Bighorn movement near California's highwaysIn Southern California, like many other parts of the world, humans have substantially changed the landscape in which wildlife live and move. Though it still looks very open to us, the Mojave Desert is much more developed and divided than when bighorn first evolved here. Using GPS data, we've been piecing together the bighorn view of the landscape and learning how human-made barriers have limited their movement.
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Bighorn vs barriers
This study focuses on the boundary of the Mojave National Preserve, where two major highways (I-15 and I-40) form a risky barrier to wildlife movement. A number of bighorn populations live very near these highways, though you're unlikely to spot them driving by at 70 mph!
Bighorn used to move across the valleys where these interstates were built. Now, the risk of crossing has proven too high, and we see very few animals attempt these movements. The loss of this behavior has reduced gene flow between populations, and at a larger scale, this effects the connectivity of the overall network of desert bighorn populations in the region.
If we want to improve movement across these barriers, where should we focus and where might bighorn go if we succeed?
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Ram 1707 regularly moved across low-desert, but the risk of crossing roadways caught up to him and he was struck and killed by a vehicle in the Mojave National Preserve. Photo: National Park Service
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Turning movement into models
We track bighorn movement with GPS collars. The collar communicates with satellites to estimate where the bighorn sheep is at every hour throughout the day. We can download those locations without having to recapture the animal.
Next, we describe bighorn movement using models. In ecology, models can be something simple like a diagram of how a process works, or something more complex, like a mathematical formula. Our models break-down bighorn movement into a step-by-step process and use statistics to estimate what features of the landscape bighorn prefer or avoid. These models are called step selection functions, because they determine what habitat bighorn select as they make steps from location to location.
A world without barriers
A good model can make predictions that match up pretty well to real-life. And they allow us to predict what might happen in new situations. There's no pre-highway GPS data to tell us how bighorn used to travel throughout this region. So we're using our models to simulate where bighorn would go if highway barriers were to suddenly disappear.
Here's what we're hoping our simulated sheep (simu-sheep?) will tell us:
- Where would bighorn most likely cross the valley that's currently blocked by the highway?
- What habitat do we expect them to use on the other side? And how often?
- How much does this differ from current conditions?