why do we need mitigation measures? Dr Rodney van der Ree, Part 2

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, reduce the amount and quality of habitat and increase the risk of local extinction. Around the world, management agencies and conservation organisations currently spend considerable amounts of money annually on engineering solutions to increase the permeability of roads for wildlife. A large body of research and monitoring has clearly demonstrated that many species of wildlife will use these structures to cross the linear infrastructure. This research has also shown that the rate of use of the structures varies according to species and depends on the characteristics of the structure, including its dimensions, position in the landscape and surrounding habitat. However, the effectiveness of these measures at reducing the risk of population extinction remains unclear.

We evaluated the use and effectiveness of wildlife crossing structures (tunnels, culverts, overpasses) by reviewing studies published in the refereed scientific and the grey literature and assessed the extent to which studies could demonstrate an increase in population viability (see van der Ree et al 2007, van der Ree et al. 2008). We included papers from journals, conference proceedings, university theses and reports from management agencies and consultants. Research published in English and other languages (French, Spanish, Dutch and German) was included. The majority of studies found an effect at the level of the individual animal, with individuals of many species detected using the structures. One study of the 24 found in the refereed scientific literature demonstrated a positive effect at the population level. Published in 1989, Mansergh and Scotts demonstrated how an under-road tunnel provided connectivity and restored the social organisation of the mountain pygmy-possum (Burramys parvus). The mountain pygy-possum is a small critically endangered marsupial from south-eastern Australia whose habitat coincides with alpine ski resorts. In contrast, many studies in both the refereed and non-refereed literature were limited by inadequate replication; insufficient detail of the physical characteristics of the crossing structure, road or traffic; and a failure to adequately describe the population of wildlife and habitat adjacent to the road.

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