MARY LOUISE KELLY, HOST:
A couple hundred years ago, the American heartland was home to massive herds of migrating bison - tens of millions of them crossing the plains, their hoofs sounding like distant thunder. Today, the only place in the lower 48 U.S. states with continuously free-ranging bison is Yellowstone National Park. NPR's Nate Rott reports a new study looks at how those bison are shaping the park's grasslands.
(SOUNDBITE OF BISON GRUNTING)
NATE ROTT, BYLINE: Bison eat a lot - grass, sedges, weeds, leafy plants. They typically forage 9 to 11 hours a day...
(SOUNDBITE OF BISON GRUNTING)
ROTT: ...Giving folks at the National Park Service ample opportunity to make recordings like this.
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ROTT: There are about 5,000 Bison in Yellowstone that have the ability to free range within the park's boundaries.
BILL HAMILTON: It's very small in comparison to the extent that we believe it happened when there were 30 million in North America.
ROTT: Bill Hamilton is a professor of biology at Washington and Lee University.
HAMILTON: But it's about a 50-mile range, and your average bison travels a thousand miles in a year. So they're doing a lot of moving.
ROTT: Scientists have long wanted to know what kind of impact all that moving and eating has on the park's ecology. Hamilton and others have been doing long-term monitoring since 2015, and this new study, published in the journal Science, focuses on the park's grasslands.
HAMILTON: We're seeing positive effects. It's not a change in plant productivity. There isn't more biomass being made at the end of the year. But it's a higher-quality food when the grazing is taking place.
ROTT: Soil microbes increase. The nitrogen cycle speeds up. Grasses in areas that have been grazed grew back with a 150% increase in protein, which herbivores need to survive winter. Big picture, Hamilton says...
HAMILTON: Allowing large numbers of herbivores to move across the landscape is a very important feature of how grassland ecosystems function.
LUKE PAINTER: The problem is that bison generally are confined to small areas.
ROTT: Luke Painter is an ecologist at Oregon State University who does a lot of work in Yellowstone National Park. He's there as we're talking.
PAINTER: And that's the same problem here. I mean, they're actually not migrating very much.
ROTT: Relative to how much they moved historically. Painter was not involved in this new study, but he's published others looking at how Yellowstone's bison populations have had not-so-great impacts on willow and aspen saplings, which grow near water.
PAINTER: They trample the banks. They eat the willows. And that would be OK if they really did migrate and then they didn't come back for a while. But that's not happening.
ROTT: Painter says to really see broad-scale ecological benefits beyond what the new study shows in Yellowstone, bison need to be able to roam across much larger landscapes. That's a hard thing to do with all of the people, homes and farms that now cover much of the American heartland and West.
Nate Rott, NPR News.
(SOUNDBITE OF HERMANOS GUTIÉRREZ'S "BLOOD MILK MOON") Transcript provided by NPR, Copyright NPR.
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