Spatial game between gray wolves, caribou, and moose in the boreal forest

Background/Question/Methods

The behavioral game taking place between predators and prey largely determines their spatial distributions. One possible, and surprising, outcome of this behavioral response race is the leapfrog effect, whereby predators match the distribution of their prey’s resources, while prey undermatch their own resources to reduce risk. We developed a model based on game theory to identify conditions under which spatial structure in the strength of leapfrog effects may arise in a simple predator-prey system, and to determine how the addition of a second prey could alter habitat selection by all three species. We then tested model predictions using field observations in a system where gray wolves (Canis lupus) coexist with woodland caribou (Rangifer tarandus caribou) and moose (Alces americanus). Our theoretical model considers that the distribution of all species among three land cover types is such that all individuals of each prey species experience the same ratio between food intake and predation risk, while all predators have the same prey intake rate. As in the wolf-caribou-moose system, our model assumes that the two prey consume different types of food, each of which is most abundant in different land cover types.

Results/Conclusions

We show that spatial constraints, such as territoriality among predators, can induce spatially structured leapfrog effects. Also, predator presence can increase prey species co-occurrence relative to expectations based on food distribution alone. Leapfrog effects may even vanish for a given prey when an alternative prey invades the system. In this case, leapfrog effects might still be present in the system, but they may involve only one prey and the predator. Our empirical assessment of the behavioral response race between wolves, caribou and moose support some of the model’s predictions. For example, caribou selected conifer stands rich in lichen in low wolf-use areas. In high wolf-use areas, however, caribou selectively moved into recent cutovers rather than in lichen-rich areas. In contrast, wolves consistently selected open conifer stands with lichen. Also, in winter, leapfrog effects were only detected between caribou and wolves, but not between moose and wolves. Our study demonstrates the major impact that food web complexity can have on the spatial game between predator and prey in heterogeneous landscapes.