Proceedings of the 2013 International Conference on Ecology and Transportation (ICOET 2013)
EVALUATION OF DESERT BIGHORN SHEEP OVERPASSES ALONG
US HIGHWAY 93 IN ARIZONA, USA
Jeffrey W. Gagnon ([email protected]), Arizona Game and Fish Department, Wildlife Contracts
Branch, 5000 W. Carefree Highway, Phoenix, AZ 85086, USA
Chad D. Loberger, Arizona Game and Fish Department, Wildlife Contracts Branch, 5000 W. Carefree
Highway, Phoenix, AZ 85086, USA
Scott C. Sprague, Arizona Game and Fish Department, Wildlife Contracts Branch, 5000 W. Carefree
Highway, Phoenix, AZ 85086, USA
Mike Priest, Arizona Game and Fish Department, Wildlife Contracts Branch, 5000 W. Carefree
Highway, Phoenix, AZ 85086, USA
Kari Ogren, Arizona Game and Fish Department, Wildlife Contracts Branch, 5000 W. Carefree
Highway, Phoenix, AZ 85086, USA
Susan Boe, Arizona Game and Fish Department, Habitat Branch, 5000 W. Carefree Highway, Phoenix,
AZ 85086, USA
Estomih Kombe, Arizona of Transportation, Research Center, 206 S. 17th Ave, Phoenix AZ 85007
Raymond E. Schweinsburg, Arizona Game and Fish Department, Wildlife Contracts Branch, 5000 W.
Carefree Highway, Phoenix, AZ 85086, USA
ABSTRACT
More than 20 years ago planning began for the improvement and realignment of US Highway 93 (US 93)
near Hoover Dam on the Arizona-Nevada border. In those early planning stages Arizona Game and Fish
Department (AGFD) and other stakeholders expressed concern for existence the Black Mountain desert
bighorn sheep (Ovis canadensis nelson) herd, the largest contiguous population remaining in the
southwest. The impending realignment, widening and increased traffic would block sheep from food,
water, mates and lambing grounds and restrict genetic interchange, threatening the herd’s existence.
Further, sheep-vehicle collisions would increase, reducing both motorist and sheep safety. On September
11, 2001, commercial truck traffic across Hoover Dam was stopped because of terrorism concerns, and
Federal Highway Administration (FHWA) and Arizona Department of Transportation (ADOT) began the
upgrade of the first 2 miles (3 km) of US 93 to bypass the dam. Concurrently, final planning for the
adjacent 15 miles (24 km) was expedited to ensure its completion by the time the Hoover Dam Bypass
opened. Construction for both sections included complete realignment near Hoover Dam and upgrading
the 17 miles (27 km) to a four-lane divided highway. To address concerns for the local sheep population,
a Technical Advisory Committee (TAC) was formed. Also concurrently, an evaluation of three wildlife
underpasses built for desert bighorn sheep along State Route (SR) 68 showed minimal use by desert
bighorn sheep and pointed to the need for better designs. In the absence of data, overpasses seemed likely
the best option for this species. Given this sparse information, combined with results from monitoring
wildlife crossings on SR 260 for other species, the TAC understood the importance of proper placement
and design for successful wildlife crossings. Therefore, in 2004, AGFD/ADOT/FHWA initiated a study
along US 93 to identify exactly where sheep crossed Hwy 93. Ultimately, GPS movement data revealed
three locations to construct desert bighorn sheep overpasses. With their completion in fall of 2010, these
overpasses linked by fencing to four bridges and numerous culverts reestablished wildlife connectivity
across Hwy 93.
