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. 91ÖÆƬ³§'s Nate Rott reports a new study looks at how those bison are shaping the park's grasslands.
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NATE ROTT, BYLINE: Bison eat a lot - grass, sedges, weeds, leafy plants. They typically forage 9 to 11 hours a day...
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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, 91ÖÆƬ³§ News.
(SOUNDBITE OF HERMANOS GUTIÉRREZ'S "BLOOD MILK MOON") Transcript provided by 91ÖÆƬ³§, Copyright 91ÖÆƬ³§.
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