Population, individual and behavioural approaches to understanding the implications of habitat change for arctic ground squirrels

Abstract

The ecological niche describes the entire set of resources and environmental conditions suitable for species to occur and persist. In northern ecosystems, rapid climate change appears to be altering these conditions and increasing the likelihood of shifts in distribution and abundance of species, with unknown consequences for resilience of ecosystem processes. Arctic ground squirrels are a widespread northern species, acting as agents of community disturbance and prey for many northern predators. Understanding the effects of habitat characteristics on occurrence and population persistence of arctic ground squirrels will improve projections of how habitat change may affect their ecosystem role. I compared arctic ground squirrels inhabiting an alpine ecotone extending across shrub, shrub-tundra and tundra habitats in southwest Yukon, and considered a variety of ecological factors determining distribution, population dynamics and individual behaviour. Populations in high suitability habitats (tundra) have greater viability and persisted at higher density with greater survival compared with low suitability habitats (more shrubs), possibly driven by a reduced risk of predation associated with lack of visual obstruction. Surprisingly, intermediate levels of shrub cover appeared to support the lowest densities and lowest survival in adult females. The apparent low suitability of shrub-tundra was also characterized by temporary habitat associations by non-resident individuals. Trends in behaviour and individual condition between habitats were inconsistent with those observed at the population level. These patterns resulted from either differences in individual state between habitats, or possibly maladaptive behaviours in transitional shrub-tundra habitat. Specifically, giving-up densities, which indicate costs of foraging and individual condition, were not consistent with predictions based on the observed population processes. Individuals in shrub habitat had higher giving-up-densities, lower adult female mass, and lower juvenile growth rates compared with shrub-tundra. Other factors showed no clear association with density or apparent habitat suitability, including reproductive output and juvenile coccidial parasite load. In general, the increasing occurrence of dense shrub in the Arctic is likely to reduce the density and restrict the distribution of arctic ground squirrels. If these low suitability habitats become more isolated, the potential for local extinction of arctic ground squirrel populations may increase.