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Following overpass construction, AGFD, funded through ADOT Research Center, began evaluation of
the overpasses. . The objectives of our study were to evaluate; 1) wildlife use of overpasses, bridges,
culverts and escape ramps, 2) post-construction sheep-vehicle collision rates, and 3) highway
permeability for sheep. To meet these objectives we used video surveillance and still camera systems to
assess wildlife movement and behavior associated with the overpasses, culverts, escape ramps and cattle
guards. We also continue to collect wildlife-vehicle collision data to compare with pre-construction
levels. Additionally, we placed GPS collars on sheep to evaluate overpass and funnel-fencing use. On
February 1, 2011, we recorded the first crossings of a wildlife overpass by desert bighorn sheep. Sheep
continue to use the three overpasses and we’ve recorded more than 1000 crossings Sheep-vehicle
collisions are below pre-construction levels. We will provide detailed results of wildlife crossing use and
sheep-vehicle collisions from the first two years of the US 93 evaluation.
INTRODUCTION
Direct and indirect highway impacts have been characterized as some of the most prevalent and
widespread forces altering natural ecosystems in the U.S. (Noss and Cooperrider 1994, Trombulak and
Frissell 2000, Farrell et al. 2002). The direct impact of collisions with motor vehicles is a significant
source of mortality affecting wildlife populations. An estimated 500,000 (Romin and Bissonette 1996) to
700,000 (Schwabe and Schuhmann 2002) deer (Odocoileus spp.) alone are killed annually on U.S.
highways. Wildlife-vehicle collisions cause human injuries and deaths, tremendous property damage and
substantial loss of recreational opportunity and revenue associated with sport hunting (Reed et al. 1982,
Schwabe and Schuhmann 2002), and disproportionately affect threatened and endangered species (Foster
and Humphrey 1996).
Forman (2000) and Forman and Alexander (1998) estimated that highways have affected >20% of the
U.S. land area through habitat loss and degradation. Perhaps the most pervasive impact of highways on
wildlife is barrier and fragmentation effects resulting in diminished habitat connectivity and permeability
(Noss and Cooperrider 1994, Forman et al. 2003). Highways block animal movements between seasonal
ranges or other vital habitats (Trombulak and Frissell 2000). This barrier effect fragments habitats and
populations, reduces genetic interchange (Epps et al. 2005, Riley et al. 2006), and limits dispersal of
young (Beier 1995), all serving to disrupt viable wildlife population processes. Long-term fragmentation
and isolation renders populations more vulnerable to stochastic events that may lead to extinctions
(Hanski and Gilpin 1997).
Wildlife passage structures have shown tremendous benefit in promoting wildlife passage for a variety of
wildlife species (Farrell et al. 2002, Clevenger and Waltho 2003, Dodd et al. 2007a), by reducing the
impact of roadway traffic on wildlife movements versus those that would occur during at-grade crossings
(Gagnon et al. 2007a,b). Wildlife crossings, in conjunction with fencing have reduced the incidence of
wildlife-vehicle collisions while promoting permeability (Clevenger et al. 2001, Dodd et al. 2007b,c,
Dodd and Gagnon 2011).
Desert bighorn sheep occur throughout much of northern Mexico and the southwestern U.S. and are
distributed in naturally fragmented populations (Krausman and Leopold 1986, Bleich et al. 1990, 1996,
Andrew et al. 1999). Population persistence as it relates to bighorn sheep population size is a
controversial topic (Berger 1990, Krausman et al. 1993, 1996, Berger 1999, Wehausen 1999). In general,
no specific population size ensures population persistence (Thomas 1990), but small bighorn sheep
populations (Berger 1990) occupying marginal or comparatively poor habitat, or small patches of suitable
habitat (Gross et al. 1997, McKinney et al. 2003) may require management intervention to ensure longterm persistence. Habitat patch size may be the primary correlate of bighorn sheep population
performance and persistence (Gross et al. 1997, Singer et al. 2001, McKinney et al. 2003), but factors
other than patch size may influence extinction and colonization (Fleishman et al. 2002). As such, bighorn
Gagnon, Loberger, Sprague, Priest, Ogren, Boe, Kombe, and Schweinsburg
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conservation efforts should emphasize preventing habitat fragmentation and loss and restoring habitat
(Fahrig 1997).
