Predation of Desert Bighorn Sheep by Mountain Lions in Grand Canyon National Park

Science Center Objects

Desert bighorn sheep populations in the southwestern United States are subject to non-native disease outbreaks, habitat loss, and genetic isolation that can threaten their long-term sustainability.  In some regions of the southwest, mountain lion predation on desert bighorn sheep has been found to be the primary source of mortality.  Grand Canyon National Park is home to one of the largest desert bighorn sheep populations, but little past research has been conducted within the park to establish population structure, distribution, gene flow, disease outbreaks, or mortality rates.  The USGS Southwest Biological Science Center is working with the Grand Canyon National Park Science Center to assess the role mountain lion predation may have on mortality rates and the distribution of desert bighorn sheep within the park. 

Background & Importance

Desert bighorn sheep herd along the banks of the Colorado River surrounded by tall grass and large rocks

Desert bighorn sheep, Ovis canadensis nelson, herd along the banks of the Colorado River in the western portion of Grand Canyon National Park (September 2016). (Credit: Kirsten Ironside, USGS. Public domain.)

Desert bighorn sheep were extirpated from a significant part of their historic range during the late 1800s and early 1900s primarily by exposure to non-native disease and over-hunting. Major restoration efforts have been undertaken by state game management agencies and the U.S. Fish and Wildlife Service, but often meet with little success due to ongoing disease outbreaks transmitted from domestic sheep. Although a large number of pathogens afflict bighorn sheep, microbial respiratory diseases caused by Pasteurella spp. and Mannheimia spp. are implicated in most historic and current outbreaks of pneumonia. Disease is and will likely continue to be an important limiter to bighorn sheep populations, even in large National Parks.

Predation by mountain lions, Puma concolor, is recognized as a potentially major threat to desert bighorn sheep, Ovis canadensis nelson, populations. Although of secondary importance to disease, mountain lion predation has been implicated in reducing bighorn sheep populations in no less than 10 studies. Mountain lion predation is often thought to exacerbate the effects of disease and declining habitat quality (such as drought), but predation alone can have substantial effects when either individual mountain lions specialize in killing sheep or mountain lions switch prey when other prey (typically mule deer, Odocoileus hemionus) change in availability.

General Methods

Desert bighorn sheep were fitted with GPS collars with satellite uplink capability. These tracking collars collect 6 locations per day and typically collect data for approximately 1.5 years.  Collars are equipped with mortality sensors so investigations into the cause of death can be made.  Twenty-five study animals across the park have been monitored via GPS telemetry to evaluate seasonal habitat use, distribution, and survivorship. 

We are developing and refining mountain lion habitat selection and prey selection models based on existing data obtained from mountain lions GPS radio-collared and monitored on the South and North Rims of the Grand Canyon between 2003 and 2014, totaling 37 study animals. These selection models are linked to habitat and prey variables. We are working on overlaying spatially-explicit probabilities of mountain lion occurrence and predation on spatially-explicit probabilities of seasonal bighorn sheep occurrence to determine, in turn, spatially explicit probabilities of exposure by sheep to risk of predation.   

Important Results

Scientists taking nasal swabs from a desert bighorn sheep ewe

Nasal swabs being taken from a desert bighorn sheep ewe. (Credit: Kirsten Ironside, USGS. Public domain.)

An unanticipated result of this project is helping improve our understanding of how topography influences GPS collar’s ability to acquire locations.  Due to the extreme topography of the Grand Canyon, there are many locations in the inner canyon and side canyons were terrain blocks satellite signals causing failed GPS fix attempts from the collars we deployed both on mountain lions and desert bighorn sheep.  Because many of the GPS collars we deployed had high rates of missed fixes, we developed a model to correct for the effects of the missing locations.  More information on this can be found at https://www.sciencebase.gov/catalog/item/55f998b7e4b05d6c4e50146f.

Desert bighorn sheep ram surrounded by large rocks in the Grand Canyon

Desert bighorn sheep ram in the Grand Canyon (September 2016). (Credit: Kirsten Ironside, USGS. Public domain.)