Rocky Mountain bighorn sheep in Idaho have experienced a significant loss of genetic diversity and connectivity over the past 100 years, according to a study released by the University of Idaho and the Idaho Department of Fish and Game on May 6.
The findings highlight concerns about the long-term survival and adaptability of these native animals. Researchers say that reduced genetic variation may make populations more vulnerable to environmental changes and disease.
The study compared DNA from bighorn sheep skulls collected between 1897 and 1985 with samples gathered from the same region between 2000 and 2017. The historical data showed that about 35% of Idaho’s bighorn sheep carried DNA linked to desert lineages, suggesting frequent interbreeding between subspecies. In contrast, this genetic signal is now nearly absent at about one percent, which researchers attribute to habitat fragmentation and local or regional extinction.
“Comparisons between historical and modern data reveal a consistent decline in genetic diversity,” said Frances Cassirer, senior wildlife research biologist at the Idaho Department of Fish and Game. “Historical populations showed higher genetic variation and contained more unique genetic variants, many of which have been lost.” Cassirer said these patterns indicate bottlenecks caused by population reductions in previous decades.
Researchers identified six distinct groups within today’s Salmon River region population—each largely corresponding with management units overseen by state officials. Some gene flow remains in central areas, but certain groups are genetically isolated. The East Fork population on Salmon-Challis National Forest is particularly distinct due to its long-term separation and small size.
“Genetic diversity is essential for the health and long-term survival of species. Idaho’s native bighorn sheep still retain moderate genetic variation, but isolated populations are at risk of further loss. Preserving this diversity is critical because these sheep have experienced local adaptation,” Cassirer said.
Bighorn sheep were once widespread across western North America but suffered major declines due to disease from domestic livestock, overhunting, and competition for habitat during the late nineteenth and early twentieth centuries. While some recovery has occurred since then, many herds remain small or fragmented.