Culverts- the unexpected partner for wildlife connectivity

It’s no surprise that wildlife-vehicle collisions are a problem. If you drive, you’ve witnessed roadkill.  

In the U.S alone, more than one million collisions with large animals occur every year, leading to 200 human fatalities, 26,000 injuries and $8 billion in damages. If you want to make these numbers look minuscule, check the number of all animals killed by vehicles, smaller critters included. 

We have to help the bears, elk, deer, coyotes, skunks and more safely cross the road, and an unexpected hero is rising to the challenge. Culverts, as in the extraordinarily ordinary pipes that manage water drainage and prevent flooding by conveying water beneath roads and other structures. 

How Culverts Help

Although it wasn’t the intended use, culverts provide a great opportunity for wildlife to cross the road without having to dip and dodge speeding cars. The discovery of just how often animals use culverts has given scientists, conservationists and engineers the perfect opportunity to further advance wildlife connectivity by designing culverts with animals in mind. 

When it comes to culverts, one size does not fit all. There are a number of design considerations at play to ensure these passageways save as many lives as possible. Size, structure and maintenance are all important. For example, a study found that deer chose not to use tunnels with cobbled floors. If the arch is curved, count Eastern Gray Squirrels out. If the culvert resembles a box shape, you might expect to see some great blue herons. (Who knew how particular animals can be?)

What Designs Work?

Animals are selective when it comes to culverts, and considering species’ needs help ensure that the animals can use these culverts as intended. 

Reptiles and amphibians prefer soil moisture and humidity. To create this in the culvert, the top can be grated to allow sunlight to pass through. If this isn’t possible, the structure’s ends should be grated past the edge of the roadway. 

Semi-aquatic species, such as the river otter, beaver and muskrat, prefer a crossing when there is aquatic habitat present or nearby. Ripe vegetation should be maintained throughout the culvert to encourage these species to come on through. 

For small and low mobility species, cover is critically important. This serves to both moderate their internal body temperature as well as make them feel safe from predators.

Wildlife corridors are on the rise. Congress committed $350 million to wildlife crossings. Other bills, including the Wildlife Movement Through Partnerships Act, will help. 

And, as it turns out, the choice of culverts can make a big difference. Road builders, county engineers, DOT officials and others should choose culverts with critters in mind. Doing so can help the human world live in better harmony with all the other species on this planet.