Under what conditions do fences reduce the effects of transportation infrastructure on population persistence?

Transportation infrastructure impedes the movement of animals, enhances their mortality due to collisions with vehicles, and decreases habitat size. We study the first two effects, using a spatially explicit individual-based model of population dynamics. We discuss the suitability of fences. Fences can either enhance or reduce survival probability, depending on the degree of road avoidance and the proportion of animals killed, of those that try to cross the road. There is a lower value of traffic mortality below which a fence is always harmful and an upper value of traffic mortality above which a fence is always beneficial. Between these two values the suitability of fences depends on the degree of road avoidance. The lower the degree of road avoidance and the higher the amount of traffic on the road, the more likely it will be that fences are beneficial. We recommend the use of fences when traffic is so high that animals never, or almost never, succeed in their attempts to cross the road, or the population of the species of concern is declining and traffic mortality is known to contribute to the decline. We discourage the use of fences when population size is stable or increasing or if the animals need to access resources on both sides of the road, unless fences are used in combination with wildlife crossing structures.