You can hide but you can't run: apparent competition, predator responses and the decline of Arctic ground squirrels in boreal forests of the southwest Yukon

Throughout much of North America's boreal forest, the cyclical fluctuations of snowshoe hare populations (Lepus americanus) may cause other herbivores to become entrained in similar cycles. Alternating apparent competition via prey switching followed by positive indirect effects are the mechanisms behind this interaction. Our purpose is to document a change in the role of indirect interactions between sympatric populations of hares and arctic ground squirrels (Urocitellus parryii plesius), and to emphasize the influence of predation for controlling ground squirrel numbers. We used mark-recapture to estimate the population densities of both species over a 25-year period that covered two snowshoe hare cycles. We analysed the strength of association between snowshoe hare and ground squirrel numbers, and the changes to the seasonal and annual population growth rates of ground squirrels over time. A hyperbolic curve best describes the per capita rate of increase of ground squirrels relative to their population size, with a single stable equilibrium and a lower critical threshold below which populations drift to extinction. The crossing of this unstable boundary resulted in the subsequent uncoupling of ground squirrel and hare populations following the decline phase of their cycles in 1998. The implications are that this sustained Type II predator response led to the local extinction of ground squirrels. When few individuals are left in a colony, arctic ground squirrels may also have exhibited an Allee effect caused by the disruption of social signalling of approaching predators.

Life history correlates of alternative migratory strategies in American Dippers

Partial migration is thought to be a critical step in the evolution of avian migration, but data on the life history correlates of alternative migratory strategies are extremely limited. We have studied a partially migratory population of American Dippers since 1999. This population is composed of sedentary individuals (residents) that maintain the same territory year round and altitudinal migrants that share winter grounds with residents, but move to higher elevations to breed. We used seven years of data on individually marked birds to (1) determine if individuals consistently use the same migratory strategy, (2) determine if offspring have the same strategy as their parents, and (3) estimate reproductive and survival rates of the two migratory strategies. We evaluate hypotheses for the persistence of partial migration and discuss their implication for the evolution of migration in sedentary populations. Individual American Dippers rarely switched migratory strategy (4/169 monitored more than one year). An individual's strategy, however, was not always that of its parents, indicating that, while migratory behavior may have a genetic component, environmental or social conditions probably influence the migratory strategy that an individual adopts. Sedentary dippers consistently had higher annual productivity (approximately 1.4 more fledglings/year) than migratory dippers, but mark-recapture models suggested that migratory dippers may have slightly higher survival than residents (approximately 3.4%). Migrants were estimated to have lower lifetime reproductive success than residents because their higher survival was insufficient to offset their lower productivity. Our data suggest that alternative migratory strategies in American Dippers are unlikely to be a fixed genetic dimorphism that persists because the two strategies have equal fitness, or because the relative fitness of the two strategies fluctuates over time. Migratory strategies in American Dippers are more likely to be condition dependent, and the two strategies persist because migrants "make the best of a bad job" by moving to higher elevations to breed. Because migrants obtained no fitness benefits by moving to seasonal breeding territories, our data are consistent with the hypothesis that migration could evolve in sedentary populations if competition for limited resources forces some individuals to seek breeding opportunities outside their initial range.

Does body condition affect fecundity in a cyclic population of snowshoe hares?

Snowshoe hares (Lepus americanus) undergo a 10-year cycle in abundance, with cyclic changes in reproduction occurring 3 years prior to numeric changes. Reproduction may be associated with body condition, which might change with nutrition or predation pressure. We describe hare body condition (as a mass - skeletal size relationship) through a cycle in the southern Yukon from 1989 to 1996, test the effects of food and predation risk on body condition, and examine whether changes in body condition are related to cyclic reproductive changes. Hare body condition was lowest during the decline phase but rapidly improved during the low phase. Although yearling hares were in poorer condition than adults, changes in age structure cannot explain the cyclic fluctuation in condition. Food addition and predator reduction both resulted in better body condition. Body condition did not affect reproduction. The highest natality occurred when hares were in intermediate condition, while the lowest natality occurred when hares were in the best condition. Although changes in food and predation risk affect hare body condition, we found no relationship between body condition and cyclic reproductive changes. Rather, during times of nutritional deficit, female hares may maintain mass during gestation and lactation, but at a proximate cost to their offspring. Thus, inferences based on indices of condition incorporating body mass may be misleading.

Survival of juvenile hares during a cyclic population increase

Snowshoe hares (Lepus americanus) are multilittered synchronous breeders that produce up to four distinct litters of young each summer. I used radiotelemetry to determine the effects of juvenile cohort (i.e., litter group) and food availability on postweaning survival of hares in the southwestern Yukon during the increase phase of a hare cycle. During the study, I monitored 86 juvenile hares from control areas and areas in which supplemental food was provided. Twenty-eight-day survival did not differ between food addition and control areas for any juvenile cohort, and survival rates of juveniles (all cohorts combined) did not differ significantly from those of adults (juveniles: 0.91 per 28 days; adults: 0.93 per 28 days). However, when examined by juvenile cohort, survival of third and fourth litters was significantly lower than that of adults and first and second litters. These differences were the result of differential survival among the juvenile cohorts during a 3-month period in the fall (September-November). Predation was the primary proximate cause of death for weaned juvenile hares, accounting for 86% of deaths.

How red-backed voles find habitat patches

The purpose of this study was to discover how red-backed voles (Clethrionomys gapperi pallescens) found preferred habitat patches. Two hypotheses tested were that voles discover habitat patches by chance, and that they perceive them from a distance and travel towards them. The distance at which voles detected a wooded habitat was determined by measuring the accuracy of orientation at different distances. Animals were released in an unfamiliar grassland at one of five distances (0-20 m) from the boundary of an adjacent unfamiliar woodland and tracked using the spool and line technique. At all distances, the voles oriented towards the woods. However, their reaction was much weaker at greater distances. Thus, reaction to the woodland was not an "all or nothing response": the closer the voles came to the woods, the straighter were their paths and the more directly they oriented towards the woods. This adds to our understanding of how habitat fragmentation affects voles. A between-patch distance of 20 m does not completely isolate red-backed voles, but a distance equivalent to a home-range diameter (60-70 m) likely would.