Throughout the Southwest, including Arizona, most populations of desert bighorn sheep are small (<100)
and isolated (Krausman and Leopold 1986). Berger (1990) concluded that desert bighorn populations
with fewer than 50 individuals tend to go extinct, but that extirpation of populations is not caused by food
shortages, weather, predation, or interspecific (e.g., with cattle, burros, etc.) competition. Desert bighorn
sheep populations may be fragmented and isolated by anthropogenic influences such as highways, fences,
railroads, agricultural developments, canals, and housing developments (Leslie and Douglas 1979,
Gionfriddo and Krausman 1986, Rodriguez et al. 1996, McKinney and Smith 2007). Traditional
management techniques such as habitat protection and improvement and maintenance of dispersal
corridors are important in conservation of bighorn sheep populations (Schwartz et al. 1986). However,
Beier and Loe (1992) suggested preservation of natural wildlife movement corridors and creation of new
corridors may not preserve connectivity if the location of these corridors are not scientifically based.
Though wildlife passage structures have been widely used in North America to enhance permeability and
reduce wildlife-vehicle collisions for a range of wildlife species, limited information exists on the efficacy
of passage structures in promoting permeability for desert bighorn sheep populations. Evaluation of
underpasses by Bristow and Crabb (2008) indicated little use by sheep, particularly ewes, leading to the
incorporation of overpasses on US 93 with their locations defined by GPS movements to overcome
potential design and placement issues.
To gain adequate insight into the efficacy of overpasses in promoting habitat connectivity for desert
bighorn sheep, long-term monitoring is essential to determine adaptation of sheep to the newly built
structures over time. Along SR 260 in central Arizona, elk and deer did not immediately use certain
structures possibly due to their design or placement (Dodd et al. 2007a). However, after a 4 year period
of monitoring, most structures were used by elk and deer regularly and in fact structure use increased over
time (Gagnon et al. 2011). Had researchers drawn final conclusions from only a couple years of
monitoring, recommendations and implementation of future structures would have been severely limited.
The expectation is that long term monitoring of the structures will provide data to validate their design
and justify the investment.
BACKGROUND AND STUDY AREA
U.S. Highway 93 (US 93) is the primary transportation route between Phoenix and Las Vegas, Nevada,
and has been congressionally designated as one leg of the CANAMEX (Canada to Mexico) Trade
Corridor upon which commercial and non-commercial traffic is projected to increase dramatically in the
future. US 93 currently crosses Hoover Dam (Milepost [MP] 0) on the Colorado River 70 miles
northwest of Kingman, Arizona, and 20 miles (32 km) southeast of Las Vegas. The study area extends
between US 93 MP 0 and 17, with a large majority of the Lake Mead National Recreation Area, and small
portions of Bureau of Reclamation at the dam site and BLM lands at the far southern extreme
Construction of a new US 93 highway alignment has been ongoing since 2003 between MP 0 and 2 to
bypass Hoover Dam to address traffic volume congestion and security issues; the overall project was
completed and the highway reopened in October 2010. US 93 traffic volumes on the new bridge over the
Colorado River, included as part of the Hoover Dam bypass project were forecast at 9,300 AADT in 1997
and are anticipated to increase to 16,400 by 2017 (Hoover Dam Bypass EIS; ADOT). US 93 from MP 217 was recently reconstructed from a 2-lane to a 4-lane divided highway with a standard median width
(108 ft (32 m) centerline to centerline).
