Future studies need to respond to these study design issues. Despite reports that underpasses were used by a range of species, there is concern that studies have suffered from design limitations, with many being of relatively short duration and having no control or reference sites to indicate whether the frequency of use of the structures is more or less what could be expected if structures were functioning effectively (van der Grift et al., 2013 van der Ree et al., 2007). Many studies have been conducted to determine the effectiveness of these structures, with a particular focus on their frequency of use (e.g., Clevenger et al., 2001 Clevenger & Waltho, 2000 Taylor & Goldingay, 2003). Structures built under the road (underpasses), which includes modified drainage culverts, have been the most widely installed and studied structures (Denneboom et al., 2021 Taylor & Goldingay, 2010).
HIGHWAY UNDERPASS WILDLIFE DRIVER
Road crossing structures for wildlife are intended to serve five objectives: reduce vehicle strike and road‐kill improve driver safety enable wildlife to disperse to maintain gene flow enable seasonal migration and enable home range movements (Denneboom et al., 2021 Kusak et al., 2009 Sawyer et al., 2012 Simpson et al., 2016 Taylor & Goldingay, 2010).
HIGHWAY UNDERPASS WILDLIFE INSTALL
The most frequent response has been to install fencing to exclude wildlife from the roadway (Clevenger et al., 2001) and structures to enable animals to cross safely under or over a road (Denneboom et al., 2021). Government road agencies have responded with greater effort during the last 20 years to reduce the effects of roads on wildlife. Some wildlife species are at particular risk from these types of disruptions (e.g., species unable to avoid moving vehicles rare species with low reproductive rates Fahrig & Rytwinski, 2009) and may require intervention. Roads lead to direct mortality of animals through vehicle strike and may disrupt wildlife populations in ways that are not easily observed, such as by preventing animals from dispersing across a formerly interconnected landscape (e.g., Olsson et al., 2008 Sawyer et al., 2016).
The impact of roads on wildlife populations is a global concern because these networks occur across most parts of the globe and are increasing in their density and reach (Laurance et al., 2014). Habitat adjoining underpasses exert a strong influence on their use and require greater consideration to maximize underpass use. Our results provide no consistent support for the “prey‐trap hypothesis.” Instead, our study confirms the generic value of underpasses for a range of medium‐large mammals as well as one large reptile. However, its activity coincided less than expected with the activity of the mammals most at risk to it. We found the red fox ( Vulpes vulpes) had high activity in some underpasses. The “prey‐trap hypothesis” postulates that predators will exhibit increased activity at underpasses as a consequence of prey being funneled. The short‐beaked echidna ( Tachyglossus aculeatus) had a high probability of detection in a single underpass. The vulnerable koala ( Phascolarctos cinereus) was detected infrequently in underpasses and in the adjoining forest. At each location, a different small macropod species, including one regionally threatened, showed a higher probability of detection in one underpass compared with several of the forest sites. The lace monitor ( Varanus varius) was detected in all underpasses, often once per week during spring and summer, and infrequently in the forest. Three species of large macropod (wallabies and kangaroos) were frequently detected in the underpasses, with some underpasses traversed 1–4 times per week, and in many cases exceeded detections in the forest. We studied 5–7 road underpasses at two locations in eastern Australia over 2–3 years, comparing camera trap detections of animals in underpasses with those at nearby forest sites. In Australia, most studies of underpasses have been of relatively short duration and without reference sites to give context to the measured rates of use. Research into the effectiveness of road underpasses and overpasses for wildlife has been concentrated in North America and Europe. Road networks continue to expand globally with predictable effects on ecological systems.
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