California Department of Fish and Wildlife

Overview of Sierra Nevada Bighorn Sheep Research Projects

Project Overview:
Can Mammals Mediate Climatically-Induced Vegetation Transitions in Alpine Ecosystems of The Western United States?

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 Overview:
Determining the Relationship Between Human Off-Trail Travel, Packstock and Sierra Nevada Bighorn Sheep In Sequoia-Kings Canyon National Parks: Are There Negative Effects?

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

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 Overview:
Recovery of Sierra Nevada Bighorn Sheep in Yosemite National Park: Tracking Distribution in Real-Time

Yosemite Conservancy logo

Project Coordinator: Tom Stephenson
Project Status: In-Progress
Completion Date: Fall 2013
Recovery Plan Actions Supported by Project: 5.3, 6.3
Resulting Publications:

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 Overview:
Use of Fecal DNA to Detect Range Expansion of Bighorn Sheep in the Sierra Nevada

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 Overview:
Using Photogrammetric Techniques to Quantify Bighorn Sheep Habitat Characteristics

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

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 Overview:
A Review of the Experimental Scientific Literature Concerning Respiratory Disease Interactions between Domestic Sheep and Bighorn Sheep

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

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 Overview:
Fire Ecology and Subsequent Sierra Nevada Bighorn Sheep Habitat Improvement

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:

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 Overview:
Extinction Vortex: Demographic, Environmental and Genetic Effects on Bighorn Population Demographics

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:

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 Overview:
Domestic Sheep Disease Risk to Bighorn Sheep

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:

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 Overview:
Responses of Sierra Nevada Bighorn Sheep to Risk of Predation by Mountain Lions; Testing the "Range Abandonment" Hypothesis

Project Coordinator: Jeff Villepique
Project Status: In-progress
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. 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 Overview:
Sexual Segregation and Winter Foraging Ecology

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

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 Overview:
Genetic Diversity, Heterozygosity, and Gene Flow in Sierra Nevada Bighorn Sheep Relative to Desert Bighorn Sheep in California

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

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 Overview:
Modeling Risks of Disease Transmission from Domestic Sheep to Bighorn Sheep

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

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 Overview:
The Evolutionary Uniqueness of Sierra Nevada Bighorn Sheep

Project Coordinator: John Wehausen
Project Status: Ongoing
Completion Date:
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.