The sheep population on the proposed study area is a sub-unit of the largest extant desert bighorn sheep
population in Arizona, encompassing the Black Mountains, and has served as a source herd for numerous
reintroductions of bighorn into several sites in Arizona and the Southwest. Elevations within the study
area range from 637 ft (194 m) at the Colorado River to 4,957 ft (1510 m) on Mount Wilson. This
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Gagnon, Loberger, Sprague, Priest, Ogren, Boe, Kombe, and Schweinsburg
topography is well suited to desert bighorn sheep and includes rugged mountainous terrain with steep
talus slopes and cliffs to dry washes among rolling hills (Cunningham and Hanna 1992). The sheep
population here provides enjoyment through wildlife watching on the study site and along the Colorado
River, as well as providing recreational opportunity to those who hunt sheep in the area. Desert bighorn
sheep have been a focal species of concern during all planning efforts for the reconstruction of US 93.
As planning progressed for the reconstruction of US 93 and the preferred alignment alternative was
selected, McKinney and Smith (2007) conducted an assessment of bighorn sheep trans-highway
movements relative to the proposed reconstruction plans. They focused on identifying potential locations
for future passage structures to enhance highway permeability for sheep and to reduce the incidence of
wildlife-vehicle collisions. Potential fragmentation of the desert bighorn population on the eastern side of
the study area from the larger population in the Black Mountains to the west was of particular concern.
McKinney and Smith (2007) instrumented 36 desert bighorn with Global Positioning System (GPS)
receiver collars, funded with the assistance of ADOT and the Federal Highway Administration, from
2004-2006 and obtained more than 73,000 fixes. They documented 5 areas corresponding to ridgelines
where sheep concentrated their movements and three of these locations were ultimately selected for
wildlife overpasses. These recommendations were incorporated into the 2008 Preliminary Bridge
Selection Report for US 93 MP 2-17, which also included two new mainline bridges (Table 1, Figures 1
and 2) for DESERT BIGHORN SHEEP passage. Concurrently, funnel-fencing, escape ramps and
installation of double-wide cattle guards were identified in the 2008 Environmental Assessment
Reevaluation for US 93, Hoover Dam to Milepost 17. The project team used this information to complete
planning and begin construction in early 2009. Construction of all wildlife crossings and funnel-fencing
wrapped up on March 15, 2011.
Table 1. US 93 Bridge and Wildlife Crossing Structures Implemented with Highway
Reconstruction between MP 3 and MP 17*.
Structure Name
Structure Type
Milepost
Length ft (m)
Overpass
12.2
202.0 (61.6)
Devils Wash Bridge Northbound (NB)
Bridge
8.0
290.0 (88.4)
Devils Wash Bridge Southbound (SB)
Bridge
8.0
268.8 (81.9)
Overpass
5.1
202.0 (61.6)
White Rock Canyon Bridge SB
Bridge
4.2
216.5 (66.0)
White Rock Canyon Bridge NB
Bridge
4.2
213.8 (65.2)
Overpass
3.3
202.0 (61.6)
Wildlife Crossing #1
Wildlife Crossing #2
Wildlife Crossing #3
*Source: ADOT 2008
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The US 93 project is located in the northwestern corner of Arizona along a 17-mile (27 km) stretch of
roadway near Hoover Dam that included the reconstruction of Hoover Dam Bypass (MP 0-2) and a
separate project that includes MP 2-17. For this study we evaluated the latter stretch of US 93, including
the three overpasses and two bridges along with select culverts linked with 7’ (2.1 m) woven wire fence
(Figure 1)
Figure 1. US Highway 93 study area showing landownership, mileposts (red numbers), 0.1-mile
segments (black numbers), and location of bridges and wildlife overpasses.
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Figure 2. Aerial and ground view of wildlife crossings along US Highway 93 near Hoover Dam, AZ.
This includes the overpasses at MP 3.3 (top), MP 5.2 (second from top), MP 12.2 (middle), and
White Rock bridge at MP 4.0 (second from bottom) and Devils Wash bridge (MP 8.0 (bottom).
.
