Overview of Sierra Nevada Bighorn Sheep Research Projects

Project Coordinator: Liz Siemion
Project Status: In-Progress
Completion Date: Summer 2027
Recovery Plan Actions Supported by Project: 2.1, 2.2.1, 6.8
Resulting Publications: TBA

Predation is often regarded as a top-down process in which predator densities constrain prey densities. Prey populations can respond to a variety of extrinsic forces that determine the extent to which predators regulate their dynamics, so a bottom-up perspective that addresses predation rate – the rate at which prey are killed by predators (kills/prey/time) – its effect on prey vital rates, and its interaction with other extrinsic processes may better quantify predation effects on prey. In this project, I will explore how prey dynamics influence predation rates at the community, population, and individual levels and assess whether the dominant level at which predation operates shifts due to changes in the environment. Mountain lion predation is bolstered by a large mule deer population and poses a significant threat to the federally endangered Sierra Nevada bighorn sheep (SNBS). To understand the fine-scale spatiotemporal circumstances leading to elevated predation rates, we aim to 1) quantify the roles that bottom-up and top-down processes play in shaping predation rate; 2) determine how environmental factors affect densities of mule deer and SNBS, and the spatial partitioning of those species across the landscape; and 3) assess the sensitivity of predator-prey interactions to shifting prey ecology at the community, population, and individual levels and evaluate how the environment mediates each process. With this framework, we can inform where predator-related management efforts will be most effective in aiding the recovery of Sierra Nevada bighorn sheep.

Project Coordinators: Jaron Kolek
Project Status: In-Progress
Completion Date: Spring 2024
Recovery Plan Actions Supported by Project: 2.4, 5, 6.4, 7
Resulting Publications: TBA

Recreation is commonly perceived as compatible with wildlife conservation, in contrast to more well-known threats such as human development. However, there is a growing body of evidence indicating that recreation can negatively impact wildlife. The effects of recreation on wildlife include increased flight and vigilance, changes in spatial or temporal habitat use, elevated stress hormone levels, as well as declines in abundance, occupancy, and density. For small, fragmented populations and at-risk species like Sierra Nevada bighorn sheep, outdoor recreation may be an added complication and an important consideration in recovery efforts.

We seek to understand how recreational use influences Sierra bighorn habitat selection and behavior during two vulnerable time periods. We will investigate bighorn behavior during the winter and during parturition, as these are periods when sensitivity to disturbance may be intensified. To understand winter recreation effects, we will study Sierra bighorn selection of winter home ranges and movement patterns in response to backcountry skiers and snowboarders. Additionally, we will build on the work of Amy Sturgill, Jackie Leary, and the Pine Creek Recreational Use Monitoring project to investigate long term changes in the selection of parturition sites, as well as habitat selection and movement patterns within parturition ranges, concurrent with the increasing popularity of recreation in Pine Creek Canyon.

Project Coordinator: Yasaman Shakeri
Project Status: In-Progress
Estimated Completion: Fall 2025
Recovery Plan Actions Supported by Project: 2.1, 2.2, 6.8
Resulting Publications: TBA

Mountain lions and Sierra bighorn sheep have historically lived alongside each other, but the ecological balance has been shifted. This shift may be affecting recovery of endangered Sierra bighorn. In an intact ecosystem, carnivores and ungulate populations regulate themselves through competition and predation. Prior to European colonization, the eastern Sierra was home to multiple predators, including wolves and grizzly bears, which competed with mountain lions and kept mountain lion populations in check. With the extirpation of wolves and grizzly bears, mountain lions have been released from competition and have become the apex predator in this ecosystem. In the absence of other predators, mountain lion populations are regulated by their prey source, which mainly consists of deer, but can also be subsidized by other species, such as rabbits, beavers, coyotes, and bighorn sheep.

To understand how mountain lions affect Sierra bighorn recovery, we need to understand the prey base as well as the densities of mountain lions in the recovery zone. We will investigate mountain lion prey consumption, and whether there are changes to their diet by season, sex, and the presence of kittens. There are currently 27 mountain lions with GPS collars in the eastern Sierra, and we are monitoring these individuals to estimate survival and population growth. We will also investigate if bighorn sheep in poor body condition take more risks when foraging, exposing them to predation by mountain lions.

Project Coordinator: Seth Rankins
Project Status: In-Progress
Completion Date: Fall 2025
Recovery Plan Actions Supported by Project: 5, 6.3, 6.9
Resulting Publications: 

Rankins, S. T., T. R. Stephenson, and K. L. Monteith. 2024. Sociality modulates nutritional carrying capacity of an endangered species. Frontiers in Ecology and Evolution 10.3389/fevo.2024.1417970.

