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Does the configuration of road networks influence the degree to which roads affect wildlife populations?


Roads act as barriers to animal movement, thereby reducing the accessibility of resources on the other side of the road. Roads also increase wildlife mortality due to collisions with vehicles, and reduce the amount and quality of habitat. The purpose of this study was (1) to determine whether or not the configuration of road networks has an influence on the degree to which roads detrimentally affect wildlife populations and (2) to identify characteristics of road network configurations that make road networks less detrimental to the persistence of animal populations. To explore these questions, we used a spatially explicit individual-based stochastic-simulation model of population dynamics. A measure assumed to reduce the effects of the road network is the bundling of roads and traffic in order to keep as large areas as possible free from disturbances due to traffic. However, the suitability of this measure may be questionable because a group of several roads bundled together, or an upgraded road with more traffic on it, creates a stronger overall barrier effect that may be more detrimental to population persistence than the even distribution of roads across the landscape. Our modelling results clearly supported the bundling concept. Population persistence was generally better (and never lower) when all traffic was put on one road than when it was distributed on several roads across the landscape. If traffic cannot be combined on one road, the model results suggested it is better to bundle the roads close together than to distribute them evenly across the landscape. We also were interested in the question of whether the effect of a road network was determined by the number and size of the pieces (“patches”) that it fragments a landscape into or by the total length of roads in the landscape. We expected that the effect of a road network would be the more detrimental the more patches it creates. The results were surprising: The expectation that fragmenting the landscape into more patches would be more harmful to population persistence (while total road length is kept constant) was contradicted by the model results in the case where the degree of road avoidance by the animals was low. This implies that for animals that do not very strongly avoid roads, it is more important to preserve core habitats at a sufficient distance from roads than to keep the number of patches low. Our results are an important step towards a network theory for road ecology and towards the design of lessdetrimental road networks. Empirical studies comparing landscapes with differing road network configurations should be conducted in the future to validate the predictions and to provide a basis for developing more practical models for use in planning and designing of highway networks. Keywords: barrier effect, bundling of roads, core habitat, landscape connectivity, landscape fragmentation, population viability analysis (PVA), road avoidance, road configuration, roads, spatially explicit population model (SEPM), traffic mortality.

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