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METHODS
With the extensive desert bighorn sheep research conducted along US 93 by Cunningham and Hanna
(1992) and McKinney and Smith (2007), our proposed research focuses on evaluating the effectiveness of
the proposed US 93 wildlife overpasses. This evaluation will add greatly to our understanding of desert
bighorn sheep-highway relationships and the effectiveness of passage structures to promote permeability
(McKinney and Smith 2007). The specific objectives and associated procedures of our proposed US 93
research project include:
1) Investigation of wildlife-vehicle collision patterns and rates along US 93.
2) Evaluation of wildlife use of overpasses the two bridges, escape ramps and select culverts using
video camera and Reconyx© still camera surveillance.
Wildlife-vehicle collision patterns
Collisions along US 93 cause significant mortality for desert bighorn sheep in the vicinity of Hoover Dam
(Cunningham and Hanna 1992, Figure 3). ADOT constructed passage structures and ungulate-proof
fencing intended to reduce sheep-vehicle collisions. Given the reduction in traffic, particularly semitrailer trucks, during the restricted access period to Hoover Dam, we compared the post-construction
collision rate to that documented prior to 9-11-2001 before truck traffic was diverted. We used two
methods to document sheep-vehicle collisions. First, AGFD Region III, Department of Public Safety
(DPS) highway patrolmen and ADOT maintenance personnel submitted sheep/vehicle collision records.
Second, we searched the highway corridor for evidence of wildlife-vehicle collisions every two weeks.
The database included non-duplicate records with date, time, location of the sheep-vehicle collision (to
the nearest 0.1 mile), sex of the animal(s) involved when known, and the reporting agency. We compared
the collision numbers to 11 sheep-vehicle collisions/year (Cunningham and Hanna 1992).
Figure 3. Desert Bighorn Sheep Killed in a Sheep-Vehicle Collision along US 93.
Wildlife use of structures
Factors that influence wildlife crossing use have been reported for many species, but not desert bighorn
sheep. Monitoring the three overpasses adds to our general knowledge when designing future bighorn
sheep crossings, as well as for other species in that habitat type. For this study, we evaluated crossing
structure use with an array of video and rapid-fire still cameras (Table 2, Figure 2 and 3). These
structures included all overpasses and bridges from MP 2-17, along with select culverts, escape ramps and
cattle guards (Table 2). Complex camera and trigger arrays detected approaching and crossing animals
used to calculate passage rates (Dodd et al. 2007). Passage rates are calculated as a ratio of crossings to
Gagnon, Loberger, Sprague, Priest, Ogren, Boe, Kombe, and Schweinsburg
approaches to provide an unbiased comparison of structure use. To minimize premature conclusions in
this report, the Research Teams limited our results to structure use only. Passage rate analysis will be
reported on following the ensuing long-term monitoring project (Gagnon et al. 2011). In the event of a
video system failure, rapid-fire still cameras were used at the overpasses to document crossing wildlife.
Still cameras were also installed at the culverts, escape ramps, and cattle guards.
Table 2. Structures monitored with video and/or still frame cameras within the study area.
Structure name
Milepost
Video camera #
Still camera #
Wildlife Crossing #1 (overpass)
12.2
X
1
Devils Wash Bridge
8.0
3
Wildlife Crossing #2 (overpass)
5.1
X
1
White Rock Canyon Bridge
4.2
2
Wildlife Crossing #3 (overpass)
3.3
X
1
Culvert 13.1
13.1
1
Culvert 6.0
6.0
1
Culvert 5.1
5.1
1
Escape Ramp Overpass #1 NW
12.2
X
Escape Ramp Overpass #2 NW
5.1
X
Escape Ramp 3.3 NW
3.3
1
Escape Ramp 3.3 NE
3.3
1
Escape Ramp 3.3 SW
3.3
1
Escape Ramp 3.3 SE
3.3
1
Escape Ramp Kingman Wash
2.2
1
Escape Ramp Sugar Loaf NE
1.1
1
Kingman Wash SE Cattle Guard
2.2
1
Kingman Wash SW Cattle Guard
2.2
1
Figure 3. During construction of the wildlife overpasses AGFD worked with ADOT and FNF
Construction to incorporate these seamless state-of-the-art video surveillance systems into the
overpass infrastructure.