Sierra Nevada bighorn sheep live in a harsh environment with limited forage availability. While the rugged eastern escarpment of the Sierra Nevada mountains provides seemingly endless expanses of suitable bighorn habitat, it is currently unknown how many bighorn the forage could sustainably support. Historical records suggest that at one time the Sierra Nevada supported approximately 1,000 Sierra bighorn. However, the environment that Sierra bighorn live in today is different. In addition, previous research exploring overwinter nutrition of wild herbivores in alpine environments has demonstrated that fat deposits gained from summer foraging are just as important as the forage available on winter ranges. In fact, these fat stores may allow some bighorn to winter at high elevation.

We will be using a combination of field sampling and remotely-sensed data to develop “foodscapes”, maps of the temporal and spatial configurations of forage throughout Sierra bighorn range. By combining our foodscapes with movement and body condition data from GPS-collared bighorn, we will be able to create a range-wide nutritional carrying capacity estimate that incorporates both forage and stored fat deposits. Furthermore, our foodscapes will allow us to test hypotheses pertaining to risk-sensitive foraging behavior of Sierra bighorn, thereby advancing our knowledge of foraging ecology of Sierra bighorn sheep.

Project Coordinator: Kristin Denryter
Project Status: Complete
Completion Date: Summer 2022
Recovery Plan Actions Supported by Project: 5, 6.3, 6.9
Resulting Publications:

• Denryter, K., T. R. Stephenson, K. L. Monteith. 2024. Migratory behaviours are risk-sensitive to physiological state in an elevational migrant. Conservation Physiology 12:1-12.
• Denryter, K., M. M. Conner, T. R. Stephenson, D. W. German, K. L. Monteith. 2022. Survival of the fattest: How body fat and migration influence survival in highly seasonal environments (PDF). Functional Ecology DOI: 10.1111/1365-2435.14151.
• Denryter, K., D. W. German, Stephenson, T. R., K. L. Monteith. 2020. State- and context-dependent applications of an energetics model in free-ranging bighorn sheep. Ecological Modelling 440:109349.
• Wildlife Society Poster 2017: Integrating nutrition, resource use, and population demographics to inform conservation of endangered species (PDF)

Nutrition underpins growth, survival, and reproduction and ultimately is the fundamental building block of populations. Generally, implications of nutrition to conservation of wildlife have not been realized because data needed to establish cause-and-effect links between nutrition and population trajectory are underappreciated or only exist conceptually. We aim to bridge the fields of nutrition and population ecology to generate innovative tools to quantify the fundamental building block of populations and assess potential benefits (e.g., population growth) of conservation actions that manipulate food supplies. By combining information on available food supplies, empirically validated measures of nutritional condition (i.e., body fat), GPS locations, and demographic rates (e.g., survival, reproduction) for 14 herds of federally endangered Sierra Nevada bighorn sheep, we will: (1) evaluate the capacity of available food supplies to meet nutritional requirements and support population growth; (2) map animal-centric foodscapes (nutritional landscapes) and assess habitat selection across these foodscapes; (3) evaluate nutritional condition of an endangered species, and link nutritional condition to foodscapes and demographic rates; and (4) unite these data in an integrated population model (developed from population-specific demographic rates) to understand how habitat and associated food supplies influence population trajectories. Foodscapes generated in this study will transcend limitations of conventional habitat maps, allow for the identification of nutritionally valuable habitats, and provide the basis for understanding population-level consequences of habitat change. They can also be used as a model in the recovery and conservation of other species. Our work will identify cause-and-effect relationships among food supplies, nutrition, demography, and population growth that can guide on-the-ground recovery efforts for an endangered species.

Project Coordinator: Derek Benjamin Spitz
Project Status: Complete
Completion Date: Spring 2016
Recovery Plan Actions Supported by Project: 3.1, 4 , 5.1
Resulting Publications:

• Phd Thesis UMT (PDF)
• Spitz, D. B., M. Hebblewhite, and T. R. Stephenson. 2017. ‘MigrateR’: extending model-driven methods for classifying and quantifying animal movement behavior (PDF). Ecography 40:788-799.
• Spitz, D., M. Hebblewhite, T. R. Stephenson and D. W. German. 2018. How plastic is migratory behavior? Quantifying elevational movement in a partially migratory alpine ungulate, the Sierra Nevada bighorn sheep (Ovis canadensis sierrae) (PDF). Canadian Journal of Zoology 96:1385-1394.

