Roads Don't Just Cross Habitat—They Divide It

When we think about the danger roads pose to wildlife, we usually picture an animal trying to cross the road and getting hit by a vehicle. It's the most obvious—and tragic—part of the problem. But the more I learn about wildlife and roads, the more I realize that a road can affect an animal without ever touching it.

A road doesn't just create a dangerous place to cross. It can divide the habitat animals depend on, separating one part of their world from another.

Imagine a large forest stretching across a landscape. To us, it looks like one continuous forest. An animal living there might move freely from one part to another, following food, water, shelter, or other animals. Then a highway is built through the middle of it. The trees are still there. The forest hasn't disappeared. But suddenly, moving from one side to the other means crossing a road.

For some animals, that may not be much of an obstacle. A bird can fly over it. A deer may cross when traffic is light. But for many other animals, a busy road can become something they avoid. Traffic, noise, headlights, pavement, and the danger of being hit can make crossing difficult or dangerous. Over time, animals may simply stop using parts of their former habitat. That means a road can make a habitat smaller without actually removing much of it.

A forest on one side of a highway and a forest on the other side may look perfectly connected to us. But if an animal rarely crosses between them, those two areas aren't functioning as one continuous home anymore. Food might be on one side. Water might be on the other. A nesting area might be separated from a feeding area. A young animal looking for territory or a mate might find its path blocked.

This is one reason the effects of roads are harder to understand than simply counting roadkill. We can see the deer beside the highway. We can see the turtle that didn't make it across. But we can't see the animal that turned around.

We don't see the animals that stop using an area because getting there has become too dangerous. We don't immediately see what happens when a population becomes separated from another population. And we don't always recognize that a piece of habitat can still be there while becoming much less useful to the animals that depend on it.

Scientists call this habitat fragmentation, but the basic idea is pretty simple: when we divide a natural area into smaller pieces, we can make it harder for wildlife to move through the landscape.

That movement matters. Animals need to find food and water, raise young, establish territories, and find mates. Some species also need to move seasonally between different parts of their range. When roads interfere with those movements, the effects can accumulate over time. The Federal Highway Administration identifies habitat fragmentation and barriers to wildlife movement as important ecological effects of roads.

There can be another problem too. A road doesn't only create a strip of pavement. It creates a disturbed edge through whatever habitat surrounds it. For some species, that may not matter much. For others, the quieter and more protected conditions farther from roads are important. The Federal Highway Administration notes that the effects of roads can extend beyond the actual area occupied by the pavement, including changes to the surrounding habitat.

And then there is the traffic itself. Noise, headlights, movement, and the constant presence of vehicles can change how animals use the area around a road. Research summarized by the U.S. Geological Survey has found that roads can reduce habitat quality and interfere with wildlife movement, even when animals aren't being killed directly.

Of course, not every road has the same effect. A quiet country road is very different from a six-lane interstate. The amount of traffic, speed of vehicles, width of the road, surrounding landscape, and the species involved all make a difference. A bird may barely notice a highway that is a major obstacle for a turtle or salamander.

That's an important part of this issue. There isn't one universal way that wildlife experiences a road. Every species has its own needs, and every landscape is different. The good news is that once we understand the problem, there are ways to help.

Wildlife crossings are one of the most obvious examples. You've probably seen photographs of large bridges covered in vegetation that allow animals to cross over highways. There are also tunnels, underpasses, modified culverts, and fencing systems that help guide animals toward safer crossing points.

These structures aren't simply places for animals to cross a road. Their larger purpose is to reconnect parts of the landscape that the road has separated. The Federal Highway Administration identifies both reconnecting habitat and reducing wildlife-vehicle collisions as important goals of wildlife crossings. And I think that distinction is important.

If we only think about roadkill, our goal is to prevent animals from being hit. But if we think about the larger problem, our goal becomes something more: keeping wildlife connected to the places they need to live. The next time you drive through a forest, wetland, or open stretch of countryside, it's easy to see the road as simply a way to get somewhere. That's what roads are for us. But wildlife sees the landscape differently.

A road may be a dangerous place to cross. It may separate one part of an animal's range from another. It may make an important source of food or water harder to reach. It may separate animals from potential mates or isolate one population from another. And sometimes, the animal doesn't have to die for any of that to happen.

That's what I think we miss when we look only at the animals lying beside the road. We see the immediate consequence, but not always the larger change taking place around it. Roads are an important part of our lives. We need them, and they're not going away. But that doesn't mean we have to accept that wildlife simply has to adapt to whatever we build.

Maybe the better question isn't only, "How do we stop animals from getting hit?"

Maybe it's also, "How do we keep the places they depend on connected?"

That's a bigger challenge. But it also gives us something to work toward.

Because the road doesn't just cross the habitat.

It can divide it.

For the wild.

References

Federal Highway Administration. Wildlife Crossing Structure Handbook: Design and Evaluation in North America. Federal Highway Administration — Wildlife Crossing Structure Handbook

U.S. Geological Survey. Wildlife Friendly Roads: The Impacts of Roads on Wildlife in Urban Areas and Potential Remedies. U.S. Geological Survey

Federal Highway Administration. Wildlife Crossing Structure Handbook, Chapter 4: Function of Wildlife Crossings and Associated Measures. Federal Highway Administration — Wildlife Crossings