Conservation and spatial use analyses for the recovery of bighorn sheep in the Peninsular Ranges

Abstract

The status of wild sheep in North America typifies the plight of many wildlife species in modern times: wild sheep have declined to 10-40% of their numbers during pristine times and on a global scale approximately 31% of Caprine are considered threatened or critical. As human populations and the number of threatened and endangered wildlife species increase, research into the causes of wildlife population declines and tools to aid recovery are urgently needed. We conducted two studies of endangered desert bighorn sheep (Ovis canadensis nelsoni) in the Peninsular Ranges of Southern California with the primary goal of furthering recovery efforts for this species. First, in order to evaluate a captive breeding program for Peninsular bighorn, we developed the following criteria to provide a standard means of evaluating ongoing captive breeding and reintroduction programs: (1) survival and recruitment rates in the captive population, (2) survival of released animals, (3) recruitment of released animals, (4) growth rate of the reintroduced or augmented population, and (5) establishment of a viable wild population. In assessing the Peninsular bighorn sheep program, we found that while reintroduction did not result in population growth or establishment of a viable population, it helped prevent extirpation of the reinforced deme, preserved metapopulation linkage, and aided habitat preservation. Chronic low recruitment and low adult survivorship precluded achievement of criteria 3-5. Environmental conditions in the release area also appeared to hinder program success. We suggest that periodic evaluations are useful for improving the success of individual captive breeding and reintroduction programs, as well as for meta-analyses needed to refine reintroduction science as a recovery tool for threatened or endangered populations. Wildlife habituated to the presence of humans have been recognized as a new dilemma facing wildlife managers. Our second study involved examining the habitat use, home range size, and nutritional levels of Peninsular desert bighorn sheep along an urbanwildland interface during two time periods (1981-82 and 1995-98). We found that bighorn sheep monitored during 1995-98 used habitat within (P <0.001) and closer to (P < 0.001) urban environments than bighorn sheep monitored in 1981-82. Females monitored in the 1990s had smaller home ranges (P = 0.03), and selected habitat farther from escape terrain, natural water sources, and hiking trails (P <0.05) than females monitored in the 1980s. Habitat selection patterns were similar among captive-reared and wild-reared bighorn, as well as males and females within the 1990s. Bighorn use of urban areas increased 5-fold between our study periods; however, this population has declined precipitously in recent times and urbanization appears to be contributing directly and indirectly to adult and juvenile mortality. We recommend excluding bighorn from urban areas in order to encourage more natural resource use patterns. Understanding the implications of wildlife-human interactions may require long-term studies because the results of these interactions are often not obvious or intuitive.