Despite their potential conservation importance, the demographic implications of migratory behavior remain poorly understood. Sierra Nevada bighorn sheep are partially migratory. In summer, Sierra bighorn share high-elevation summer ranges, but in winter some individuals migrate to lower elevation for winter while others remain resident at high-elevation. Lower elevations have better forage in winter, but these areas also carry an increased risk of predation from Sierra bighorn's primary predator, the cougar. This project explores expected differences in winter conditions which may result in demographic differences between migrants and residents. The project is using novel techniques for identifying altitudinal migration and applying these tools to Sierra bighorn. Differences in status-specific winter resource use and selection by migrants and residents are developed using resource selection functions across three scales. Causes and consequences of migratory behavior in Sierra bighorn are tested to understand migratory behavior.

Project Coordinator: Shannon Forshee
Project Status: Complete
Completion Date: Spring 2018
Recovery Plan Actions Supported by Project: 5.1, 6.1, 6.3
Resulting Publications:

• Master's Thesis. University of Montana. 2018. Life on the edge: Risk of predation drives selection of habitat and survival of neonates in endangered Sierra Nevada bighorn sheep (PDF)

CDFW has made significant progress towards the recovery of Sierra Nevada bighorn sheep, yet juvenile recruitment, a key vital rate of population growth, continues to fluctuate annually and among geographically distinct herd units. To quantify these fluctuations, we monitored neonate and juvenile survival and examined how lactating Sierra bighorn balance forage and predation risk during the neonatal period. Using resource selection function models, we found lactating females selected for habitat that, despite decreased access to high quality forage, reduced the risk of predation by mountain lions. Despite the efforts of lactating females to protect neonates from predation risk, predation was the strongest factor contributing to survival of neonates across subpopulations. We determined that neonates become less vulnerable as they age, were most vulnerable if they were born before the peak birth pulse (April), and if lactating females selected habitat farther from the safety of escape terrain.

Project Coordinators: Amy Sturgill and Jackie Leary
Project Status: In-Progress
Completion Date: Ongoing
Recovery Plan Actions Supported by Project: 2.4, 5, 6.4, 7
Resulting Publications:

In October of 2014 the Recovery Program initiated a study in Pine Creek Canyon to address concerns of increased recreational use. Over the last ten years, the Pine Creek area of the Wheeler Ridge herd unit has become an increasingly popular destination for hikers, sightseers, and, in particular, rock climbers. Pine Creek Canyon is also routinely used as lambing habitat by Wheeler Ridge ewes. The overall goal of this study is to determine if there are any detectable effects from changes in human recreational use on bighorn at Wheeler. To do this, daily use was monitored using infrared trail counters at popular climbing trailheads. Monitoring sites were selected because they experience the highest use in proximity to parturition and lambing sites, as well as for ease of access for checking and maintaining trail counters. We discovered these sites were visited frequently, more than 500 visits/month, during lambing season in April and May of 2015. In addition, recreationists and bighorn overlapped throughout the winter of 2014-15 in Pine Creek, but this was likely driven by the drought conditions allowing above average winter use by humans. In more average weather years overlapping use of recreationists will likely be reduced, as the same weather that tends to push bighorn down to wintering grounds also halts recreational use in the canyon. By continuing to collect this data we hope to gain a further understanding of interannual variation to identify the most likely periods of overlap between bighorn and recreationists. In addition, this study supplies critical baseline data of both human and bighorn use of the area that may be used to determine if increased recreation causes bighorn to shift their habitat use in the future.

Project Coordinator: Christian John
Project Status: In-Progress
Completion Date: Spring 2022
Recovery Plan Actions Supported by Project: 5, 6.3, 6.9
Resulting Publications:

• American Geophysical Union oral presentation 2018: Long-term Projections for Migration Phenology in Sierra Nevada Bighorn Sheep
• John, C., T. Avgar, K. Rittger, J. A. Smith, L. W. Stephenson, T. R. Stephenson, and E. Post. 2024. Pursuit and escape drive fine-scale movement variation during migration in a temperate alpine ungulate. Nature Portfolio 14:15068.

Elevational migration facilitates intake of newly emergent and thus highly nutritious and digestible plant tissue as snowpack recedes and plant emergence advances upslope. Snow cover, plant phenology, and their interaction over variable landscapes are of considerable importance to the ecology of the endangered Sierra Nevada bighorn sheep, which is a partially migratory, mid- to high elevation specialist. Constraints imposed by snow cover, and its influence on green-up phenology, as well as trends in these related to climatic warming, on the timing and location of seasonal foraging and range use patterns of Sierra bighorn, may pose consequences for survival and reproductive performance. Down-scaled climate models predict reduced snow pack, upslope retreat of snow pack, and longer snow-free seasons across the Sierra Nevada over the next century of warming. Unfortunately, the frequency of snow and cloud cover in the high Sierra precludes the opportunity to take advantage of satellite-derived measurements of plant growth. Furthermore, the relatively infrequent measurements taken at the satellite level fail to capture the narrow window of plant growth characteristic of high elevation habitats. Here, we use programmable time-lapse cameras to monitor fine-scale ecological processes at the plot level. Imagery collected from these cameras is compiled and analyzed using cutting edge computer vision and machine learning approaches to estimate snow cover and plant growth across the course of snowy and growing seasons. Specifically, this project will contribute highly spatio-temporally resolved data necessary for investigating effects of snow cover and plant phenology on Sierra Nevada bighorn spatial distribution, survival, and reproduction. The project seeks to address the following three questions:

