degree of diet selectivity. Most species, therefore, must balance trade-offs associated with these constraints such as obtaining enough food while minimizing predation risk (Lima and Dill, 1990), or decisions about selecting species with differing degrees of nutritional value (Stephens and Krebs, 1986). I am using a combination of descriptive and experimental methodologies, combined with modelling techniques, to increase our understanding of plant-herbivore interactions and their influence on herbivore population dynamics, using the collared pika (Ochotona collaris) as a model species. Pikas are small (~160 g) lagomorphs that live in alpine talus slopes. Pikas are asocial and each individual collects vegetation for two distinct diets: a summer diet that is consumed immediately, and a winter diet that is stored in haypiles under boulders (Millar and Zwickel, 1972; Conner, 1983). Because of predation risk, pikas rarely venture more than 10 m from talus into meadows to collect vegetation (Holmes, 1991), and this behaviour creates a strong gradient of grazing pressure, with most foraging activity occurring within 3 m of talus (Huntly, 1987; McIntire and Hik, 2005). Insatiable hoarders of vegetation (McKechnie et al., 1994), pikas provide an opportunity to test experimentally hypotheses regarding diet selection as a function of various constraints. Since 1995, most individuals (> 95%) within the study area have been captured, weighed, sexed, and marked with uniquely coloured ear tags. This detailed, long-term demographic dataset will provide context for this study and a means of validating population models. The objectives for this project are 1) to determine how collared pikas make foraging decisions with respect to predation risk and forage quality, 2) to incorporate these constraints into a plant-herbivore modelling approach to determine their effects on population dynamics, and 3) to compare the plant-herbivore model to spatial demographic models of population dynamics.