Abstract
Wildlife crossing structures (WCSs) enhance connectivity between habitats of wild animals fragmented by fenced motorways, but factors affecting their use by targeted species remain understudied, particularly in areas recently recolonized by large carnivores. We investigated the use of WCS—6 overpasses (width 30-45m), 5 large underpasses (width 33–114 m) and 4 small underpasses (width 15–19 m)—located along the A4 motorway in the Lower Silesian Forest (western Poland), a large forest tract recently recolonised by wolves (Canis lupus). Identifying and counting tracks of mammals left on sand-beds as well as individuals recorded by camera traps were used to determine species diversity, number and activity patterns of mammals on WCS, and to reveal seasonal and temporal changes of WCS use over 3 years of study (2010–2013). WCSs were mostly used by wild species (51.5%), followed by humans (34.8%), livestock and pets (13.7%). Among wild species, ungulates were the most common (77.4% of crossings), while lagomorphs and carnivores were recorded less often (15% and 7.6% of crossings, respectively). The number of species and crossings of wild mammals, especially wild ungulates and wolves, was substantially higher on overpasses (mean effective number of species (Hill numbers): 0D = 7.8, 1D = 4.1 and 2D = 3.3) than on underpasses (0D = 6.3, 1D = 2.9 and 2D = 2.3) and was not affected by distance between WCS and human settlements or WCS width. There was a higher diversity of wild species and more crossings under large extended bridges than on smaller underpasses. The number of species and number of crossings of wild mammals, domestic animals and people increased from 2010 to 2013. There was a significant difference in activity patterns, with almost all wild species being nocturnal, in contrast to people and dogs. There was no relationship between crossing time and rates of wild carnivores and potential prey. We conclude that overpasses, even with steep entrance slopes (25–26.5%) or integrated with moderately used gravel roads, maintain movement of wild terrestrial mammals much better than underpasses, and the presence of wolves does not hamper the movement of other wild species. As there are significant temporal changes in use of WCS by mammals, we recommend monitoring WCS in all seasons for at least 3 years as a minimum standard for the post-investment assessment of WCS utilization by animals.
Highlights
Roads have profound effects on wildlife populations (Laurance et al 2014; Ibisch et al 2016), due to physical disturbance, traffic mortality and barrier effects (Huck et al 2010; Colchero et al 2011; Gibson et al 2013; Poessel et al 2014)
Our study provided further evidence that to sustain movement of a wide range of wild mammals across fenced motorways, overpasses are superior to underpasses
Overpasses located on the A4 motorway have a width of 30–45 m, which is far below the size recommended for such structures in Europe (i.e. 80 m, Smith et al 2015)
Summary
Roads have profound effects on wildlife populations (Laurance et al 2014; Ibisch et al 2016), due to physical disturbance, traffic mortality and barrier effects (Huck et al 2010; Colchero et al 2011; Gibson et al 2013; Poessel et al 2014). Various measures have been proposed to mitigate the negative impact of roads on wildlife, including warning signs, reduction of traffic volume, vehicle speed limits, systems of animal detection, deterring of animals through olfactory, audio and visual cues, management of road verges, fencing and placement of wildlife crossing structures (hereinafter WCS) (Forman et al 2003; D’Angelo and van der Ree 2015; Huijser et al 2015; Smith et al 2015). Despite the popularity of WCS all over the world, and critical financial investments in their planning and building, we are still far from understanding factors affecting their use by wild animals, especially those for large mammals (van der Ree and van der Grift 2015a, b)
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