Nostalgia may be one good reason for restoring bison to the North American plains, but now there's a scientific incentive as well: Bison appear to help keep grassland ecosystems healthy. Findings reported in today's Science  linking bison grazing to plant diversity in a Kansas tallgrass prairie offer hope to land managers trying to preserve the last remnants of native U.S. grasslands.
About 10% of North America's original tallgrass prairie remains in scattered patches across the plains states. Ecologists know that fires--whether set by lightning or by people--kept the bison-filled prairies from turning into forests for millennia. Scientists at Kansas State University hoped to better tease out the delicate relationship between burning, grazing, and a new threat to prairies worldwide: atmospheric nitrogen from car tailpipes and fertilizers, which helps some species flourish at the expense of others. So in 1986 the researchers launched an experiment on 20 grassland plots, plying some with heavy doses of nitrogen, torching others once a year, doing both to a third set of plots, and leaving the rest alone. They also mowed some plots each June to simulate bison grazing.
The team found that by 1994, the burning and added nitrogen had taken a heavy toll. Taller, warm-season grasses thrived on the burnt, nitrogen-rich plots, but plants that prefer cooler digs were decimated, leaving these plots with roughly 5.6 species per 10 square meters--66% fewer than the control plot. In plots that had also been mowed, however, the number of species didn't fall off at all. The researchers got similar results when they compared burned and unburned Konza Prairie watersheds where bison had been reintroduced in 1987. The reason, says study leader Scott Collins of the National Science Foundation, seems to be that mowing opens up a "big, thick canopy" formed by the taller warm-season grasses, allowing sunlight to get through so that "a lot more species can coexist."
The study suggests that cattle grazing might also help prairies. But prairie-reserve managers shouldn't move too quickly, as bison tend to range more widely than cattle, and any grazing "can be very bad for grasslands" that are already degraded, Collins says. The best approach, he and others say, is to allow moderate bison or cattle grazing on healthy prairies and track how the ecosystems respond.