1. How do topography, geography, and climate interactively influence plot-scale snowmelt and plant growth in the Sierra Nevada?
2. What is the importance of fine-scale heterogeneity in forage resource availability for movement in Sierra bighorn?
3. How can we expect environmental conditions, and consequently movement patterns in Sierra bighorn, to change over the next century?

Project Coordinator: Rob Klinger (USGS), Tom Stephenson (CDFW)
Project Status: In-Progress
Completion Date: Spring 2014
Recovery Plan Actions Supported by Project: 6.9
Resulting Publications:

We are proposing to model: (1) range shifts for five alpine mammal species and (2) the degree to which plant-animal interactions may alter what is often assumed to be an inevitable, climatically driven transition of alpine meadows to woody dominated communities in the Sierra Nevada mountain range. Meadows provide critical habitat for alpine mammals, including Sierra Nevada bighorn sheep, a federally listed endangered species, and American pika, which has been recently petitioned for listing. However, there is an acute lack of data on potential impacts of climate shifts on bighorn sheep and pika, the other three mammals, and more generally, the alpine zone in the Sierra Nevada. We will use remote sensing data, downscaled climate data, rangewide surveys of the mammals, and experiments on plant-animal interactions to model the relative influence of climate, topography, and vegetation on the distributions of the five mammal species, and reciprocally, controls mammals may have on vegetation transitions. The critical aspect of the study is the testing of multiple models of projected vegetation transitions with and without variables representing shifts in mammal ranges and feedbacks from herbivory and granivory. This synthetic approach will allow us to evaluate the degree to which mammals can “manage their own habitat” in the face of climatic shifts. Research on alpine ecosystems is a top priority by USGS and other international and national organizations. The study is a cooperative effort between five federal and state agencies and three universities that would directly address four goals of the U.S. Climate Change Science Program, two strategic directions of the USGS Science Strategy, and numerous aspects of the USGS Global Change Science Strategy.

Project Coordinator: Tom Stephenson
Project Status: Complete
Completion Date: Spring 2014
Recovery Plan Actions Supported by Project: 1.2, 2.4, 5.3, 6.3, 6.4
Resulting Publications:

• Klinger, R. C., Few, A.P., Knox, K. Hatfield, B. E., Clark, J,, German, D. W., and Stephenson, T. R. Evaluating Potential Overlap Between Pack Stock and Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae) in Sequoia and Kings Canyon National Parks, California (PDF).

Project objectives are to (1) quantify the degree of current and potential spatial overlap between Sierra Nevada bighorn sheep and areas grazed by packstock; (2) determine the effect of packstock on vegetation structure and species composition, with special focus on forage resources jointly used by bighorn and packstock; (3) refine our understanding of diet and foraging behavior of Sierra bighorn on alpine and subalpine summer range; (4) use resource selection functions (RSF’s) to develop bighorn distribution models; 5) examine the response of bighorn sheep to off-trail human use. Achieving these objectives will allow us to evaluate the potential behavioral, nutritional, distributional, and demographic effects of packstock on bighorn sheep. This is a cooperative project among CDFW, USGS BRD, and Sequoia-Kings Canyon National Park.

Project Coordinator: Tom Stephenson
Project Status: Complete
Completion Date: Fall 2015
Recovery Plan Actions Supported by Project: 5.3, 6.3
Resulting Publications:

• 2018 Status of Sierra Nevada Bighorn Sheep Herds in Yosemite National Park (PDF)

