Predicting the effects of restoring linear features on woodland caribou populations

Abstract

Predator-prey dynamics are increasingly being modified by the alteration of natural habitats. Such alteration has led to increased predation rates and local extirpation of woodland caribou (Rangifer tarandus caribou) in western Canada. Linear features such as roads or seismic lines (narrow corridors used for petroleum exploration that are cleared of vegetation) increase predation rates on caribou by increasing wolf (Canis lupus) movement rates and by facilitating access into caribou habitat. Linear feature restoration is therefore hypothesised to help reverse caribou declines. However, with the high financial cost to restore approximately 350,000 km of seismic lines within western Canada’s boreal forests, theoretical predictions can clarify the efficacy of such actions. We use a mathematical model based on coupled ordinary differential equations representing predator-prey dynamics to estimate equilibrium densities of caribou, moose (Alces alces) and wolves under various parameter scenarios. Changes in equilibrium density serve as a proxy for the expected effect of linear feature restoration on population densities. Our model captures dynamical feedbacks between caribou and wolf densities, and also includes moose, which are the wolf’s primary prey species. With our best estimates of parameter values, caribou density increased 2.51-fold if all linear features were restored and 1.61-fold if only seismic lines were restored. As a comparison, simulated predator reductions increased caribou densities 3.92-fold, nearly twice the total response of linear feature restoration. The effect of restoration was increased if caribou group size was smaller, yet was less pronounced if carrying capacity for ungulates was higher. By varying the parameter values and fixing population densities, our approach allowed us to partition the caribou populations’ response from restoration into the contributions of the various mechanisms and feedbacks. In particular, contrasting simulation results where wolf densities are kept fixed with those where they are free to respond to prey allowed us to disentangle the relative importance of wolf functional and numerical response. This novel approach indicates that most of the increase in caribou densities after restoration was due to reduced foraging efficiency of wolves, especially reduced habitat overlap. Overall, our results suggest that restoration could substantially benefit caribou populations, but only if all linear features are restored, which is far from realistic. Linear feature restoration alone may not lead to population recovery, and should therefore be coupled with other direct management actions to successfully recover caribou.