Pennsylvania State University scientists, led by Associate Professor Eric Post and graduate student Christian Pederson, said most computer models indicate shrubs will thrive as a result of global warming. And since shrubs have an increased ability over grasses and other small plants to absorb carbon dioxide, many scientists believe shrubs will thereby lessen the impact of climate change in the Arctic.

But Post and Pederson argue grazing by muskoxen and caribou will reduce the carbon-mitigating benefit of the plants.

"If you imagine a chessboard on which the dark squares are shrubs and the light squares are grasses, warming alone would tend to increase the size of the dark squares until the chess board is completely filled in," said Post. "Our experiment suggests that herbivores, like caribou and muskoxen, will slow this process, inhibit it, or perhaps even increase the size of the white squares on the chessboard."

The research appears in the early online edition of the Proceedings of the National Academy of Sciences.