Conservation Corridors

Infrastructure: Not Just for Humans Anymore?

Toads making their way through a concrete-walled tunnel

A toad tunnel in Wales

Toad tunnels might seem like kind of a wacky idea at first glance. That’s especially true if you’re looking at the one that’s referenced most often: it’s located in Davis, California, and has a cutesy little wooden house topped with solar panels near the entrance.

Toad Hollow: a pair of small green houses no more than three feet tall grace the entry to the Davis, CA toad tunnel.

Toad Hollow

Tunnels built to let amphibians cross highways without getting hit by passing cars have been used in Europe for decades, according to a National Geographic article on the subject. During breeding season, usually March or April, hundreds of frogs, toads, and salamanders may have cross roads to get to their habitual spawning grounds.

There are definitely reasons to be skeptical about amphibian tunnels. If they’re not built right, the critters are likely to skip the entrance altogether. Scott Jackson, a biologist quoted in the article mentioned before, says tunnels should be about two feet by two feet and “fitted with an iron grate [across the top] to allow sufficient air, light, and moisture into the tunnel.” If there’s not enough moisture along a tunnel to keep an amphibian happy, it might just turn around and look for a different route.

A tunnel under a road in the process of being built

The SPLAT project’s tunnel, under construction

The morbidly-named SPLAT project on Vancouver Island, BC has spent the last four years working with the B.C. Ministry of Transportation to design and build an experimental tunnel, after a previous five years of catching amphibians in pit traps and carrying them across. The SPLAT blog details the methods they’ve tested over the years for creating a comfortable passageway and funneling amphibians into it rather than the more dangerous overland routes. So far, they’ve recorded nearly 300 frogs, newts, and salamanders moving through it, and they’re continuing to improve on their design.

A map of the Appalachian trail, a red line extending from Maine to Georgia.

The Appalachian Trail is one of the largest and best-known conservation corridors in the US.

Amphibian tunnels are a miniature example of a wildlife corridor, or conservation corridor. Conservation corridors are usually long and narrow areas of protected or restored wilderness, which connect what are known as habitat fragments—wildlife habitats that have been blocked off by human activity. The idea is for populations to be able to mingle and breed, as well as to clear paths for north-south migrations.


Engineering has historically been focused on improving life for humans, and taking care of the rest of the world has been a specialized goal for naturalists, biologists, and environmental activists. However, recognition is growing across industries that ignoring environmental concerns is not a great long-term strategy. A Google search for “green engineering” brings up about 402,000 hits, for example. This might, of course, be partly because of how many companies are realizing that “going green” is a wonderful way to market to the growing number of people who are worried about the environment but aren’t quite sure what to do about it—but that’s another blog post.

Bringing environmentalism into engineering, and vice versa, does involve some growing pains. Changing longstanding beliefs and values of any culture is not easy work. On the other hand, engineers often have a great talent for getting enthusiastic about new ideas and ways of doing things. The authors of Cradle to Cradle tell a story about an engineer working on the ecologically-conscious redesign of Ford’s River Rouge plant who went from refusing to “[talk] to no eco-architects about no eco-architecture” to being one of the project’s most dedicated supporters. I, for one, look forward to watching what comes from this unlikely-but-necessary marriage.