MINNEAPOLIS (AP) -- University of Minnesota officials hope a herd of bison will help restore a threatened ecosystem in a nature reserve.

More than 30 bulls were released into the 200-acre Cedar Creek Ecosystem Science Reserve in East Bethel on June 13. There are no wild bison in the area. The herd of two-year-old bulls is from a ranch in Wisconsin.

Cedar Creek director David Tilman says the bison program is part of a larger initiative to restore the oak savanna ecosystem.

Post-doctoral ecology researcher Chad Zirbel says bison feed on grass, which creates room for oak trees to grow.

Researchers have planted 660 oak seedlings in the enclosure. They plan to monitor the plants' growth as the bison graze.