Sierra Nevada bighorn sheep have increased to more than 400 animals since receiving federally endangered status in 1999. However, the Northern (Yosemite) Recovery Unit, which includes eastern portions of Yosemite National Park, remains below recovery goals and continues to be threatened by disease because of the proximity to active domestic sheep allotments. Since 2002, CDFW has radio-collared numerous bighorn sheep in the Mount Gibbs and Mount Warren herd units. Repeatedly bighorn rams have traveled long distances that place them at risk of disease from domestic sheep. In addition, bighorn captured east of the Park are using areas within the Park with much greater frequency. With the aid of radio-telemetry, the first recent documentation of lambing within the Park occurred in 2006.Technology has recently become available that allows for deployment of GPS collars on wildlife that enable real-time transmission of location data using the satellite phone network. Such collars will be deployed on a portion of resident bighorn and additional animals translocated to augment the existing herds in the Northern Recovery Unit. Satellite collars offer tremendous improvements in the ability to monitor bighorn recovery. Movements of bighorn that may pose a disease risk can be monitored daily. Acquiring remote locations will improve the efficiency of field observations to determine lambing success and the collection of forage and fecal samples that are analyzed to quantify nutritional status and genetics. GPS locations will be used to develop resource selection models that clarify suitable bighorn habitat within the Park. Furthermore, location data will be placed on a website accessible to the public. This is a cooperative project among CDFW and Yosemite National Park with funding from the Yosemite Conservancy. Static maps of real-time locations. Yosemite Pages where you can explore the project in more detail, including Google Earth video maps of bighorn sheep travel paths.

Project Coordinator: John Wehausen
Project Status: Ongoing
Completion Date:
Recovery Actions Supported by Project: 6.1, 6.3
Resulting Publications:

When bighorn sheep colonize new range, they initially occur at very low density and consequently are very difficult to find and document visually. However, they potentially leave considerable evidence of their existence in the form of dropping. Because deer droppings cannot be reliably distinguished from bighorn sheep dropping visually, there was a need for a simple method to distinguish the droppings of these two species. A relatively simple genetic PCR method using fecal DNA was recently developed by this recovery program and has been applied successfully to droppings from a variety of areas. Its use is expected to increase in the future.

Project Coordinator: Erin Latham
Project Status: Complete
Completion Date: Fall 2010
Recovery Actions Supported by Project: 5.3, 6.3
Resulting Publications:

• Masters Thesis (PDF)

While repeat photography of ground photos is not a new technique in land cover change analysis, it mostly has been limited to qualitative analysis due to the non-uniform distortion inherent in ground photography. Using modern photogrammetric techniques early twentieth century ground photos of the eastern slope of the Sierra Nevada Mountains will be re-photographed and then accurately rendered in coordinate space. The goal is to rectify these photos accurately enough to perform quantitative analysis alongside air photos from the 1940s, 1970s, and present day. This is a collaborative project with Lacey Greene. The analysis will focus on the encroachment of Pinyon Pine and the resulting loss of open habitat for Sierra Nevada Bighorn Sheep. Data protoccols for repeated photographic documentation and subsequent quantitative analysis will also be developed.

Project Coordinator: John Wehausen
Project Status: Complete
Completion Date: 2011
Recovery Actions Supported by Project: 2.3, 5.4
Resulting Publications:

• Wehausen, J. D., S. T. Kelley, and R. R. Ramey II. 2011. Domestic sheep, bighorn sheep, and respiratory disease: a review of the experimental evidence (PDF). California Fish and Game 97(1). (2011).

The threat of a respiratory disease epizootic resulting from contact between Sierra Nevada bighorn sheep and domestic grazed on adjacent public lands was one of primary reasons listing Sierra Nevada bighorn sheep as a federal endangered species. Following that listing the seriousness of that threat has been questioned by the domestic sheep industry. This research is a thorough review of the scientific literature concerning experimental data that pertain to this question. It categorizes those results in a way not done previously, and addresses the topic from the standpoint of multiple different questions and hypotheses.

Project Coordinator: Lacey Greene
Project Status: Complete
Completion Date: Spring 2010
Recovery Plan Actions Supported by Project: 1.2, 2.2, 5.3, 6.3
Resulting Publications:

• P - Greene, L., Hebblewhite, M., and Stephenson, T.R. Short-term vegetation response to wildfire in the eastern Sierra Nevada: Implications for recovering an endangered ungulate (PDF) . Journal for Arid Environment, Vol. 87, pp. 118-128. Elsevier; 2012.
• Masters Thesis (PDF)

One of the limiting factors for Sierra Nevada bighorn sheep identified in their recovery plan is the loss of open habitat. Open habitat is important for bighorn sheep because it allows bighorn to detect predators visually. It is widely assumed that open habitat in lower elevation ranges has been declining due to Pinyon pine (Pinus monophylla) encroachment. The SNBS Recovery Plan suggests using prescribed fire to combat this encroachment and restore open habitat. Open habitat loss will be quantified using historic images of the Eastern Sierra in collaboration with Mike Dodd. Measurement of forage and visibility changes caused by fire are ongoing in the area burned by and adjacent to the Seven Oaks Wildfire of July 2007. Bighorn sheep habitat selection for burned and unburned habitat is being quantified. Fecal nitrogen levels will be used as a surrogate for diet quality to determine the effect of this habitat selection. Results will help direct management use of prescribed fire to restore Sierra Nevada bighorn sheep habitat.