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Figure 4. Structure surveillance system components: video camera with infra-red lights (top left),
four-feed video output (top right), video system trigger with still camera (bottom left), and still
image (bottom right).
RESULTS
Wildlife-vehicle collision patterns
In the first two years following construction, we documented seven sheep-vehicle collisions, all between
MP 0-4, which included two rams and five ewes (Table 3), therefore we realized a 45% collision year one
and a 82% reduction in year two compared to the baseline of 11 sheep-vehicle collisions/year
(Cunningham and Hanna 1992). The highest percentage of collisions occurred in July (43%) and
November (29%).
Table 3. Desert bighorn sheep-vehicle collisions following construction of wildlife crossings and
fencing along US Highway 93 in Arizona from March 15, 2011 to March 15, 2013.
Date
Mile Post
Sex
7/14/2011
2.2
Male
7/15/2011
2.0
Female
7/18/2011
3.1
Female
10/29/2011
0.9
Female
11/25/2011
0.8
Male
5/1/2012
3.1
Female
11/12/2012
0.8
Female
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We suspect the collisions were the result of sheep circumventing the ungulate-proof fencing in three
possible ways: 1) passing through holes in the fence, 2) jumping up escape ramps (the wrong direction),
and 3) jumping over single-wide cattle guards at the Kingman Wash traffic interchange. We are currently
collaborating with ADOT to restrict this access points.
Wildlife use of structures
We observed 1742 sheep using overpass structures and 179 using bridges to cross from one side of US 93
to the other (figure 5). Rams used all overpasses relatively equal while ewes and lambs used Overpass #3
most. Over time, use of structures vary, however use of bridges remains relatively low (Figure 6). Eight
other species utilized the overpasses, most notably coyote (Canis latrans). We did not document any
cougar (Puma concolor), major predator of desert bighorn sheep at the overpasses.
Figure 5. Desert bighorn sheep using various large wildlife crossings along US Highway 93 in
Arizona. Ewes and lambs (upper left) and a ram (upper right) cross US 93 via overpass structures
and sheep use White Rock Canyon (lower left) and Devils Wash (lower right) bridges to cross
under US 93.
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Figure 6. Graph depicting 1,921 desert bighorn sheep crossing US Highway 93 via three overpasses
and two bridges from March 2011 to March 2013.
Culverts
We documented 199 sheep approaching the three monitored culverts. Four sheep (2%) successfully
crossed US 93 using one of these structures at MP 6.0. The typical observed sheep behavior was an
approach to the structures followed by a retreat. On a few occasions, sheep entered the culvert and
quickly exited, leaving the area without crossing (Figure 7), perhaps because of traffic noise overhead.
Figure 7. This sequence of photos shows a typical failed attempt of a desert bighorn sheep ram to
cross under US Highway 93 using the culvert at MP 6.0
Escape ramps
We documented 150 sheep at the 8 escape ramps we monitored. Of the sheep that used the escape ramps,
322 of 337 (96%) used them in the proper direction while 44 of 1312 (3%) traversed them backwards and
accessed the ROW (Figure 8). Throughout the project, research team personnel added PVC to reduce
sheep movements in the wrong direction (Figure 8). However, no sheep have used monitored ramps to
enter the ROW since May 3, 2012 when the research team adjusted the PVC and metal pipes that had
been added to prevent sheep from ROW access to a height of 16 inches (0.4 m). In addition, the added
bar does not appear to prevent sheep from using the ramps to exit the ROW. Since May 3, 2012, 96% of
sheep that approach the ramp from the ROW side exit the ROW via the ramp. ADOT Maintenance began
replacing all PVC with adjustable metal pipes in January 2012
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Figure 8. Progression of modifications to eliminate desert bighorn sheep from entering the right of
way via escape ramps while continuing to allow exiting along US Highway 93 in Arizona. Sheep
entering ROW via escape ramp without modification (upper left), failed attempt at access to ROW
with low visibility smooth wire modification (upper right) and a sheep successfully exiting the ROW
via escape ramp under (lower left) and over (lower right) a bar provided as a visible obstruction.
