Click on the 'Listen' button above to hear this interview. 

Exactly one year ago today, former President Barack Obama named the bison our country's national mammal. 

"We used to have more than 50 million bison in North America," explains Amy Martin, who has been reporting on bison in Montana for her podcast Threshold. "They were here for hundreds of thousands of years before the humans crossed over the Bering Land Bridge, and they were really a keystone species for the entire continent." 

Between settlers trying to clear land for agriculture, and hunting the giant mammals for sport, the bison nearly became extinct by the 20th century. Early conservationists realized they needed to do something to save the animal and today, thanks to extensive re-population efforts, there are now more than 20,000 wild bison in the U.S. — and hundreds of thousands being raised for meat.

But as Martin explains on the first episode of Threshold, those efforts have created their own problems. In Yellowstone Park, the national park service slaughtered 1,200 bison this winter — about a fifth of the park's herd — to prevent overgrazing and other imbalances to the ecosystem. 

The entire first season of Threshold is devoted to the story of the American bison, and the ongoing saga of restoring our national mammal. Here, Martin discusses her findings — click on the 'Listen' button above to hear our conversation, and check out episode one of the podcast here.