Project Coordinator: Heather Johnson
Project Status: Complete
Completion Date: Spring 2010
Recovery Actions Supported by Project: 3.1, 4 , 5.1, 6.1, 6.2
Resulting Publications:

• Johnson, H. E., M.H. Hebblewhite, T.R. Stephenson, D.W. German, B.M. Pierce, V. B. Bleich. Evaluating apparent competition in limiting the recovery of an endangered ungulate (PDF). Oecologia. 171(1):295-307. doi: 10.1007/s00442-012-2397-6. Jan. 2013.
• Johnson, H. E., L. S. Mills, J. D. Wehausen, T. R. Stephenson and G. Luikart. Translating Effects of Inbreeding Depression on Component Vital Rates to Overall Population Growth in Endangered Bighorn Sheep (PDF). Conservation Biology, Volume 25, No. 6, 1240–1249. 2011
• Johnson, H. E., L. S. Mills, J. D. Wehausen, and T. R. Stephenson. 2010. Combining ground count, telemetry, and mark-resight data to infer population dynamics in an endangered species (PDF). Journal of Applied Ecology 47:1083-1093.
• Johnson, H. E., L. S. Mills, T. R. Stephenson, and J. D. Wehausen. 2010. Population-specific vital rate contributions influence management of an endangered ungulate (PDF). Ecological Applications 20:1753-1765.

An extinction vortex is the single greatest threat to endangered species; when demographic, environmental, and genetic stochasticity interact with each other and with deterministic factors, such as habitat quality, to reinforce the demise of a small population. To successfully escape an extinction vortex and enable species recovery, all processes that affect endangered populations must be comprehensively assessed and incorporated into conservation plans. This project will apply a combination of demographic, habitat, and genetic techniques to identify stochastic and deterministic factors limiting the recovery of endangered Sierra Nevada bighorn sheep. A spatially explicit, individual-based viability model will be built to examine the relative impact of demographic, habitat, and genetic factors on the performance of Sierra Nevada bighorn, their synergistic effects on population persistence, and the benefit of different management activities for stimulating sub-species recovery. Results from this study will elucidate critical aspects of Sierra Nevada bighorn ecology, develop a framework for quantifying and alleviating extinction processes in endangered populations, and provide recovery strategies for Sierra bighorn that can be implemented by the program.

Project Coordinator: Maya Leonard-Cahn
Project Status: Complete
Completion Date: Fall 2009
Recovery Plan Actions Supported by Project: 2.3, 5.4, 6.6
Resulting Publications:

• Cahn, M. L., M. M. Connor, O. J. Schmitz, T. R. Stephenson, J. D. Wehausen and H. E. Johnson. Disease, Population Viability, and Recovery of Endangered Sierra Nevada Bighorn Sheep (PDF). The Journal of Wildlife Management 75(8):1753–1766; 2011; DOI: 10.1002/jwmg.232. The Wildlife Society; 11/2011.

Mitigating disease risks from domestic sheep is critical for Sierra Nevada bighorn sheep population recovery and long-term species management. A two-pronged approach is being used to understand this complex issue. First, a population viability model that integrates an epidemiological structure has been developed to estimate the effects of disease transmission on quasi-extinction. Second, spatially explicit habitat models are being combined with cost surface analysis to evaluate transmission risks associated with varying land use regimes. By integrating these components, a quantitative framework is being developed to understand the multiple risks and consequences of domestic sheep grazing for Sierra Nevada bighorn sheep persistence. The overarching objective is to provide meaningful risk assessment criteria and mitigation strategies for management actions.

Project Coordinator: Jeff Villepique
Project Status: Complete
Estimated Completion: Fall 2014
Recovery Plan Actions Supported by Project: 2.1, 2.2, 6.8
Resulting Publications:

Villepque, J.T., B. M. Pierce,V. C. Bleich,and R. T. Bowyer. 2011. Population of mule deer (Odocoileus hemionus): lack of evidence for switching prey (PDF). The Southwest Naturalist 56(2):187-192.

Villepique, J. T., V. C. Bleich, B. M. Pierce, R. A. Botta, T. R. Stephenson, and R. T. Bowyer. Evaluating GPS collar error: A critical evaluation of Televilt POSREC-Science™ collars and a method for screening location data (PDF). California Fish and Game 94: 155-168.

