Why Did The Amphibian Cross The Road?

Roadkill is an all-too-common occurrence in many countries. Indeed, the death of non-human animals en masse on human roadways can have staggering effects on populations. Some species are more affected by this than others. Amphibians are particularly vulnerable given their relatively slower movement coupled with their large migrations. Because of this, “road mitigation” techniques have been designed to help amphibians and other non-human animals safely cross roads and avoid being hit by vehicles. 

The most popular road mitigation techniques include fences to keep animals from entering the road, and tunnels, offering them the ability to move under roadways rather than across them. Such measures are used across the world, yet only a handful of studies have looked at how effective they are in achieving their goals of reducing deaths and offering animals safe passage. Focusing on common toads in the Netherlands, this study evaluated the effectiveness of road mitigation techniques in protecting toad populations during the animals’ breeding season migration. 

In carrying out their study, the authors focused on a one-kilometer stretch of road running through a forest with one of the largest-known common toad populations in the Netherlands. To track the toads, the authors used a “capture-mark-recapture” design, where toads were humanely trapped, tagged with a biodegradable sticker that held onto them for about a week, and then recaptured to track how many of them had used the tunnels versus the road. In addition to counting the trapped toads, the authors also did “road surveys,” counting toads — dead or alive — along four stretches of road. Dead toads were moved to avoid double-counting and living toads were released in the nearest breeding pond. The study lasted for three consecutive mating seasons (February/March-April of 2013, 2014, and 2015). 

The researchers found that only about a third of the toads used the tunnels to cross the pavement. 28% still ended up on the road, and migrating population numbers decreased by nearly three-quarters after the fences and tunnels were installed. They tested five possible explanations for the low tunnel use:

1. Some toads simply arrived at the road further away from the tunnels than others.
2. Some toads had to cover less distance along the fence to get to a point where they could cross.
3. There was something about the tunnels themselves that discouraged toads from using them.
4. Some toads moved away from the tunnels after hitting the fence, rather than toward the tunnels.
5. Some toads were discouraged altogether by the fence.

The main reason, the authors say, for the relatively low proportion of toads who actually used the tunnels was the spacing. On average, there were only about two tunnels per kilometer along the fence, and these tunnels were spaced about 160 meters apart. By contrast, the average toad only traveled 60 meters along the fence. Tunnel design may also be an issue. For example, if the tunnels are too narrow, too long, or have what the authors call an “inhospitable micro-climate,” toads could be more likely to avoid them.

Finally, one of the more troubling findings of this study is that migrating population numbers decreased by nearly 75% after the road mitigations were installed. The authors posit two possible explanations for this. On one hand, the total size of breeding waters available to the toads decreased by nearly 80% relative to before the tunnels and fences were installed, meaning there were fewer breeding opportunities available. Another possibility is that the drop occurred because so few toads were able to successfully cross to the breeding waters.

The authors offer some recommendations for improving the number of toads able to safely cross the roadway. These include extending fences along the road well past the point where toads are expected to cross; increasing the number of tunnels and the frequency with which toads encounter them along the fence; and constructing breeding waters on both sides of the road to reduce the need for toads to cross in the first place. Additionally, they call for more comprehensive baseline studies of toads’ migration behavior at roadways to determine when and why they don’t use tunnels, as well as to understand how so many toads are still ending up on the pavement despite mitigation measures.

While tunnels and fences are at least somewhat effective in reducing roadkill, they are far from perfect. This is relevant for animal advocates and policymakers who wish to design better solutions for human-wild animal cohabitation. In the long term, designing effective measures to protect wild animals will not only benefit them, but will also help future generations of humans by preserving biodiversity. Studies like this reinforce the importance of sound policy and balancing non-human animals’ interests with our own.