Response of carnivores to existing highway culverts and underpasses: implications for road planning and mitigation

Abstract

Roads with high traffic volumes are a source of animal mortality, can disrupt normal animal movements and dispersal, and may represent a potentially serious threat to wildlife population stability and viability. Retrofitting existing structures built for other purposes (e.g., drainage culverts or small below-grade access roads) to facilitate wildlife crossing by animals and to reduce mortality may be expensive if modifications to the existing structures themselves were involved. However, the environmental context surrounding these structures may influence the willingness of animals to cross, and management of some of these attributes may enhance the attractiveness of these structures. Culverts and underpasses are two common structures along roads in Portugal. We quantified the response of small and medium-sized carnivores to the presence of both types of existing passages by determining: (1) frequency of use; (2) whether use differed by type of passage, and if so; (3) by examining if associated environmental attributes might explain the differences observed. We surveyed 57 different passages along 252 km of highway with a total sampling effort of 2,330 passage trap-days. The mean passage rate for carnivores combined was 0.7 complete passages per crossing structure per day. Crossings by weasel, polecat, otter, and wildcat were infrequent or absent. Red fox, badger, genet and Egyptian mongoose used the crossing structures regularly and without obvious preference; stone marten preferred underpasses. Regression analyses showed the frequency of use by carnivores varied with structural, landscape, road-related features, and human disturbance with 17 of 26 (65%) attributes being significant. Larger passages with vegetation close to the passage entrances, favorable habitat in the surrounding area, and low disturbance by humans were important key features to regular use by the guild of species studied. Mitigation planning in areas with ecological significance for carnivores will be beneficial. Structural attributes and human disturbances are more difficult or expensive to change, even though related significantly to crossing use. Management of vegetation at passage entrances and restricting human use near passages in carnivore suitable areas may substantially improve crossing attractiveness for the guild of carnivore species.

Ecology and conservation of the polecat Mustela putorius (Linnaeus, 1758) in Portugal: a review

“…In a two-year study, involving 260 km of highways and 314 km of national roads, Grilo et al (2009) found that only 2.5% out of 806 detected casualties of carnivores were of polecats and suggested that this might be positively correlated with the species abundance in the region. This conclusion agrees with both Grilo et al (2008), in which from a total of 1,940 track records of carnivores detected in highway culverts and underpasses (marble dust track pads) only one corresponded to a polecat, and Mateus et al (2010), a study conducted in the same area, to assess cost-effectiveness of monitoring methods of carnivore use of drainage culverts, in which not a single polecat was recorded.…”

“…Culverts and highway underpasses are often used by carnivores to cross roads and can be vital in maintaining landscape connectivity . To increase the probability of usage by carnivores, Grilo et al (2008) recommended several measures to be considered by highway managers: (1) construction of large passages; (2) prioritise mitigation measures in areas with ecological significance for carnivores (e.g. natural forest areas and sites with streams and riparian vegetation); (3) planting vegetation at passage entrances, to guide animals towards existing structures; and (4) restrict human use of passages.…”

Surveying drainage culvert use by carnivores: sampling design and cost–benefit analyzes of track-pads vs. video-surveillance methods

“…Despite the growing interest in measuring the effectiveness of crossing structures for fauna to increase road permeability (e.g., Yanes et al 1995;Clevenger et al 2001;Grilo et al 2008;Mata et al 2008), there is an uncertainty when determining the most appropriate method to best evaluate their effectiveness. In fact only one study has compared the passage delectability among the methods, as well the cost-benefit for each method (Ford et al 2009).…”

“…Track detection in varying substrates, such as marble dust or sand, is a common method because of its low cost and effort, and odorless condition (Yanes et al 1995), besides its ability to distinguish between most species (e.g., Cain et al 2003;Grilo et al 2008;Mata et al 2005;Ng et al 2004;Rodriguez et al 1996). However, it is difficult to differentiate between age classes (adults vs. juveniles) and counts may be underestimated when footprints do not leave species distinguishable tracks (e.g., wolf vs. dog, cat vs. wildcat).…”

Habitat or matrix: which is more relevant to predict road-kill of vertebrates?

“…Efficient passages should have large size (wider than long) with vegetation close to the passage entrances (Beckmann et al, 2010;Grilo et al, 2008), and riparian forest strips could act as functional corridor to cross roads depending on matrix permeability particularly to birds (Vergara, 2011). However, there are some restrictions to apply this proposal in the case of Brazilian transportation and environment agencies, and to additional costs to highway manager.…”