Demographic differences in diet breadth of Canada lynx during a fluctuation in prey availability.

Population dynamics of specialist carnivores are closely linked to prey availability, but the extent of variability in diet breadth of individual carnivores relative to natural variability in the abundance of their primary prey is not well understood. Canada lynx (Lynx canadensis) specialize on snowshoe hares (Lepus americanus) and exhibit cyclic fluctuations in abundance that lag 1-2 years behind those of snowshoe hares. Declining hare densities spur demographic changes in lynx, but it is unclear whether a corresponding increase in diet breadth occurs: (1) broadly across a lynx population; (2) only among individuals who are able to effectively switch to alternative prey; or (3) only among individuals who cannot capture sufficient primary prey. We measured stable isotope ratios of lynx muscle tissue spanning a cyclic increase and decline in hare density (1998-2001) in Fort Providence, NT, Canada. We found that lynx cohorts responded differently to hare population change, with yearling animals having broader diets at low hare densities, while adults and dependent juveniles maintained a constant diet through the initial decline in hare density. This result was consistent irrespective of lynx sex and indicates that yearling lynx likely are forced to adopt a broader diet when primary prey densities decline. Our results imply that select cohorts of specialist carnivores can exhibit high dietary plasticity in response to changes in primary prey abundance, prompting the need to determine whether increased diet breadth in young lynx is a successful strategy for surviving through periods of snowshoe hare scarcity. In this way, cohort-specific niche expansion could strongly affect the dynamics of organisms exhibiting population cycles.

Amphibian chytrid fungus and ranaviruses in the Northwest Territories, Canada.

Pathogens can cause serious declines in host species, and knowing where pathogens associated with host declines occur facilitates understanding host-pathogen ecology. Suspected drivers of global amphibian declines include infectious diseases, with 2 pathogens in particular, Batrachochytrium dendrobatidis (Bd) and ranaviruses, causing concern. We explored the host range and geographic distribution of Bd and ranaviruses in the Taiga Plains ecoregion of the Northwest Territories, Canada, in 2007 and 2008. Both pathogens were detected, greatly extending their known geographic distributions. Ranaviruses were widespread geographically, but found only in wood frogs. In contrast, Bd was found at a single site, but was detected in all 3 species of amphibians in the survey area (wood frogs, boreal chorus frogs, western toads). The presence of Bd in the Northwest Territories is not congruent with predicted distributions based on niche models, even though findings from other studies at northern latitudes are consistent with those same models. Unexpectedly, we also found evidence that swabs routinely used to collect samples for Bd screening detected fewer infections than toe clips. Our use and handling of the swabs was consistent with other studies, and the cause of the apparent lack of integrity of swabs is unknown. The ranaviruses detected in our study were confirmed to be Frog Virus 3 by sequence analysis of a diagnostic 500 bp region of the major capsid protein gene. It is unknown whether Bd or ranaviruses are recent arrivals to the Canadian north. However, the genetic analyses required to answer that question can inform larger debates about the origin of Bd in North America as well as the potential effects of climate change and industrial development on the distributions of these important amphibian pathogens.

Hierarchical habitat selection by barren-ground grizzly bears in the central Canadian Arctic.

Using resource selection functions, we examined habitat selection patterns of barren-ground grizzly bears (Ursus arctos) in the central Canadian Arctic among and within home ranges. There was no difference between the sexes with regard to habitat selection patterns at the home range level (Wilks' λ, approx. F 11,11=1.27, P=0.37). Bear home ranges contain more esker habitat, tussock/hummock successional tundra, lichen veneer, birch seep, and tall shrub riparian areas relative to the proportional availability of habitats in the study area. We observed differences in habitat selection within home ranges among levels of sex/reproductive status (Wilks' λ, approx. F 20,412=3.32, P<0.001) and by season (Wilks' λ, approx. F 30,605=2.71, P<0.001). Eskers and tall shrub riparian zones were the habitats most preferred by bears throughout the year. Tussock/hummock successional tundra was also favored by males at varying times during the year and lichen veneers were favored in spring and autumn by most bears. Females with cubs tended to avoid the highest ranked habitat for males throughout the year. This pattern of habitat selection was not observed for females without accompanying young. Results of this study underline the importance of scale dependence in habitat selection. Failure to view habitat selection as a hierarchical process may result in a narrow and possibly misleading notion of habitat selection patterns.