Facultative migration is a strategy used by many herbivores to optimize energetic gain in response to annual variations in the environment. While it is known that Ontario woodland caribou (Rangifer tarandus) exhibit significant variation in migration, the potential for facultative migration has yet to be explored. Understanding both the inherent variation and associated drivers of this behaviour is important in order to better understand habitat use, and maintain effective range management. The Hudson Bay lowlands lend ideal conditions to investigate variation in migration flexibility due to the gradient of both environmental and behavioural variation that is found from the bottom of James Bay to the top boundaries of the province. We used GPS collar data from 109 caribou across the Hudson Bay lowlands to first identify the presence of facultative migration. We then compiled environmental data, including estimates of vegetation, snowcover, and human disturbance, and ran a logistic regression against migration events to determine what environmental conditions affect the probability of migration. In the event of migration, we also identified linear covariates of migration distance, to examine potential environmental drivers of the magnitude of migration.
Results/Conclusions
We found that approximately 18% of sampled caribou exhibited facultative migration, switching between migratory and sedentary behaviour at least once. These individuals were found across the entire study range, demonstrating that individuals even at the far north of the range abstained from migration at times. Among those individuals, similar temporal variation suggested that annual environmental fluctuations impact migration at a population level. This flexibility may be key in adapting to changing environments. Probability of migration increased with winter severity, resource availability, and distance from human disturbance such as roads and utility corridors. Migration distance, in comparison, was less affected by resources, and only showed patterns of increase only with increased winter severity and with moving away from areas of human disturbance. These results help identify environmental impacts which caribou may be more vulnerable to, and illustrates how their behaviour might change in response to those impacts. Spatial trends were also observed in migration, with both probability and distance increasing northward. A consequent cluster analysis found that these spatial trends were comparable to the regional ecotypes in patterns of migration distance, but not probability; this indicates that ecotypes may not differ in migration as distinctly as they are currently treated.
