Not a problem. Wood Bison are so big nothing messes with the Mama. Not wolves or grizzlies. It's what they call preditor resistant. Yes, preditor's could kill the babies if they could get past the Mama, but not likely.
As for them being here. The villege Elders talk about their father's father, (Their Grandfather) killing Bison so large it could feed the entire village for several weeks. The miners in the Yukon Flats are constantly finding Bison bones and horns from Wood Bison. We are not talking about fossels, we're talking about actual bones preserved in the peat bogs. Dating of these finds gives the dates ranging from 250 to 150 years ago. That means the Wood Bison was killed out about the time of the War of Northern Aggression.