Sierra Nevada Bighorn Sheep Recovery Program

Bishop, CA, United States

Sierra Nevada Bighorn Sheep Recovery Program

Bishop, CA, United States
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Bleich V.C.,Sierra Nevada Bighorn Sheep Recovery Program | Marshal J.P.,University of Witwatersrand
Journal of Arid Environments | Year: 2010

Provision of surface water has been a long-standing management strategy to enhance habitat for large mammals in southwestern North America. In this paper, we use a resource selection function (RSF) developed from telemetered mountain sheep (Ovis canadensis) in three occupied mountain ranges in the Sonoran Desert, California, USA, to examine the effects of water development on habitat quality within those ranges. Further, we apply that model to four nearby and similar mountain ranges, but for which telemetry data are not available, and again examine the effects of water development. When distance to water was decreased to 2,000 m from an average of 3033 m (±522 [SD]) in three occupied mountain ranges, availability of high-quality habitat increased by as much as 92%. When distance to water was decreased to 2,000 m from an average of 3660 m (±799 [SD]) in three mountain ranges not occupied permanently by mountain sheep, and one occupied range for which telemetry data were not available, the proportion of high-quality habitat resulting from application of our model indicated increases that varied from 116 to 508%. We conclude that development of additional sources of surface water can increase availability of high-quality habitat for mountain sheep inhabiting Sonoran Desert mountain ranges, and that the technique has implications for population persistence and conservation of those large, specialized ungulates. © 2009 Elsevier Ltd. All rights reserved.


Johnson H.E.,University of Montana | Mills L.S.,University of Montana | Stephenson T.R.,Sierra Nevada Bighorn Sheep Recovery Program | Wehausen J.D.,University of California
Ecological Applications | Year: 2010

To develop effective management strategies for the recovery of threatened and endangered species, it is critical to identify those vital rates (survival and reproductive parameters) responsible for poor population performance and those whose increase will most efficiently change a population's trajectory. In actual application, however, approaches identifying key vital rates are often limited by inadequate demographic data, by unrealistic assumptions of asymptotic population dynamics, and of equal, infinitesimal changes in mean vital rates. We evaluated, the consequences of these limitations in an analysis of vital rates most important in the dynamics of federally endangered Sierra Nevada bighorn sheep (Ovis canadensis sierrae). Based on data collected from 1980 to 2007, we estimated vital rates in three isolated populations, accounting for sampling error, variance, and covariance. We used analytical, sensitivity analysis, life-stage simulation analysis, and a novel non-asymptotic simulation approach to (1) identify vital rates that should be targeted for subspecies recovery; (2) assess vital rate patterns of endangered bighorn sheep relative to other ungulate populations; (3) evaluate the performance of asymptotic vs. non-asymptotic models for meeting short-term management objectives; and (4) simulate management scenarios for boosting bighorn sheep population growth, rates. We found wide spatial and temporal variation in bighorn sheep vital rates, causing rates to vary in their importance to different populations. As a result, Sierra Nevada bighorn sheep exhibited population-specific dynamics that did not follow theoretical expectations or those observed in other ungulates. Our study suggests that vital rate inferences from large, increasing, or healthy populations may not be applicable to those that are small, declining, or endangered. We also found that, while asymptotic approaches were generally applicable to bighorn sheep conservation planning, our non-asymptotic population models yielded unexpected results of importance to managers. Finally, extreme differences in the dynamics of individual bighorn sheep populations imply that effective management strategies for endangered species recovery may often need, to be population-specific. © 2010 by the Ecological Society of America.


Johnson H.E.,University of Montana | Hebblewhite M.,University of Montana | Stephenson T.R.,Sierra Nevada Bighorn Sheep Recovery Program | German D.W.,Sierra Nevada Bighorn Sheep Recovery Program | And 2 more authors.
Oecologia | Year: 2013

Predation can disproportionately affect endangered prey populations when generalist predators are numerically linked to more abundant primary prey. Apparent competition, the term for this phenomenon, has been increasingly implicated in the declines of endangered prey populations. We examined the potential for apparent competition to limit the recovery of Sierra Nevada bighorn sheep (Ovis canadensis sierrae), an endangered subspecies under the US Endangered Species Act. Using a combination of location, demographic, and habitat data, we assessed whether cougar (Puma concolor) predation on endangered bighorn sheep was a consequence of their winter range overlap with abundant mule deer (Odocoileus hemionus). Consistent with the apparent competition hypothesis, bighorn sheep populations with higher spatial overlap with deer exhibited higher rates of cougar predation which had additive effects on adult survival. Bighorn sheep killed by cougars were primarily located within deer winter ranges, even though those areas constituted only a portion of the bighorn sheep winter ranges. We suspect that variation in sympatry between bighorn sheep and deer populations was largely driven by differences in habitat selection among bighorn sheep herds. Indeed, bighorn sheep herds that experienced the highest rates of predation and the greatest spatial overlap with deer also exhibited the strongest selection for low elevation habitat. Although predator-mediated apparent competition may limit some populations of bighorn sheep, it is not the primary factor limiting all populations, suggesting that the dynamics of different herds are highly idiosyncratic. Management plans for endangered species should consider the spatial distributions of key competitors and predators to reduce the potential for apparent competition to hijack conservation success. © 2012 Springer-Verlag.