Once the bars were included and adjusted to approximately 16” (0.4 m), sheep use of the ramps in
the wrong direction ceased and proper use continued.
Kingman Wash cattle guards
After construction was completed on US 93, we observed sheep in the ROW near the Kingman Wash
Traffic Interchange on several occasions. The Research Team observed that sheep were walking the
concrete edge of the cattle guard vault. To close off this access point, ADOT extended the fence into the
cattle guard and added a “painted cattle guard” to the ROW side of the existing cattle guard. We
continued to see sheep in the ROW and monitored the two southern-most Kingman Wash cattle guard and
noted sheep continuing to enter the ROW area by leaping the cattle guard (Figure 9). They landed on the
“painted cattle guard”. The cattle guards at the Kingman Wash Traffic Interchange remain a concern, so
we are working with ADOT to block these access points.
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Figure 9. Interactions of sheep at single cattle guards located at the Kingman Wash transportation
interchange. Sheep considering jumping (left) and successfully jumping (right) a cattle guard.
AGFD and ADOT are working toward a solution to reduce sheep crossings, potentially an electric
mat, that will be incorporated in fall 2013.
DISCUSSION
Preliminary analysis of sheep-vehicle collisions suggested that ungulate-proof fencing along with
wildlife crossings have limited the number of sheep-vehicle collisions along US 93. When compared to
Cunningham and Hanna’s (1992) sheep-vehicle collision rate of 11/year, we found a 45% and 82%
reduction in collisions during years one and two, respectively. We believe the addition of PVC at the
escape ramps accounted for the reduction in collisions between years one and two as sheep movement in
the wrong direction was eliminated. ADOT is working to replace the PVC with an adjustable metal pole.
Aside from the escape ramps, we determined sheep entered the ROW through holes in the fence and
single-wide cattle guards at the Kingman Wash Traffic Interchange. This information has allowed us to
close gaps in the fence and we are now considering and “electrified” cattle guards at the Kingman Wash
Traffic Interchange. Once all issues are addressed, we believe effectiveness of mitigation measure will
exceed the 82% reduction realized in year two.
Our video and rapid-fire still camera systems enabled us to document the use of all three overpasses 1,742
times by sheep including rams, ewes, and lambs. These preliminary results, when compared to Bristow
and Crabb (2008) who documented only 32 ram crossings of wildlife underpasses on nearby SR 68,
indicate that wildlife overpasses are highly effective for promoting sheep permeability. Preliminary
results also indicated that the use of all 3 wildlife crossing structures has increased sharply during the last
few months monitored, which indicates that sheep are becoming more accustomed to the structures and
use will continue to increase. This upward trend is similar to that found for elk by Gagnon et al. (2011),
who found that elk used passing structures in increasing numbers as time passed. However, this increase
may be due to seasonal or behavioral parameters that can only be identified through long-term
monitoring. We also found that sheep will pass under large-span bridges, but at significantly reduced
frequencies relative to overpass utilization. We have only documented the use of a culvert by four sheep
to successfully cross US 93.
Our long-term evaluation of the US 93 wildlife crossings and fencing will include assessments of
permeability through calculation of passage rates similar to those for elk and Coues’ white-tailed deer
(Odocoileus virginianus cousei) completed by Dodd et al. (2007b) and Dodd and Gagnon (2011). This
assessment will allow for comparison of the ability of desert bighorn sheep to cross the highway before
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during and after construction. Currently, approximately 38 desert bighorn sheep are collared along US 93
and these collars are collecting a location every two hours for two years. Further, our video surveillance
assessment will take into account unsuccessful crossings and determine passage rates and crossing
probabilities per Dodd et al. (2007a) and Gagnon et al. (2011). These passage rates will be used to
compare success of overpasses of different size while controlling for species distribution. This long-term
analysis is crucial to determining if wider more expensive 100’ (30 m) overpasses are necessary or if the
less expensive 50’ (15 m) overpasses are adequate.
