Those conservation efforts are on exhibit at Brookfield Zoo, where a female calf was born on May 16. That means the herd of six is now a herd of seven.
Zookeepers knew Leotie, 3, was pregnant, but were surprised when the baby showed up -- suddenly.
"She gets up and she's immediately part of the herd," Amy Roberts, curator of mammals, said. "Yes, we weren't there but I imagine it was less than 20 minutes that she was up and completely active and able to follow mom."
The 40-pound calf doesn't have a name yet, but she does have a personality. She's rambunctious and full of energy. She's the first bison born at Brookfield Zoo in 40 years.
"The first since the 1970's. We chose not to breed for a long time because we really didn't have the best exhibit for them. Now we have this big, large, beautiful, appropriate exhibit we plan to start breeding bison again," Roberts said.
In the mid-1800s, as many as 50 million bison roamed the western plains. But by 1890, that number was down to less than 1,000. The animal had been slaughtered by hunters.
"This is the story of the American West and how we nearly drove the bison to extinction and now how they've come back from the brink of extinction," Roberts said.
The giant bison were shot dead for their hides- not their meat- and then the babies were left on the prairie to die. But the killing of bison was eventually outlawed.
"I hope we've learned our lesson that even if something seems countless and there seems to be so many of them that everything is not inexhaustible and you have to be careful and mange your resources," Roberts said.
About 500,000 bison live on ranches, tribal lands and protected park. Or, about 500,001 -- with the Brookfield baby.