Understanding the relationship between Sierra Nevada Bighorn Sheep and their primary predator, the mountain lion, is a key component of managing the recovery of Sierra Bighorn Sheep. Declines in bighorn numbers during the 1990s were attributed by Dr. Wehausen not only to direct mortality by mountain lions, but also to decreased recruitment resulting from abandonment by females of high-quality, low-elevation winter ranges in response to risk of predation by mountain lions. This hypothesis also holds that in choosing to trade safety from mountain lions for quality low-elevation forage, Sierra bighorn suffered increased mortality as a result of harsh winter conditions at high elevation.

We are testing this hypothesis by evaluating responses of Sierra Bighorn Sheep to mountain lions, through direct observation of behavior, as well as evaluation of movements determined from GPS collars deployed on both sheep and lions. The objective is to determine the behavioral and spatial response by bighorn sheep to proximity of mountain lions as well as to instances when mountain lions kill conspecifics. In addition to evaluating this possible causal link in the Wehausen hypothesis, we are investigating alternative explanations for shifts in range use by bighorn. These include applying well-established relationships between forage quality and habitat selection by herbivores, as well as influences of snow cover on selection, to conditions specific to the Eastern Sierra. These modes of inquiry will help to elucidate the impact of mountain lions on Sierra Nevada Bighorn Sheep populations, and provide insights into alternative factors that may drive selection of winter range, while providing insights into the relationship between predator and prey.

Project Coordinator: Cody Schroeder
Project Status: Complete
Completion Date: Spring 2007
Recovery Plan Actions Supported by Project: 6.3, 2.2.3
Resulting Publications:

• Masters Thesis (PDF)
• Schroeder, C. A., R. T. Bowyer, V. C. Bleich, and T. R. Stephenson. 2010. Sexual segregation in Sierra Nevada bighorn sheep, Ovis canadensis sierrae: ramifications for conservation (PDF). Arctic, Antarctic, and Alpine Research 42: p 476-489.

A critical aspect of the ecology of mountain sheep (Ovis canadensis) is the differences in behavior and foraging ecology between sexes. Ecological variables between adult male and adult female Sierra Nevada Bighorn Sheep were compared and evaluated to better understand sexual segregation and foraging ecology during winter. Females foraged in larger groups that were closer to escape terrain than did males. Foraging areas used by males had a higher biomass of vegetation and were less open than areas used by females. Males foraged more efficiently when in larger groups, whereas females foraged more efficiently when closer to escape terrain. Male and female sheep differed in their dietary niches and in bite rates. Male mountain sheep traveled farther per day and in less rugged terrain than did females. These differences in behavior and space use are likely a result of allometric differences and associated life-history strategies between the sexes.

Project Coordinator: John Wehausen
Project Status: Ongoing
Completion Date:
Recovery Actions Supported by Project: 6.1
Resulting Publications:

• Epps, C. W., J. D. Wehausen, P. J. Palsbøll, and D. R. McCullough. 2010. Using genetic tools to track desert bighorn sheep colonizations (PDF). Journal of Wildlife Management 74:522-531.
• Epps, C. W., J. D. Wehausen, V. C. Bleich, S. G. Torres, and J. S. Brashares. 2007. Optimizing dispersal and corridor models using landscape genetics (PDF). J. Applied Ecology 44:714-724.
• Appendix H in the Recovery Plan for the Sierra Nevada Bighorn Sheep (PDF), U. S. Fish and Wildlife Service. 2007.
• Epps, C. W., P. J. Palsbøll, J. D. Wehausen, G. K. Roderick, and D. R. McCullough. 2006. Elevation and connectivity define genetic refugia for mountain sheep as climate warms (PDF). Molecular Ecology 15:4295-4302.
• Epps, C. W., P. J. Palsbøll, J. D. Wehausen, G. K. Roderick, R. R. Ramey II, and D. R. McCullough. 2005. Highways block gene flow and cause rapid decline in genetic diversity of desert bighorn sheep. Ecological Letters 8:1029-1038.
• Wehausen, J. D., R. R. Ramey II, and C. W. Epps. 2004. Experiments in DNA extraction and PCR amplification from bighorn sheep feces: the importance of DNA extraction method (PDF). Journal of. Heredity. 95:503-509.

