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Open Access Publications from the University of California

Overcoming the Barrier Effect of Roads-How Effective Are Mitigation Strategies?

  • Author(s): Ree, Rodney van der
  • Gulle, Nadine
  • Holland, Kelly
  • Grift, Edgar van der
  • Mata, Cristina
  • Suarez, Francisco
  • et al.

Roads, railways and other linear infrastructure are pervasive components of most landscapes throughout the world. Combined with the effect of vehicles, they have the potential to cause mortality in wildlife, severely disrupt animal movement and increase the risk of local extinction. Management agencies and conservation organisations currently spend considerable amounts of money annually on engineering solutions to increase the permeability of roads for wildlife. We evaluated the use and effectiveness of wildlife crossing structures (e.g. tunnels, culverts, overpasses) by reviewing studies published in the refereed scientific literature, conference proceedings and consultant reports. We evaluated the scientific rigour and methodology of studies, the extent to which studies demonstrated an increase in permeability, the detail included in the reporting and the extent to which population, community and ecosystem effects were shown. One hundred and twenty three studies were reviewed and all except two found an effect at the level of the individual animal. Two studies demonstrated a positive effect for the population and thus overall, the effectiveness of mitigation measures at reducing the risk of population extinction remains unclear. The level of scientific rigour, amount of replication and description of adjacent habitat and animal populations varied considerably among studies, in many cases limiting the level of inference that could be made. In the context of evaluation, we propose that a clear distinction be made between “use” and “effectiveness” of a wildlife crossing structure. The use of a structure may be broadly defined as the rate of detections of individuals or species, while effectiveness relates to a specific question or the goal of mitigation. A large amount of effort has conclusively shown that crossing structures are used by many species of wildlife. The long-term success of mitigation will ultimately depend on their effectiveness – i.e. to what extent have they mitigated the barrier effect of roads and has this prevented the local extinction of populations due to road effects? The next phase of research must focus more explicitly on quantifying their effectiveness, relative to location- and species-specific goals.

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