Johnson H.E.,University of Montana | Scott Mills L.,University of Montana | Wehausen J.D.,University of California | Stephenson T.R.,Sierra Nevada Bighorn Sheep Recovery Program
Journal of Applied Ecology | Year: 2010

1. To successfully manipulate populations for management and conservation purposes, managers must be able to track changes in demographic rates and determine the factors driving spatial and temporal variation in those rates. For populations of management concern, however, data deficiencies frequently limit the use of traditional statistical methods for such analyses. Long-term demographic data are often piecemeal, having small sample sizes, inconsistent methodologies, intermittent data, and information on only a subset of important parameters and covariates.2. We evaluated the effectiveness of Bayesian state-space models for meeting these data limitations in elucidating dynamics of federally endangered Sierra Nevada bighorn sheep Ovis canadensis sierrae. We combined ground count, telemetry, and mark-resight data to: (1) estimate demographic parameters in three populations (including stage-specific abundances and vital rates); and (2) determine whether density, summer precipitation, or winter severity were driving variation in key demographic rates.3. Models combining all existing data types increased the precision and accuracy in parameter estimates and fit covariates to vital rates driving population performance. They also provided estimates for all years of interest (including years in which field data were not collected) and standardized the error structure across data types.4. Demographic rates indicated that recovery efforts should focus on increasing adult and yearling survival in the smallest bighorn sheep population. In evaluating covariates we found evidence of negative density dependence in the larger herds, but a trend of positive density dependence in the smallest herd suggesting that an augmentation may be needed to boost performance. We also found that vital rates in all populations were positively associated with summer precipitation, but that winter severity only had a negative effect on the smallest herd, the herd most strongly impacted by environmental stochasticity.5. Synthesis and applications. For populations with piecemeal data, a problem common to both endangered and harvested species, obtaining precise demographic parameter estimates is one of the greatest challenges in detecting population trends, diagnosing the causes of decline, and directing management. Data on Sierra Nevada bighorn sheep provide an example of the application of Bayesian state-space models for combining all existing data to meet these objectives and better inform important management and conservation decisions. © 2010 The Authors. Journal compilation © 2010 British Ecological Society.


Cahn M.L.,Yale University | Conner M.M.,Sierra Nevada Bighorn Sheep Recovery Program | Schmitz O.J.,Yale University | Stephenson T.R.,Sierra Nevada Bighorn Sheep Recovery Program | And 2 more authors.
Journal of Wildlife Management | Year: 2011

Sierra Nevada bighorn sheep (Ovis canadensis sierrae) experienced a severe population decline after European settlement from which they have never recovered; this subspecies was listed as endangered under the United States Endangered Species Act (ESA) in 1999. Recovery of a listed species is accomplished via federally mandated recovery plans with specific population goals. Our main objective was to evaluate the potential impact of disease on the probability of meeting specific population size and persistence goals, as outlined in the Sierra Nevada bighorn sheep recovery plan. We also sought to heuristically evaluate the efficacy of management strategies aimed at reducing disease risk to or impact on modeled bighorn populations. To do this, we constructed a stochastic population projection model incorporating disease dynamics for 3 populations (Langley, Mono, Wheeler) based on data collected from 1980 to 2007. We modeled the dynamics of female bighorns in 4 age classes (lamb, yearling, adult, senescent) under 2 disease scenarios: 5% lower survival across the latter 3 age classes and persistent 65% lower lamb survival (i.e., mild) or 65% reduced survival across all age classes followed by persistent 65% lower lamb survival (i.e., severe). We simulated management strategies designed to mitigate disease risk: reducing the probability of a disease outbreak (to represent a strategy like domestic sheep grazing management) and reducing mortality rate (to represent a strategy that improved survival in the face of introduced disease). Results from our projection model indicated that management strategies need to be population specific. The population with the highest growth rate (λ̂; Langley; λ̂ = 1.13) was more robust to the effects of disease. By contrast, the population with the lowest growth rate (Mono; λ̂ = 1.00) would require management intervention beyond disease management alone, and the population with a moderate growth rate (Wheeler; λ̂ = 1.07) would require management sufficient to prevent severe disease outbreaks. Because severe outbreaks increased adult mortality, disease can directly reduce the probability of meeting recovery plan goals. Although mild disease outbreaks had minimal direct effects on the populations, they reduced recruitment and the number of individuals available for translocation to other populations, which can indirectly reduce the probability of meeting overall, range-wide minimum population size goals. Based on simulation results, we recommend reducing the probability of outbreak by continuing efforts to manage high-risk (i.e., spatially close) allotments through restricted grazing regimes and stray management to ensure recovery for Wheeler and Mono. Managing bighorn and domestic sheep for geographic separation until Sierra Nevada bighorn sheep achieve recovery objectives would enhance the likelihood of population recovery. © The Wildlife Society, 2011.


