Roads and their associated traffic are modern risks for wildlife. Yet, a detailed and mechanistic understanding of individual and group decision-making processes underlying road crossing behavior in animals is still lacking. We investigated how GPS collared elk (n=12, representing >80% of the adult population) balanced the risks of vehicle traffic with the forage benefits of crossing a major highway in Yoho National Park (Canada) from September to May. Individual elk from this population cross the highway an average of 18 times per month (range: 0-64) during the winter period, with the majority of crossings occurring in April and May in early mornings and evenings.
We developed a detailed map of forage biomass in the study area by combining extensive field data with remotely sensed environmental layers. Risk was indexed by a traffic counter on the highway collecting data on the time of each passing vehicle. Using hidden Markov movement models and step-selection functions, we found that elk selected for safer and more profitable crossing opportunities at the population-level. The probability of crossing the highway increased with the average time gap between vehicles, and the amount of available biomass of herbs and shrubs at their target location. Further, we found support for an interaction between risks and rewards. When available forage on the other side of the road was higher, their probability of crossing increased multiplicatively. Variability in risks across the day and throughout the season also modulated this trade-off. We conducted these analyses in a mixed-effect framework and thus could interpret behavior for each week and each individual (each unique elk-week combination, n=436 elk-weeks). Our results suggest that elk habitat selection with respect to the highway varied strongly among individuals. Across individuals, the proportion of weeks when elk selected for safer crossings ranged from 43.8 to 66.0%, and ranged from 16.7 to 37.7% for profitable crossings. We thus plan to explain this variation by evaluating how individual- (e.g., boldness, reproductive status) and group-specific (e.g., dominance, social connectedness, group size and composition) characteristics affect how individuals trade-off available food resources and traffic levels.
Our study helps in understanding how individual and group decision-making processes affect wildlife interactions with highways. Our work also provides information on why, when and where elk cross this major road, as there is a proposal to upgrade this section of highway from two to four lanes. However, topography and landscape features impede the construction of traditional crossing structures that are present in other parts of Yoho National Park. Ultimately, our work provides guidance to managers regarding the prioritization of mitigation efforts (e.g., habitat improvements) and alternative strategies for allowing wildlife to cross this section of highway during and after highway upgrades.