RECOMMENDATIONS
Although we will hold final recommendations until the completion of the long-term monitoring study,
there are a few preliminary recommendations that need to be considered in the event that DOT’s move
forward with projects involving desert bighorn sheep prior to completion of our long-term evaluation.
Currently, overpasses appear to promote desert bighorn sheep habitat connectivity better than
underpasses. However, given the preliminary nature of these findings, planners should consult local
wildlife experts for the most recent findings and recommendations to ensure their sustained validity.
Whatever measures are considered as mitigation to reduce collisions and enhance connectivity, postconstruction monitoring should be considered to evaluate efficacy of those measures.
ACKNOWLEDGEMENTS
This project was funded by the Arizona Department of Transportation’s (ADOT) Research Center, the
Arizona Game and Fish Department (AGFD) and the Federal Aid Wildlife in Restoration Act, Project W78-R supporting AGFD research. We commend ADOT for its proactive commitment to promoting
wildlife connectivity and highway safety. The Federal Highway Administration also was instrumental to
the funding and support of the project. We sincerely thank the Arizona Desert Bighorn Sheep Society for
their financial support to help bridge the gap between during construction and long-term post-construction
monitoring. Lake Mead National Recreation Area and Bureau of Land Management Kingman Office
were instrumental in both successful implementation of mitigation measures and support of the ongoing
research projects.
BIOGRAPHICAL SKETCHES
Jeff Gagnon has worked for AZGFD since 1998, currently as a Research Biologist focusing primarily on
wildlife-highway interactions throughout Arizona, including State Route 260 and US Highway 93
wildlife crossing projects. Jeff received his B. S. and M. S. from Northern Arizona University where he
studied the effects of traffic volumes on elk movements associated with highways and wildlife
underpasses.
Chad Loberger is a research biologist for the Arizona Game and Fish Department working on projects
related to wildlife-highway relationships. Current research focuses on large ungulates interacting with
Interstates 17 and 40, and U.S. 93. He obtained his M.S. in biology from Northern Arizona University.
Scott Sprague has worked as a Research Biologist for AZGFD since 2002, focusing on wildlife-highway
interactions. He received his B. A. from Colgate University and M. S. from Northern Arizona University
where he studied the effects of roads on genetic interchange of pronghorn populations in northern
Arizona.
Mike Priest obtained his B. S. in Biology with a Fish and Wildlife Management emphasis from Northern
Arizona University. He has worked for AZGFD for four years on various projects, focusing on large
mammal research.
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Kari Ogren has worked for AZGFD for more than a decade mainly on wildlife-highway interaction
projects, primarily focusing on the tedious task of viewing and interpreting the unending supply of video
and camera surveillance data obtained during wildlife crossing monitoring.
Sue Boe received her B. S. and M. S. from the University of Minnesota - Duluth. She has worked for
AZ Game and Fish Department since 1992 as GIS analyst on a wide variety of projects, both terrestrial
and aquatic, including numerous wildlife-highway interaction projects.
Estomih Kombe completed his graduate research at Arizona State University and in 1995 joined ADOT
Research Center as a Project Manager for transportation research projects. Estomih has been involved
with several collaborative wildlife-highway research projects with AGFD. Prior to this, Estomih was a
member of the academic staff of the Faculty of Engineering at the University of Dar Es Salaam in
Tanzania, where he taught Project Management and Industrial Safety classes.
Ray Schweinsburg has been a research program supervisor with the department for 20 years. He
received his Ph.D. from the University of Arizona and currently focuses on enhancing wildlife habitat
connectivity throughout Arizona.
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