Johnson H.E.,University of Montana | Mills L.S.,University of Montana | Wehausen J.D.,University of California | Stephenson T.R.,Sierra Nevada Bighorn Sheep Recovery Program | And 2 more authors.
Conservation Biology | Year: 2011

Evidence of inbreeding depression is commonly detected from the fitness traits of animals, yet its effects on population growth rates of endangered species are rarely assessed. We examined whether inbreeding depression was affecting Sierra Nevada bighorn sheep (Ovis canadensis sierrae), a subspecies listed as endangered under the U.S. Endangered Species Act. Our objectives were to characterize genetic variation in this subspecies; test whether inbreeding depression affects bighorn sheep vital rates (adult survival and female fecundity); evaluate whether inbreeding depression may limit subspecies recovery; and examine the potential for genetic management to increase population growth rates. Genetic variation in 4 populations of Sierra Nevada bighorn sheep was among the lowest reported for any wild bighorn sheep population, and our results suggest that inbreeding depression has reduced adult female fecundity. Despite this population sizes and growth rates predicted from matrix-based projection models demonstrated that inbreeding depression would not substantially inhibit the recovery of Sierra Nevada bighorn sheep populations in the next approximately 8 bighorn sheep generations (48 years). Furthermore, simulations of genetic rescue within the subspecies did not suggest that such activities would appreciably increase population sizes or growth rates during the period we modeled (10 bighorn sheep generations, 60 years). Only simulations that augmented the Mono Basin population with genetic variation from other subspecies, which is not currently a management option, predicted significant increases in population size. Although we recommend that recovery activities should minimize future losses of genetic variation, genetic effects within these endangered populations-either negative (inbreeding depression) or positive (within subspecies genetic rescue)-appear unlikely to dramatically compromise or stimulate short-term conservation efforts. The distinction between detecting the effects of inbreeding depression on a component vital rate (e.g., fecundity) and the effects of inbreeding depression on population growth underscores the importance of quantifying inbreeding costs relative to population dynamics to effectively manage endangered populations. ©2011 Society for Conservation Biology.


Greene L.,University of Montana | Hebblewhite M.,University of Montana | Stephenson T.R.,Sierra Nevada Bighorn Sheep Recovery Program
Journal of Arid Environments | Year: 2012

We studied short-term changes in vegetation for two years following a summer wildfire on the winter ranges of Sierra Nevada bighorn sheep (Ovis canadensis sierrae). Forbs dominated burned areas and shrubs dominated unburned areas. Green forage (new-growth of all forage classes) biomass rebounded quickly; within two years green forage biomass was equal in burned and unburned areas, although total forage biomass remained greater in unburned areas. Plants in the burn had slightly higher crude protein but equivalent digestibility and phenology as plants in unburned areas. This, in combination with the shift toward more forb biomass, likely increased forage quality in burned areas. Forage models developed from ground-based measures of biomass performed better than the NDVI and were able to capture changes in forage composition, emphasizing the importance of field sampling to model vegetation. Based on microhistological analyses of fecal pellets, Sierra Nevada bighorn sheep whose winter ranges were extensively burned consumed more forbs than those with less burned habitat. Visibility was greater in burned areas compared with unburned areas, suggesting that burns may decrease predation risk from stalking predators. In conclusion, wildfire may have beneficial effects for Sierra bighorn by increasing forb availability, forage quality and visibility. © 2012 Elsevier Ltd.


