Space use patterns and tropic interactions among woodland caribou, wolves, and moose across an anthropogenic disturbance gradient
Woodland caribou populations in the boreal forest are declining across North America as well as exhibiting dramatic range contraction. Multiple causal factors may be implicated, including climate change, declining food resources, enhanced predation risk due to apparent competition, and avoidance of areas of heavy human use. We used a combination of methods to measure key ecological variables affecting space use by each species across a substantial anthropogenic disturbance gradient in northern Ontario. Remote-sensing data were gathered on physical features, forest stand type, snow depth, NDVI, and linear features like roads and hydro lines. Field teams estimated stand-specific variation in food abundance and quality, whereas caribou diet was assessed using animal-borne video and DNA bar-coding of faeces. Substantial numbers of caribou and wolves were equipped with GPS radio-collars to assess spatial distribution and patterns of habitat selection and aerial surveys were used to estimate spatial variation in moose populations. These data were used to estimate models of movement by caribou across heterogeneous landscapes to assess the risk of encounter with wolves as well as long-term rates of energy gain. A spatially explicit population viability model was then used to evaluate the impact of anthropogenic disturbance of the boreal forest through road construction and forest harvesting.
Our results suggest substantial variation in multiple variables across the anthropogenic disturbance gradient. Abundance of predominant food types in caribou diets varied dramatically across stand types (particularly lichens), being particularly abundant in conifer-dominated stands that had not experienced recent anthropogenic disturbance. Moose distribution was heavily concentrated in forest stands regenerating from forestry and areas dominated by mixtures of deciduous and coniferous species. Wolf populations were highest in sites with substantial moose populations and the spatial distribution of wolves was closely associated with habitats preferred by moose as well as linear features. Caribou movements were best described by models incorporating a reasonably localized positive response to food abundance but a negative response to predation risk and preferred moose habitat. As a result, the spatially-explicit PVA suggests that anthropogenic disturbance can appreciably increase the risk of caribou declines due to increased predation risk and to a lesser extent resource limitation. Research strategies that link parameter estimation along spatial gradients for key ecological variables combined with mechanistic models of population interactions may be one of the best tools available to explore the potential consequences of long-term changes in the environment.