Determination of critical habitat for the endangered Nelson's bighorn sheep in southern California

Abstract

The United States Fish and Wildlife Service's (USFWS) designation of critical habitat for the endangered Nelson's bighorn sheep (Ovis canadensis nelsoni) in the Peninsular Ranges of southern California has been controversial because of an absence of a quantitative, repeatable scientific approach to the designation of critical habitat. We used 12,411 locations of Nelson's bighorn sheep collected from 1984–1998 to evaluate habitat use within 398 km2 of the USFWS‐designated critical habitat in the northern Santa Rosa Mountains, Riverside County, California. We developed a multiple logistic regression model to evaluate and predict the probability of bighorn use versus non‐use of native landscapes. Habitat predictor variables included elevation, slope, ruggedness, slope aspect, proximity to water, and distance from minimum expanses of escape habitat. We used Earth Resources Data Analysis System Geographic Information System (ERDAS‐GIS) software to view, retrieve, and format predictor values for input to the Statistical Analysis Systems (SAS) software. To adequately account for habitat landscape diversity, we carried out an unsupervised classification at the outset of data inquiry using a maximum‐likelihood clustering scheme implemented in ERDAS. We used the strata resulting from the unsupervised classification in a stratified random sampling scheme to minimize data loads required for model development. Based on 5 predictor variables, the habitat model correctly classified >96% of observed bighorn sheep locations. Proximity to perennial water was the best predictor variable. Ninety‐seven percent of the observations were within 3 km of perennial water. Exercising the model over the northern Santa Rosa Mountain study area provided probabilities of bighorn use at a 30 times 30‐m2 pixel level. Within the 398 km2 of USFWS‐designated critical habitat, only 34% had a graded probability of bighorn use to non‐use ranging from ≥1:1 to 6,044:1. The remaining 66% of the study area had odds of having bighorn use <1:1 or it was more likely not to be used by bighorn sheep. The USFWS designation of critical habitat included areas (45 km2) of importance (2.5 to ≥40 observations per km2 per year) to Nelson's bighorn sheep and large landscapes (353 km2) that do not appear to be used (<1 observation per km2 per year).

Compensating for diminishing natural water: Predicting the impacts of water development on summer habitat of desert bighorn sheep

“…Virtually every habitat model developed for desert bighorn sheep has identified water as an important correlate of sheep use (McCarty and Bailey, 1994;Bleich et al, 1997;Andrews et al, 1999;Turner et al, 2004;Oehler et al, 2005;Sappington et al, 2007). Further, an analysis of 20th Century extinction patterns of desert bighorn sheep populations in California found the existence of predictable surface water, in addition to elevation and maximum precipitation, to be strongly correlated with population persistence (Epps et al, 2004).…”

“…During summer months, water sources are considered as an essential component of suitable habitat for nearly all desert bighorn sheep populations (Bleich et al, 1997;Andrews et al, 1999;Turner et al, 2004;Oehler et al, 2005;Sappington et al, 2007;but, see Warrick and Krausman, 1989) and the presence of dependable water sources has been strongly correlated with population persistence (Epps et al, 2004). Artificial water sources have been used for decades to enhance and restore habitat for desert bighorn sheep (Halloran and Deming, 1958;Weaver et al, 1958;Werner, 1984), but the benefits are not always obvious.…”

Habitat use by a desert ungulate: Predicting effects of water availability on mountain sheep

“…Despite strong evidence that water plays an important role in the life histories of arid-land ungulates and its profound influence on habitat selection by mountain sheep (Andrew et al, 1999;Bleich et al, 1997;Oehler et al, 2005;Rubin et al, 2009;Turner et al, 2004), particularly during summer (Bristow, 1998;Waddell et al, 2007), provision of water sources as a conservation strategy remains controversial (Bleich, 2005;Krausman et al, 2006). Even ''long term'' manipulative experiments have not provided unequivocal results that will assist land managers in making informed decisions about the contributions of wildlife water developments to conservation programs.…”

“…For mountain sheep (Ovis canadensis) and mule deer (Odocoileus hemionus), proximity to surface water during summer (Andrew et al, 1999;Bleich et al, 1997;Blong and Pollard, 1968;Oehler et al, 2005;Ordway and Krausman, 1986;Turner et al, 2004), visitation rates (Blong and Pollard, 1968;Hervert and Krausman, 1986;Marshal et al, 2006a;Turner, 1973), seasonality of use (Bleich et al, 1997;Bristow, 1998;Waddell et al, 2007), and water consumption rates (Hazam and Krausman, 1988;Turner, 1970Turner, , 1973 all demonstrate the importance of surface water to those ungulates. Moreover, Turner (1973) demonstrated that water content of mountain sheep diets during summer is insufficient for meeting evaporative water losses; as a result, those ruminants must rely on sources of free water to maintain water balance.…”

Books and Literature

“…In the BMEMP, service area around a water development was considered to be 1.5 miles for general wildlife use. Turner et al (2004) found that 97% of desert bighorn collar locations were within 1.86 miles of water. Some consider the highest ranking habitat to be within two miles of a permanent water source (Cunningham 1989, Hansen 1980); yet Wilson (1975) found desert bighorn up to three miles from water.…”