The Desert Bighorn Sheep is an essential part of the wildlife in northwest Arizona, yet highways have separated and endangered these populations. Due to wildlife-vehicle collisions, interference with access to adequate water, and lack of habitat connectivity, bighorn populations are at risk.

To increase genetic diversity, many sheep do not need to migrate from one subpopulation to another. However, sheep become more vulnerable to outside threats if their genetic diversity diminishes. For example, an isolated herd of fewer than 50 animals can be wiped out by disease or drought. In addition, localized catastrophes such as drought could last so long that the animals cannot replace themselves and die.

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A bighorn population would naturally replace another population that had declined before artificial barriers fractured its habitat. However, if these areas are cut off from one another, the natural process of refreshing a population is not possible.

Bighorn sheep walked across valleys when they sought mates, food, water, or whatever else, drawing them to an adjacent mountain range before humans created barriers. In addition to fragmenting wildlife habitats and blocking connectivity between populations, highways also destroy and degrade home ranges. Highway construction has destroyed or altered at least 20 percent of wildlife habitats in the U.S., according to studies conducted in 1998 and 2000. Collisions also reduce wildlife populations with vehicles on roads. Other studies estimate 500,000 and 700,000 deer are killed on U.S. highways yearly.

As part of a historic and innovative collaboration between the U.S. Fish and Wildlife Service, Federal Highway Administration, and Arizona Department of Transportation, the Arizona Game and Fish Department developed a solution that saves bighorn sheep and drivers. 3 wildlife overpasses and adjacent funnel fencing were constructed along Highway 93 north of Kingman to reduce vehicle collisions with bighorn sheep. As a result of this infrastructure, wildlife can cross over roads safely, keeping them connected to their habitat and reducing the risk of injured motorists and wildlife.

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Unlike most highway overpasses, the bighorn bridges have dirt surfaces planted with native grasses on top of concrete overpasses. The sheep are guided safely toward three bridges, two of which are 50 feet wide, and the third is 100 feet wide, by funnel fences. In addition, sheep are prevented from jumping off bridges onto roads by parapet fencing.

After the U.S. 93 reconstruction mitigation measures, the crossings are already showing positive results with active monitoring. Findings already show that Desert Bighorn Sheep vehicle collisions dropped by at least 85 percent.

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