The ABCs of Designing an Amphibian Tunnel

Over 40 years ago, people were alarmed by large numbers of roadkill or “splats” on Switzerland’s highways and started building tunnels for amphibians. Through trial and error, Europeans, the British, and more recently, North Americans have been coaxing frogs and salamanders to go under roads rather than over them.

It turns out that some species readily move through culverts and tunnels while others do not. The reasons vary.

German biologist Ulrich Sinsch discovered that newts use aboveground landmarks and celestial cues to find their way around forest and fields. Going underground through a tunnel could be very disorienting.

Cold-blooded amphibians are sensitive to temperature. They become dormant when it gets cold. British biologist Rob Oldham showed that toads were reluctant to move through tunnels that were colder than above ground temperatures.

It is possible to make tunnels less disorienting and the same temperature as the surroundings. In fact, people who have reviewed tunnels around the world suggest that large sizes (minimum 1 m in diameter) let in lots of light and airflow and work best for most species.

Other important design features include diverting runoff to prevent water from flowing though the tunnel, moist natural soil substrates, and straight walls easy for frogs to follow.

Our goal is to build a tunnel that meets all the recommendations learned from past projects and fits within the roadbed at the peak amphibian crossing point on Highway 4.

Road surveyors measured the height of the road last spring. It’s a shallow roadbed with the road surface only 60 cm higher than the normal water level in the ditches. This limits the options for installing a large enough tunnel.

We reviewed a variety of prefabricated tunnels/culverts available in B.C. We looked at both open-bottom metal culverts and concrete box tunnels. By far the best option for size and straight walls is a concrete box tunnel, 1.8 m wide by 0.9 m tall (interior dimensions). Safety standards require at least 22.5 cm of cover and 5 cm of asphalt over top of the culvert. This means we will have to dig the culvert into the ground and fill it with substrate to the height of the water level. The end result will be a tunnel that is .42 m tall. That’s taller than most Red-legged frogs jump. We hope the large width will compensate for the lack of height in letting in adequate light and air circulation.

We are working with the Ministry of Transportation to plan the installation of the experimental tunnel this winter. We will spend next year testing how well it works. Fingers crossed.

Why did the Red-legged Frog cross the road?

To get to the other side, of course! No one has been able to get a straight answer out of the frogs on this question, but a number of people have tracked their movements over wide areas. In Oregon, Marc Hayes and his colleagues caught and marked Red-legged Frogs at a breeding pond in February and then recaptured one 4.8 km away, six months later. This frog likely traveled back to breed at the same pond the following winter.

No one knows exactly why frogs move so far. Perhaps it is to find better food and shelter. Or maybe to escape from predators. It's likely a combination of factors. One thing we do know, if there is a road to cross along the way, frogs will attempt it. Unfortunately, not all of them make it, without some help.

This year, since August 31, the Wetland Stewards for Clayoquot and Barkley Sounds have safely moved 400 Red-legged Frogs across Highway 4. Most were juveniles, dispersing 500 m away from the shallow lake where they were born last spring. Some were adults returning to the breeding site.

We installed plastic fences along 90-m sections of the highway to block frog access to the road. We caught, measured and marked each individual at the fence before releasing it on the opposite side of the road. We rarely caught the same individual again. Juveniles moved on to wherever they will spend time, over the next two to four years, before returning to breed at the lake. Adults moved towards the lake where they will breed in February.

The fencing works well except that it prevents the natural migration and dispersal behaviour of frogs, unless people are involved in catching and moving them across the road. A better solution would be a raised highway or a series of tunnels to provide safe passage under the road.

The Wetland Stewards are working with the B.C. Ministry of Transportation and Infrastructure to design an experimental tunnel. Our first step was to decide where to install the tunnel. To answer that question we asked: "Where did the Red-legged Frog cross the road?" That's a relatively easy question to answer.

By surveying the 40-km highway between Ucluelet and Tofino by car on warm rainy nights from 2000 to 2004, we located the highest concentration of roadkill along a 1.5-km stretch. Searching on foot, we found the highest concentrations within that 1.5-km stretch. In 2005, we set up the 90-m barrier fences to catch frogs and pinpoint where the majority of crossings occur. Now we know where that is, and we're designing a tunnel that will fit the roadbed at that location. More about that in the next post.

The SPLAT Project

The SPLAT Project aims to prevent highway traffic from reducing populations of frogs and salamanders along Highway 4 on the west coast of Vancouver Island, B.C. Our blog is a way of sharing what we know (and don't know) about how to monitor and mitigate amphibian roadkill in our part of the world. We are keen to provide links to information from similar projects elsewhere.

Local volunteers started the SPLAT project in 2000 after noticing squished remains ("splats") of many frogs and salamanders during rainy nights.  After surveying the 40-km highway repeatedly, we identified the major concentration of "splats" within half a kilometer of a 4-ha wetland. Red-legged Frogs (Rana aurora), Pacific Treefrogs (Pseudacris regilla), Northwestern Salamanders (Ambystoma gracile) and Roughskin Newts (Taricha granulosa) breed in the wetland in late-winter and spring and then spend the rest of the year in the surrounding forests. Young of the year emerge from the wetlands in late summer. All ages risk becoming "splats" when they cross the highway to move between the breeding/rearing site and forest habitats.

Researchers and volunteers with the SPLAT Project have been moving live amphibians across Highway 4 over the past five years. We catch frogs and salamanders in pitfall traps along temporary plastic fences in three different locations. We are now working with the B.C. Ministry of Transportation to design an experimental amphibian tunnel at the location where we consistently catch the highest number of amphibians. A road survey, literature review and on-going discussion are all part of designing an appropriate tunnel. Future posts will describe this work and the actual installation process as it unfolds.

We have formalized ourselves into a non-profit society called The Association of Wetland Stewards for Clayoquot and Barkley Sounds. This project still falls under the title "SPLAT" which is affectionately known as the "Society for the Prevention of Little Amphibian Tragedies".