Gonzalez Gonzalez, Carlos Eduardo (2018-11). Desert Bighorn Sheep Restoration in Texas: Survival, Population Dynamics, and Habitat. Doctoral Dissertation.
Thesis
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.
