Underpasses as wildlife corridors
In Southern California, many highways fragment habitat used by a variety of wildlife, including bighorn sheep. The high traffic volumes make crossing the roadway especially dangerous, but there are other ways to cross these barriers...
Along each highway, underpasses of different shapes, sizes, and design provide a safe path to get to the other side. These structures were not designed for wildlife, but do animals still use them anyway?
How did the bighorn cross the road?
Our research focuses on the boundary of the Mojave National Preserve, where two major highways (I-15 and I-40) form a barrier to bighorn movement. A few brave bighorn have managed to cross both highways - did they time the traffic just right, or did they use an underpass?
We set out an array of camera traps at underpasses near bighorn habitat to document what wildlife make use of these structures. The cameras were out for three years, taking a mountain of photos and videos. Our team of students, volunteers, and early career biologists reviewed this camera data and identified the wildlife captured on camera. You can explore this data on Wildlife Insights:
If you build it, they may not come...
After three years of monitoring, we saw little evidence that bighorn sheep used underpasses or culverts to cross either highway. We suspect there are two main reasons:
1. Underpasses along I-15 and I-40 are far from areas that bighorn use frequently |
2. Underpasses near bighorn activity are rarely tall, open structures |
It's possible that bighorn sheep just didn't want to cross the highway. BUT during the study two bighorn were struck by vehicles and killed while trying to cross I-15 and I-40. Bighorn still seem to be taking the direct approach, despite the risk of traffic!
Can you teach an old bighorn new tricks?
We tried to make some underpasses more attractive to bighorn sheep. One year into the study, we added a guzzler - an artificial water source - near three underpasses along I-40. We hoped the water would bring bighorn down where they could observe the opening under the highway.
Unfortunately, it didn't work. We never photographed bighorn using the guzzlers or underpasses.
It may take time for some animals to become familiar with a new resource. Maybe many years down the road this approach might teach bighorn that they can cross safely, but maintaining water structures for years to come takes consistent effort and resources.
It may take time for some animals to become familiar with a new resource. Maybe many years down the road this approach might teach bighorn that they can cross safely, but maintaining water structures for years to come takes consistent effort and resources.
Connectivity for some
It's important to note, that many species of wildlife DID use these underpasses to cross the highway. The underpasses may not be pretty (the amount of trash is just plain gross), but for many animals they're a way to access more habitat
LEFT: Detection rates for animals observed in or at the tunnels of underpasses along I-15 and I-40. The only bighorn sheep spotted near underpass tunnels were at the mouth of a culvert along I-40. We didn't see evidence that they ever tried to go all the way through. RIGHT: Many other species, however were recorded moving through tunnels.
Time for a highway makeover
Based on what we observed, and research on bighorn sheep near highways in other states, we think there are two possible solutions to this problem:
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It's time for many highways in California to get a makeover. Well-designed crossing structures for bighorn sheep can improve habitat connectivity, gene flow, and population resiliency. The best way forward is to incorporate wildlife-friendly designs into new projects and infrastructure plans.