Effectiveness of small road tunnels and fences in reducing amphibian roadkill and barrier effects at retrofitted roads in Sweden

Abstract

Schemes to reduce road impacts on amphibians have been implemented for decades in Europe, yet, several aspects on the effectiveness of such schemes remain poorly understood. Particularly in northern Europe, including Sweden, there is a lack of available information on road mitigation for amphibians, which is hampering implementation progress and cost-effectiveness analyses of mitigation options. Here, we present data derived from systematic counts of amphibians during spring migration at three previous hot-spots for amphibian roadkill in Sweden, where amphibian tunnels with guiding fences have been installed. We used the data in combination with a risk model to estimate the number of roadkills and successful crossings before vs. after mitigation and mitigated vs. adjacent non-mitigated road sections. In mitigated road sections, the estimated number of amphibians killed or at risk of being killed by car traffic decreased by 85-100% and the estimated number successfully crossing the road increased by 25-340%. Data, however, suggested fence-end effects that may moderate the reduction in roadkill. We discuss possible explanations for the observed differences between sites and construction types, and implications for amphibian conservation. We show how effectiveness estimates can be used for prioritizing amphibian passages along the existing road network. Finally, we emphasize the importance of careful monitoring of amphibian roadkill and successful crossings before and after amphibian passages are constructed.

Keywords: Amphibians; Barrier effects; Effectiveness; Road mitigation; Roadkill; Sweden.

Figure 1. Location of the three study sites in Stockholms larger metropolitan area.

Map image credit: Lantmäteriet.

Figure 2. Maps of the three study sites (A, site 1 Skårby; B, site 2 Kyrksjölöten; C, site 3 Skeppdalsström).

Red lines denote mitigated (fenced) section, black lines are the tunnels and blue line is the road section where amphibians were counted before and after mitigation. Map image credit: Lantmäteriet.

Figure 3. Amphibian tunnel with guiding structure, fence and fence-end at the three study sites (A–B) site 1 Skårby; (C–D) site 2 Kyrksjölöten; (E–F) site 3 Skeppdalsström.

Photos: Jan Olof Helldin and Erik Jondelius.

Figure 4. Probability of getting killed for an individual of different amphibian species at different traffic intensities, as described by Hels & Buchwald (2001).

The probability of getting killed is weighted by amphibian behavior (velocity and diurnal activity) and diurnal variation in traffic intensity, and assuming that amphibians are crossing perpendicular to the road. Traffic intensity of the three study sites are indicated by vertical dashed lines.

Figure 5. The number of amphibians found along the studied road sections, divided per evening or night and 50 m road interval starting from northwest.

Upper graphs (A–C) are before mitigation, lower graphs (D–F) are with mitigation in place. Site 1: Number of dead newts (smooth + great crested) found per night; site 2: Number of live and dead common toads found per night; site 3: Number of live and dead amphibians (four species) found per evening. Red lines below x-axes after mitigation denote the mitigated sections (permanent amphibian fencing), green line below x-axis at site 2 before mitigation denotes the temporary fenced section. Due to the difference in method, the data from counts along the temporary fence at site 2 cannot be directly compared to the other data from that site.