Prior to restoration effort beginning in the late 1970s, the range of Sierra Nevada bighorn sheep declined to three native herds in the southern Owens Valley because of the extirpation of many other herds. Because epizootic disease contracted from domestic sheep was the likely cause of most if not all of those losses, it is likely that the surviving herds are derived from relatively small numbers of sheep that survived such past epizootics. Such major reductions in populations are known as population bottlenecks because they have the potential effect of greatly eroding genetic diversity. This raised a fundamental question of how much genetic diversity remains in Sierra Nevada bighorn sheep herds, and if it will limit the recovery of these sheep. Natural levels of genetic diversity vary greatly among wildlife species depending on natural population sizes, gene glow between populations, and potential bottlenecks of the more distant past. To assess the genetic diversity of bighorn sheep in the Sierra Nevada it has been necessary to sample many other bighorn sheep populations in the desert region of California and elsewhere for comparison. One of the first steps in that process was the refinement of methods of DNA extraction from bighorn sheep droppings to allow the rapid sampling of many populations. Multiple researchers have coordinated in applying fecal DNA methods developed for Sierra Nevada bighorn sheep to bighorn sheep populations throughout California, neighboring southern Nevada, and some further populations. All but a few of the bighorn sheep populations in California have been sampled and a variety of important analyses have resulted from the data developed. Bighorn sheep in the Sierra Nevada show clear evidence of a recent major bottleneck and have the lowest genetic diversity of all populations sampled to date. A recent research project that looked at individual heterozygosity levels of bighorn sheep in the Sierra Nevada relative to their physical measurements and reproductive output concluded that low heterozygosity in some individual females may be lowering reproductive success. However, that research also concluded that the size of this genetic effect was too small to influence the ability of these sheep to recover numbers needed. Future research will compare current genetic diversity with pre-bottleneck diversity in the Sierra Nevada as measured from DNA extracted from bone of early skull specimens archived at various locations.

Project Coordinator: Tom Stephenson
Project Status: Complete
Completion Date: Fall 2007
Recovery Actions Supported by Project: 2.3, 5.4
Resulting Publications:

• Clifford, D. L., B. A. Schumaker, T. R. Stephenson, V. C. Bleich, M. L. Cahn, B. J. Gonzales, W. M. Boyce, and J. A. K. Mazet. Assessing risk of disease transmission at the wildlife-livestock interface: a study of Sierra Nevada bighorn sheep (PDF). Biological Conservation. 2009, 42: 2559-2568.

Habitat sharing by livestock and wildlife may present a threat to endangered species persistence if diseases affecting domestic animal populations are transmitted to vulnerable wildlife. Respiratory disease transmission from domestic sheep has been implicated in the declines of numerous bighorn sheep populations, but direct evidence is lacking under field conditions. In California, endangered Sierra Nevada bighorn sheep number fewer than 400 individuals. Pathogens present in domestic sheep grazing in public and private areas within or adjacent to Sierra Nevada bighorn sheep populations may threaten the recovery and persistence of this unique subspecies, but the degree of risk has not been quantitatively evaluated. We used the best available spatial, demographic, and disease data to assess the risk and impact of a respiratory disease outbreak in Sierra Nevada bighorn sheep resulting from contact with domestic sheep. Probabilities of interspecies contact were derived by assessing the spatial overlap of bighorn sheep population kernel utility distributions and public domestic sheep grazing allotments when both species were present on the landscape. Demographic data and modified Reed-Frost disease models were then used to assess the population-level impacts of a respiratory disease outbreak, including herd-level mortality and the possibility of disease spread to adjacent herds of bighorn sheep. Although current management strategies, which included restricting domestic sheep grazing during the bighorn sheep rut, reduced the risk of interspecies contact to less than 2% per year, our model still predicted a 50% probability of at least one respiratory disease outbreak causing = 40% bighorn sheep herd mortality during the next 70 years. Efforts to further minimize or eliminate the potential for interspecies contact include options such as closing allotments within the current distribution of Sierra Nevada bighorn sheep, further restricting grazing during rut, and educating private parties about the disease risk posed by domestic sheep. These choices will require difficult decisions to balance trade-off costs and benefits to economic livelihoods and natural resource protection.

Project Coordinator: John Wehausen
Project Status: Complete
Completion Date: Spring 2016
Recovery Actions Supported by Project: 6.1
Resulting Publications:

Bighorn sheep in the Sierra Nevada have long been proposed to be different from adjacent desert bighorn. Recent cranial morphometric research has found them to be one of three distinguishable subspecies of bighorn sheep. The evolutionary background of such uniqueness and how that uniqueness stands up to genetic data have been obvious questions. A fledgling research project to pursue those questions became a part of the recovery program for Sierra Nevada bighorn sheep. That project has sequenced two mitochondrial genes of many bighorn sheep from the Sierra Nevada. Sampling, of wild sheep in North America and Siberia, is necessary to build an adequate database to understand where bighorn sheep in the Sierra Nevada fit into the larger evolution of wild sheep in North America. An important finding has been that the divergence of bighorn sheep from desert bighorn goes back 300-400 thousand years. This means that bighorn sheep have persisted in the Sierra Nevada through at least three ice ages. This research is nearing completion.