Study: Red squirrels cause evolutionary change

When most people think about Yellowstone National Park and animals, the big ones come to mind: bison, wolves or grizzly bears.

They’re the species that draw tourists from around the globe. They’re also the ones that raise controversy and debate. But when it comes to actual impact on the forest, scientists recently discovered a much smaller, much more common creature is causing large, evolutionary changes to the park: the red squirrel.

“It’s pretty remarkable what squirrels do,” said Craig Benkman, a zoology and physiology professor at the University of Wyoming. “It’s just spending the time to figure out how it manifests itself.”

In the rebuilding of forests, Benkman and Matt Talluto, a recent UW doctoral graduate, found squirrels drastically change how forests regrow after fire. The pair recently published their work, “Conflicting selection from fire and seed predation drives fine-scaled phenotypic variation in a widespread North American conifer,” in the Proceedings of the National Academy of Sciences.

Their research started formally about four years ago, but Benkman has wondered about squirrels and forest growth for more than a decade, he said.

He noticed striking differences between forests with and without squirrels. Those lodgepole pine forests without squirrels regrew quickly after fires; those with squirrels grew back in much more of a patchwork way with many more open meadows and sparse mountainsides. He also noticed that forests with squirrels had fewer serotinous pine cones — pine cones that need fire or extreme heat to open — versus other cones that open to seed every fall.

Lodgepole pines with serotinous cones are generally common in forests prone to fire. Trees wait to seed until a fire moves through, opening millions of cones and regrowing hundreds of thousands of trees in relatively small areas. Forests without closed cones take much longer to regrow because no seeds have accumulated over the years.

“You end up with much more open habitat, which in turn influences everything,” Benkman said. “It must influence the bird communities, mammal communities and others organisms working in Yellowstone.”

He wondered then if squirrels, and their affinity for eating serotinous cones, could be stopping trees with those cones from reproducing, which keeps the population of trees from regenerating.

Benkman then met Talluto who arrived for graduate school at UW, and they turned their sights on Yellowstone.

As Yellowstone continues to regrow after the massive 1988 fires, trees have appeared in patches, Talluto said. Biologists knew the differences were due to areas with more and less closed cones, but didn’t understand the cause of the variation. Yellowstone also made the perfect study area with relatively untouched wilderness and had large amounts of previous research.

Talluto, who now works as a postdoctoral researcher with the University of Quebec, spent three summers in Yellowstone tracking squirrel numbers and serotinous cones. The pair also used past research and modeling to reach their conclusions.

Unlike feral pigs on the east coast or zebra or quagga mussels in lakes and reservoirs, red squirrels are not an invasive species in the Rocky Mountains. Nevertheless, as fires become more frequent and more intense across the West, Talluto thinks the squirrels could play an even more important role in changing the landscape.

“In terms of using this information for actual decision making, we’re not there yet,” Talluto said. “We’re getting closer to helping make decisions. There are still unanswered questions about how will things change with climate change, or will we see a change in the number of fires. Everything you find asks more questions.”

What the team does know is that squirrels, one of the smaller, and certainly less talked about mammals in the Yellowstone ecosystem, are making substantial changes with tangible results.