Background and approach
To reconstruct interrupted national wildlife corridors, functional wildlife passages (overpasses and underpasses) that cross transportation infrastructure are necessary. The corresponding VSS norm SN 640694 recommends the implementation of wildlife overpasses. However, in practice it is not always feasible to build overpasses. Therefore, underpasses are also planned and constructed. Unlike for overpasses, the VSS or the federal government do not provide standards for dimensions of such underpasses.
24 underpasses in Germany, France, The Netherlands, Austria and Switzerland were assessed with a field study or by using existing data. In the field study, the underpasses were surveyed with camera traps for at least three weeks and frequencies of usage by wildlife per 24 hours were estimated. Furthermore, local experts were interviewed with a standardised survey. For each underpass, further local factors that could affect the usage of the underpass, like forest area, wildlife density, guiding structures, accessibility as well as anthropogenic disturbance, were recorded. The data was analysed making use of pairwise Pearson correlations, linear regression models and regression trees. Subsequently, the results were compared with current literature and discussed.
Roe deer accepted almost all assessed underpasses. Despite the regular occurrence in the surroundings of many underpasses, red deer and wild boars each passed only three structures. Animals mainly crossed at night (84%). On average, 1.1 wildlife crossings and 3.1 crossings by humans or pets were recorded per 24 hours.
The most important factors that enhanced the usage of underpasses by wildlife were the area of forest in a radius of 2 km and the area of guiding structures 100 m around the underpasses. Anthropogenic disturbance had a negative effect on the usage by wildlife. From approximately two human crossings per 24 h, less wildlife crossings can be expected. No significant correlation was found between acceptance of ungulates and the width (10-54 m), height (2-10 m) or openness (0.58-16.7) of the underpasses.
Results from the literature review highlight the importance of the location of underpasses within forested areas and a functional connectivity network as well as the strong potential effect of anthropogenic disturbances. Authors agree furthermore that many groups of animals – including ungulates – use wildlife overpasses clearly more often than underpasses. In some countries there are regulations and recommendations for the dimensions of underpasses, but these dimensions vary greatly between countries.
When planning and implementing fauna passages in national wildlife corridors, the following should be considered:
- In principle, the aim should be to implement wildlife overpasses instead of underpasses.
- Wildlife underpasses should be located within forest areas or at least be well connected to the surrounding forest habitats with guiding structures on either side of the underpass. The functionality of these elements should be secured for the long-term.
- Sources of anthropogenic disturbance within or in the direct surrounding of wildlife underpasses have to be prevented.