You can lead a Treefrog to an underpass, but you can't make it enter...

We tested the effectiveness of our new fence design using wildlife cameras. Two cameras took time-lapse photographs of our fences at the entranceway of the culvert. Surveillance started in September 2014 but we did not get a chance to look through the images until this winter.

For the most part, Northern Red-legged Frogs and Northwestern Salamanders moved without hesitation along the ground at the base of the fences into the culvert. We counted more of them moving through to the other side on peak nights than in previous years.

Northern Pacific Treefrogs, however, did not take the culvert in stride. Our September surveillance period showed dozens of juvenile treefrogs climbing the fences just outside the entranceway.

The young tree frogs were not only good climbers, they also found the weak points where the fences joined the culvert. There was a cut in the fabric on one side, and a bunched fold on the other side. At each spot the overhanging fabric came close to touching the upright wall and gave the climbing frogs a relatively easy escape hatch. It was remarkable to watch so many of them follow the same route around the overhanging lip and over the top!

After realizing the flaws, we braced the overhanging fabric with wood at the joins.

We set out the cameras again in February to see if we had made the fence more escape-proof. Granted, a week of surveillance in February is not the same as a week of surveillance in September. There were no juvenile treefrogs around to test our improvements. Instead, we watched a few adult treefrogs and a salamander climb the fences. We were happy to see that none of them escaped. We will have to wait until next September to see whether our fix is also effective for the next generation of juvenile treefrogs.

Making an escape-proof fence for treefrogs is useful in reducing the number killed by traffic. We also hoped it would guide them to take the safe passageway under the highway. It was too difficult to detect juvenile treefrogs in the grass to track the number that actually did enter and move through the culvert. Perhaps some did, however seeing so many treefrogs climbing begs the question: are culverts effective in connecting habitats across roads for these climbers? 

We're not the first to wonder about this problem… Kenneth Dodd and colleagues showed that culverts and concrete barriers helped other amphibians, but not Green Treefrogs or Squirrel Treefrogs, cross a busy highway in Florida.

Various researchers in Europe have also noted that treefrogs do not readily move through culverts. A Dutch publication on Boomkikkers (the Dutch word for treefrogs) recommends not interrupting connections between breeding and foraging habitats across the landscape when new roads are planned.

The fence installation and effectiveness monitoring were financially supported by the B.C. Ministry of Transportation and Infrastructure, the Clayoquot Biosphere Trust and the Government of Canada, as part of the National Conservation Plan.