Large herbivores have been living in Yellowstone for 2,300 years: Study

Large herbivores, such as bison and elk, have lived continuously in Yellowstone National Park for more than two millennia, a new study has confirmed.

Despite the near-extinction of bison in North American in the 19th and 20th centuries, these big plant-eaters and others have persisted in the park region since around 238 B.C., according to the study, published Wednesday in the journal PLOS ONE.

Since little was known about where and how these animals lived before European colonization, the researchers decided to figure out which large herbivores dominated the Yellowstone region.

Understanding the population makeup, they explained, could provide insight into long-term ecosystem dynamics, past herbivore communities and environmental influences in this area and elsewhere.

To paint a clearer picture of the park's past, researchers from multiple universities analyzed the steroids present in animal dung — unearthed from lake sediments that range from around 238 B.C. to the present day.

Their first task in conducting this analysis was to identify which types of steroids occur in the feces of large herbivores, including bison, elk, moose, mule deer and pronghorn.

Although they found that they could recognize moose, pronghorn and mule deer based on steroids alone, the scientists saw that bison and elk were harder to differentiate from each other.

Upon evaluating the steroids within different layers of lake sediments, they observed that either bison, elk or a combination of the two were the primary large plant-eaters that inhabited the watershed for the past 2,300 years.

Steroid levels were particularly high during the 20th century, when hunting was banned and bison and elk were discouraged from migrating in the winter, according to the scientists from Montana State University, Oklahoma State University and Ca' Foscari University of Venice, Italy.

Based on plant pollen, microalgae and plankton detected in the dung, the researchers concluded that these expanded populations likely consumed local forage plants. In turn, their dung may have fertilized the growth of certain types of algae in the lake and thereby altered the local ecosystem.

Stocks of winter hay provided by nearby park managers also kept the animals in the area longer and may have likewise caused changes in the watershed, the researchers noted.

The scientists expressed optimism that their results could help wildlife managers and conservationists understand how communities of hoofed animals shift over time.

Extending this approach of lake sediment analysis to other watershed could provide much-needed insight into past grazing habits of large herbivores in Yellowstone and elsewhere, according to the study.

"This information is critical for understanding long-term dynamics of ecologically and culturally important herbivores such as bison and elk," the authors added.