Will Bighorn Sheep Survive Climate Change?

The bighorn sheep is one of the iconic species of the American landscape, like the buffalo and the bald eagle. At a time when so many species are suffering loss of population due to human encroachment and global warming, people have become concerned about the well-being of bighorn populations.  

Dr. Tyler Creech, OSU alumnus and member of the Center for Large Landscape Conservation, led a large study of bighorn sheep living in National Park Service units – some national parks, some other kinds of land administered by the Park Service – in an area spanning Arizona, California, Nevada, and Utah. 

 The study covered ten different NPS units in all, home to 62 distinct populations of bighorns. Genetic information was collected from more than 1,600 sheep for the study, whose results have been published in the scholarly journal Frontiers in Ecology and Evolution. 

The good news is that when bighorn are able to live in areas which are relatively pristine – in and near Death Valley National Park and Grand Canyon National Park – they are still thriving. The bad news is that there aren’t very many pristine places in the American Southwest anymore.  

Even in areas which might seem almost like true wilderness, like NPS units in southeastern Utah and the southern part of the Mojave Desert, bighorn are showing signs of environmental stress brought about by human activity. 

How they defined populations 

In the past, animal populations were identified simply by where they lived, and by visible differences between them. Now, a population group can be identified more accurately by its genetic makeup. Even when two populations live in areas close by one another, if their DNA says that they are not closely related, they are counted as separate groups. 

Creech says in a press release, “We used DNA samples from bighorn sheep to tell us how genetically diverse populations are. The populations that are less genetically diverse and less connected to their neighbors are more likely to be negatively impacted by climate change.” 

Clint Epps, an Associate Professor in the Department of Fisheries and Wildlife in OSU’s College of Agricultural Sciences and co-author of the study, elaborates: “Genetic diversity allows populations to adapt to new environments. This study highlights the important role our national park units can play in keeping these populations up as the climate changes.” 

Finding out a lot from feces 

Many think that DNA information about a bighorn sheep must involve trapping animals and taking blood or tissue samples, or perhaps shooting them with a dart gun to obtain biopsies. Actually, most of the information was found in pellets of bighorn feces which were swept up for analysis. This method allowed large amounts of information to be collected in a short amount of time, with very limited effect on the sheep themselves.  

The pellets were collected by several different field studies in different parts of the study area over the last twenty years. The DNA information from the pellets allowed the team to make an estimate of the range of genetic diversity in each population, and of how often sheep from one population joined another. If a population had low diversity, or showed signs of being isolated from other populations, it would be more vulnerable to disease and less able to adapt to changing circumstances. 

Where will the sheep go 

The study estimates the effect ongoing climate change is likely to have on populations using “forward climate velocitythe speed at which a population must move to a new location to adapt to climate change.  

As temperatures rise, the area where suitable food grows and where weather will not be too hot for the sheep keeps moving uphill, and bighorn populations will have to move as well. It’s a bit like living in a town where the river is eroding the bank, so houses by the riverside must be abandoned and new ones built further inland.  

The study used two scenarios developed by the Intergovernmental Panel on Climate Change, one which presumed that greenhouse gas emissions would remain high and another where emissions were reduced. In both studies, usable bighorn habitat moved uphill; with high emissions, the movement was more rapid, which would put more stress on the populations, which would reduce the chances of the species surviving in healthy populations.  

John M. Burt