Modeling the Effects of Road Network Patterns on Population Persistence: Relative Importance of Traffic Mortality and 'fence Effect'

Roads affect animals in three adverse ways. They act as barriers to movement ('fence effect'), enhance mortality due to collisions with traffic, and decrease habitat size. We study the relative importance of the first two effects using a spatially explicit individual-based model of population dynamics. We discuss our results with respect to the suitability of fences along roads as a measure to reduce road mortality. The results reveal a much stronger effect of road mortality than of the 'fence effect'; the influence of traffic mortality is always much more significant when the proportions of individuals avoiding the road and those that are killed on the road (in relation to the number of individuals encountering roads) in the two situations compared are the same. The results indicate that putting up fences along roads might be a useful interim mitigation measure until more suitable measures will be applied. However, fences must be used with caution because they could increase extinction risk for species that have large area requirements and small population sizes. In the second part of this paper, we outline a comparison of different configurations of road networks. We ask if different spatial arrangements of the same amount of roads (e.g., 'bundling' of roads) have consequences for the strength of both the 'fence effect' and road mortality. The model results indicate longer times to extinction in case of a 'bundling' of roads but the proportion of populations going extinct within 500 time steps does not change significantly.