RATIONALE. Requirements of target species to enhance wildlife passage use
Wildlife passages (also known as ‘fauna passages’ and ‘wildlife crossings’), combined with fencing, are the most efficient mitigation measure to reduce fragmentation and the barrier effect that transport infrastructure causes for wildlife (Huijser et al., 2016; van der Grift et al., 2017). To maintain the integrity of the ecological network a multispecies approach is recommended. Nevertheless, wildlife passages are often designed for only a few target species, or to be multiuse, e.g. facilitating human use or drainage.
In general, wildlife passages exclusively constructed for wildlife have better outcomes than those for multiuse, especially for large carnivores which generally shy away from humans and traffic (Denneboom et al., 2020). Monitoring studies of multiuse passages in Sweden (Seiler & Olsson, 2009; Bhardwaj et al 2021; Knufinke, 2021) revealed that large mammals like moose and deer may take advantage of such facilities, but the frequency of their use decreases quickly with increasing traffic and human presence. Animals avoided the passages during periods of human activity and were generally cautious and alert when approaching the sites.
There are certain characteristics in wildlife passages, beside location and dimensions, that help maximize their ecological performance (Rosell et al., 1997; Smith, 2003). For example, most animal groups seem to prefer vegetated entrances to a fauna passage and short distances between the crossing structure and the surrounding vegetation (Clevenger et al., 2001; Clevenger & Waltho, 2005; Ascensao & Mira, 2007). Furthermore, many species prefer using underpasses where they can see the exit (Clevenger et al., 2001; Clevenger & Waltho, 2005) and amphibians avoid using long underpasses lacking light (Hlaváč et al., 2019).
However, a particular characteristic of a passage sometimes has contradictory results depending on the target species. Therefore, careful planning and design are needed. For example, amphibians prefer structures with presence of water both at the entrances and within the structure (Rosell et al., 1997; van der Grift et al., 2009). Water however may reduce the suitability of underpasses for small and medium mammals (Rosell et al., 1997).
Continued research is needed to more accurately define the characteristics that mark the success of wildlife passages and to determine the best design to achieve effective multispecies solutions.
Wildlife passages as part of a general landscape permeability concept
Wildlife passages should never be considered as an isolated feature. They should be an integrated part of a general permeability plan to maintain connectivity within and between animal populations and/or ecosystems. A permeability plan emphasises connectivity between habitats at regional or larger scale and considers not only the transport infrastructure but also the distribution of habitats and ecological corridors as well as other potential barriers such as built-up or fenced areas. Wildlife passages can then be regarded as critical elements to connect habitats and enhance the mobility of animals across transport infrastructure.
A permeability plan should be designed for each transport infrastructure project. All connecting elements, such as tunnels, viaducts, underpasses, overpasses, stream and river crossings and culverts designed or adapted to facilitate wildlife movement should be integrated into an assessment of connectivity. Again, the primary objective must be to maintain permeability for wildlife across transport infrastructure and to ensure the connectivity of the habitats within the landscape.
Wildlife passages are necessary if transport infrastructure:
- bisects important patches of habitat or creates a barrier to traditional migration routes (see Chapter 1 – Ecological effects of infrastructure and Chapter 4 – Integration of the infrastructure into the landscape);
- causes a significant barrier which affects ecological networks, migration corridors or other particularly vulnerable habitats, such as wetlands or other bodies of water;
- threatens any wildlife species and particularly those most sensitive to barrier effect and traffic mortality;
- results in significant damage or loss of special ecosystems, communities or species which require frequent connections between habitats on both sides of the infrastructure;
- Impairs significantly the general permeability of the landscape, i.e. the connectivity between habitats in the wider countryside;
- is fenced at some sections or for the entire length.
Density and location of wildlife passages
The total number and distribution of wildlife passages required to effectively maintain habitat connectivity is a major decision when planning mitigation measures. The number and type of measures required will depend on the target species and the distribution of habitat types in each area (Table 5.4). In some cases, a single or a few large passages could be necessary to mitigate a local problem, whereas in other situations it may be more important to provide a large number of different types of wildlife passages suitable for a variety of target species and appropriate to connect different habitat types.
- Wildlife passages should be part of an overall permeability plan where the biggest structures which allow complete restoration of connectivity (such as landscape overpasses or adapted viaducts) must be located at strategic points of the ecological network, while other, possibly multiuse structures, play an important complementary role. When determining the permeability of an infrastructure, all crossing opportunities for animals should be considered, including multiuse structures that may not have been designed for wildlife but nevertheless can provide possibilities for passages.
- Different techniques, such as graph theory, least-cost path and others may provide useful tools to visualize possible movements and flows of wildlife and help to identify the most appropriate location for wildlife passages. For a given species, a maximum ‘effective resistance’ coefficient could be established between core areas (‘nodes’) that an infrastructure fragments in order to determine the correct number and location of wildlife crossings to provide appropriate permeability of the infrastructure. The more passages are built, the smaller the ‘effective resistance’ is. This coefficient will depend on variables such as target species habitat preference, distance between the nodes or wildlife passage effectiveness.
- Identifying the species for which wildlife crossings are targeted is another basic step to decide on the number and location of the passages, as well as their dimensions and design (Table 5.5). Knowledge about the species’ habitat dependency, scale of movement, behaviour, population size and dynamics is key. Larger species with wide ranges (ungulates, large carnivores) may require a few strategically placed passages of larger dimensions, while smaller species may require more passages, even if of small dimensions, located in specific habitats. Invertebrates must also be considered everywhere.
- The square-root of the home range (HR0.5) may provide a useful measure to assess the scale of a species’ mobility and help to decide how far apart wildlife passages should be located. As mobility may differ between populations, the recommendation is to use local, empirical data obtained in field studies using techniques such as camera traps or telemetry. However, focus on a single species will not ensure proper connectivity for all wildlife. To achieve a more permeable infrastructure, a multiple species approach is needed that balances species mobility, population size and behaviour. Here, the needs of smaller, abundant species may weigh less than the requirements of larger or rare species.
- While the number and density of wildlife passages may be established on a wider scale relating to population sizes and dispersal, the specific location of the passages should be determined on a local scale based on local conditions. Different parameters such as wildlife-vehicle collision hotspots, known migratory and/or commuting routes and severance of habitats, are critical in the analyses to achieve the maximum benefit from wildlife passages for local biodiversity.