Maintaining Desert Bighorn Sheep

The Black Mountain herd is the largest extant population of desert bighorn sheep in Arizona and has acted as a source herd for hundreds of transplanted bighorn throughout the Southwest. Desert bighorn sheep generally occur in small fragmented groups or subpopulations that interconnect in what is known as a metapopulation. A “barrier”, like a roadway, a canal, or fencing blocks the movement of animals from others of their kind or essential habitats. Rams have “rut runs” that consist of movement corridors that lead from one ewe band to the next. Infrastructure can prevent rams from travel, thus disrupting the breeding cycle. Likewise, bighorn may be cut off from lambing or feeding areas or interfere with access to adequate water. To mitigate for these ill effects, transportation agencies have agreed to work with the Arizona Game and Fish Department and other natural resource agencies to identify places where bighorn cross the highway and design structures that will enable bighorn to safely cross above and below the highway.

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Location:
The area of concern lies within the Lake Mead National Recreation Area in northwestern Arizona. U.S. Highway 93 bisects the northern part of the Black Mountains separating Wilson Ridge from the remainder of the range and Black Canyon. Highway construction is occurring in desert bighorn sheep habitat from Hoover Dam south for 17 miles to the existing four-lane divided highway south of the recreation area.

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Photo: George Andrejko

Approach:
Phase I
The first phase of research took place from 2004-2006 and involved deploying GPS radiocollars on desert bighorn sheep with GPS units along 17 miles of U.S. Highway 93 in the Lake Mead National Recreation Area. Thirty-six desert bighorn provided 75,000 data points used to address the following objectives:

Identify desert bighorn distribution, movements and travel corridors in relation to the U.S. Highway 93 alignment in order to recommend mitigation to maintain or enhance permeability (passage rate) relative to the upcoming highway upgrade

Determine distribution and movements of bighorn sheep in relation to the Hoover Dam Bypass to evaluate current highway mitigation efforts

Evaluate collisions, injuries, and mortalities of bighorn sheep resulting from highway vehicle traffic.

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Three main travel corridors were identified; all associated with major ridgelines connecting Wilson Ridge with the rough terrain of Black Canyon. The Arizona Department of Transportation and Federal Highway Administration committed funds for three highway wildlife crossing structures in the same three areas desert bighorn preferred to cross. Overpasses, due to bighorn sheep behavior and construction logistics lent themselves well as proper bighorn mitigation of road development and were chosen as Arizona’s first to be built for wildlife and the first for desert bighorn anywhere. However, overpasses remain untested on bighorn sheep and additional study is underway to test their effectiveness.

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Ridgeline connecting sheep habitat on Wilson Ridge to that of Black Canyon and the rest of the Black Mountains. Note U.S. Highway 93 bisecting the ridge. Future site of a wildlife overpass

Phase II
To be able to assess the post-construction efficacy of the planned overpasses in promoting highway permeability for desert bighorn, a pre-construction evaluation of the same is needed. In November 2008, 30 more desert bighorn were caught and placed with satellite uplink radiocollars. Objectives of Phase II are:

Assess desert bighorn sheep movement, highway crossing patterns and distribution relative to US 93, and assess permeability across the highway corridor.

Investigate the spatial and temporal relationships of desert bighorn sheep highway crossing and distribution patterns to vehicular traffic volume.

Investigate wildlife-vehicle collision patterns along US 93.

Assess the impact of US 93 highway construction activities on desert bighorn sheep movements and behavioral patterns and develop mitigation strategies to minimize these impacts.

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Artist’s rendition of the same ridge as above with a wildlife overpass in place

Phase III
In the future, more desert bighorn will be radiocollared post-construction to determine if the overpasses to be built are functional and vehicle-bighorn mortality have been reduced. Cameras and video equipment installed on the overpasses will provide additional data on permeability. Animals in Banff Provincial Park in Canada experienced a “learning curve” associated with traversing an overpass. Some species took a decade to feel comfortable using the structure.

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Benefits:
The redevelopment of U.S. Highway 93 will undoubtedly reduce traffic congestion and potentially reduce wildlife-vehicle collisions. Without traffic on Hoover Dam, security issues there will be less intense. As for desert bighorn sheep, the future is very optimistic; likely less bighorn hit on the road, ways to cross the highway safely and perhaps an increase in permeability as bighorn that approached but did not cross previously, learn a way to do so without danger. All in all, this stands to be a win/win situation for both motorists and desert bighorn sheep. This undertaking has been the first time in Arizona that data from wildlife has been used to dictate construction prescriptions at the planning stage and represents the success in partnerships between transportation and natural resource agencies.

Reports:
Phase I results in final report to ADOT and FHWA

For more information contact:
Thorry Smith, Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086-5000.
E-mail: tsmith@azgfd.gov

Jeff Gagnon, Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85023
E-mail: jeff_gagnon@yahoo.com