Desert Bighorn Sheep Restoration in Texas: Survival, Population Dynamics, and Habitat
Date
2018-11-23Metadata
Show full item recordAbstract
Bighorn sheep (Ovis canadensis) once occupied mountain ranges from western Canada
to northern Mexico in North America. The distribution and abundance of mountain
sheep in North America have declined from >500,000 historically, to 185,000 in the
1990s. In Texas, there were 1,000-1,500 desert bighorn (O. c. mexicana) living in 16
mountains ranges within the Trans-Pecos region during the late 1800s. Declines
resulted from a combination of factors including competition for forage with domestic
livestock, introduced diseases from domestic animals, unrestricted hunting, and
restriction of movements by net-wire fencing. By the mid-1940s, bighorn sheep
populations were estimated at 35 individuals, and by early 1960s the last Texas native
desert bighorn was extirpated. One successful approach to the conservation of large
mammals has been their translocation into former habitats. While translocation
strategies have been successful for many species, translocations of large ungulates can
be expensive and time consuming, as well as logistically and politically challenging.
Beginning in 1957, the Texas Game and Fish Commission brought desert bighorn from
Arizona to a breeding facility to initiate a restoration process. Over the next 4 decades,
a total of 146 desert bighorn were transplanted to Texas facilities from other states.
This study was initiated to fill gaps in the autecological knowledge of desert bighorn in
order to inform management decisions and maximize the potential for long–term
success of translocated desert bighorn populations. The objectives of this study
included: (1) analysis of survival and cause-specific mortality, (2) assess various
strategies to conduct translocations of desert bighorn in Texas using a system modeling
approach, and (3) evaluation of potential desert bighorn distributions utilizing a
probability occurrence distribution model at a landscape scale within the Trans-Pecos
region of Texas. Results for the first objective, from the 172 collared individuals a total
of 57 mortalities was recorded (25 M, 32 F). Causes of mortality were: 27
undeterminable, 20 by mountain lion predation (Puma concolor), 5 were attributed to
contagious ecthyma (parapox orf virus), 1 poached in Mexico, 1 birth complication, 1
infection due to a broken jaw, 1 ingestion of toxic vegetation (cloakfern,
Astrolepis sinuate), and 1 fell from a cliff. For the second objective, results indicated
that the number of years required for the population to reach carrying capacity (1) was
reduced when proportionally more females than males were reintroduced, (2) was
reduced slightly more by shorter than by longer time lags between the initial and the
second reintroduction, although differences were negligible, and (3) was reduced when
a larger number of animals (representing a larger proportion of carrying capacity) was
reintroduced. Results for objective 3 showed slope (49.74%) to have the greatest
variability explanation followed by elevation (21.26%). The model was able to explain
95.73% of variability by using 4 variables. Distribution values for slope demonstrated
selection values ranging from 0.09 to 314, having a median of 56.6 with a lower
quartile of 38.2 and upper quartile of 76.3. Elevation values showed greater selection
for elevations between 1,200 m and 1,600 m having the median of 1,459 m. Elevation
values ranged from 721 m to 2,024 m. In conclusion, reintroductions are increasingly
used to re-establish populations of threatened species. However, many reintroduction
attempts have been unsuccessful and the main reasons of failure are seldom
understood. Monitoring should continue to provide the primary tool by which we learn
about the success or failure of conservation investments.
Citation
Gonzalez Gonzalez, Carlos Eduardo (2018). Desert Bighorn Sheep Restoration in Texas: Survival, Population Dynamics, and Habitat. Doctoral dissertation, Texas A & M University. Available electronically from https : / /hdl .handle .net /1969 .1 /174532.