PubMed | Western Illinois University, University of Alaska Fairbanks, North Carolina State University, U.S. Fish and Wildlife Service and 4 more.
Type: Journal Article | Journal: PloS one | Year: 2014

New-hoof growth is regarded as the most reliable metric for predicting age of newborn ungulates, but variation in estimated age among hoof-growth equations that have been developed may affect estimates of survival in staggered-entry models. We used known-age newborns to evaluate variation in age estimates among existing hoof-growth equations and to determine the consequences of that variation on survival estimates. During 2001-2009, we captured and radiocollared 174 newborn (24-hrs old) ungulates: 76 white-tailed deer (Odocoileus virginianus) in Minnesota and South Dakota, 61 mule deer (O. hemionus) in California, and 37 pronghorn (Antilocapra americana) in South Dakota. Estimated age of known-age newborns differed among hoof-growth models and varied by >15 days for white-tailed deer, >20 days for mule deer, and >10 days for pronghorn. Accuracy (i.e., the proportion of neonates assigned to the correct age) in aging newborns using published equations ranged from 0.0% to 39.4% in white-tailed deer, 0.0% to 3.3% in mule deer, and was 0.0% for pronghorns. Results of survival modeling indicated that variability in estimates of age-at-capture affected short-term estimates of survival (i.e., 30 days) for white-tailed deer and mule deer, and survival estimates over a longer time frame (i.e., 120 days) for mule deer. Conversely, survival estimates for pronghorn were not affected by estimates of age. Our analyses indicate that modeling survival in daily intervals is too fine a temporal scale when age-at-capture is unknown given the potential inaccuracies among equations used to estimate age of neonates. Instead, weekly survival intervals are more appropriate because most models accurately predicted ages within 1 week of the known age. Variation among results of neonatal-age models on short- and long-term estimates of survival for known-age young emphasizes the importance of selecting an appropriate hoof-growth equation and appropriately defining intervals (i.e., weekly versus daily) for estimating survival.


Pierce B.M.,Sierra Nevada Bighorn Sheep Recovery Program | Bleich V.C.,Sierra Nevada Bighorn Sheep Recovery Program | Bleich V.C.,Idaho State University
California Fish and Game | Year: 2014

Assessing mountain lion (Puma concolor) populations is difficult due to their inherently low densities, secretive nature, and a near absence of demographically closed populations. We developed and compared two methods of indexing the number of mountain lions within a subset (referred to in this paper as the core area) of a total study area. The study area was defined as the outer boundary of combined home range polygons for all collared mountain lions. Therefore, the study area was regularly occupied by uncollared individuals whose home ranges overlapped the study area boundary. We determined through intensive capture efforts and monitoring that the much smaller core area was used only by adult mountain lions that we had identified and collared and was not used in any significant manner by uncollared adults. We derived two indices to the number of lions using the core area. One index is based on location data from VHF aerial telemetry ("fixed wing index"); the second index is based on a combination of fixed wing locations and GPS collar data combined ("location data index"). The fixed wing index yields the mean number (and variance) of adult individuals located in the core region of the study area each of 15 winters during weekly telemetry flights. The location data index is based on the sum of the proportions of locations for each individual that are within the core area each winter. The two indices were highly correlated, and the trends generally were in the same direction and changes in each were of a similar magnitude. These methods are preferable to attempting total counts because the periphery of any study area will occasionally be occupied by unmarked animals. Our methods account for those individuals, but they are not afforded the same weight as mountain lions that use the area frequently or exclusively. Managers with GPS radio collar data are encouraged to delineate a core area, where all lions known to use the area are collared, and use the sum of the proportion of locations from each individual in that area to index density, population size, number of animals present, or use. © 2014 California Fish and Game.


Villepique J.T.,Idaho State University | Pierce B.M.,Sierra Nevada Bighorn Sheep Recovery Program | Bleich V.C.,Idaho State University | Bowyer R.T.,Idaho State University
Southwestern Naturalist | Year: 2011

We investigated diet of cougars (Puma concolor) in the eastern Sierra Nevada, California, following a decline in the population of mule deer (Odocoileus hemionus). Mule deer declined 84% from 1985 to 1991, a period concurrent with declines in bighorn sheep (Ovis canadensis sierrae; an endangered taxon). An index to numbers of cougars lagged behind those declines, with a reduction of ca. 50% during 1992-1996. We determined diet of cougars by analysis of fecal samples collected during 1991-1995, when the population of mule deer was <25% of its former size. Mule deer was in 79% of 178 feces in winter and 58% of 74 feces in summer. Although most (69%) fecal samples in winter were <5 km from, or within (25%) winter range of bighorn sheep, none contained evidence of bighorn sheep. One fecal sample in summer contained remains of bighorn sheep, indicating that those ungulates were not an important component of the diet during our investigation. Copyright © 2011 BioOne All rights reserved.

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