But in patches that were open to grazing—and also enclosed to raise temperatures—the mix of plants was much the same as today.
Musk-oxen seem to be responsible for most of the grazing, the study found.
"Not only are animals like caribou and musk-oxen themselves affected by climate change, they also affect how the Arctic responds to climate change," Post added.
(Read: "Fewer Caribou Born as Warming Causes Missed Meals" [May 12, 2008].)
"For decades the Arctic has been considered to be regulated 'bottom up,'" meaning that scientists thought that fewer nutrients in the soil determined how Arctic plants grow, said plant ecologist Laura Gough of the University of Texas in Arlington.
But the new study shows that "mammals have the potential to counteract effects of warming on Arctic systems," added Gough, who was not involved in the new study.
"It is true that herbivores are relatively few in number in [the] tundra compared with warmer areas, but their effects can be intense," she said.
Although large grazers are sparsely spread across the Arctic—altogether there are about five million caribou and tens of thousands of musk-oxen—they eat enough plants to shape the ecosystem.
Shrubs are darker and absorb more heat, so if they spread due to continued global warming, this would create a "positive feedback" that makes the warming worse.
By preventing or delaying this change, grazers could help temper the warming of the planet, said Arctic ecologist Greg Henry of the University of British Columbia in Vancouver, Canada.
"This could be a very important finding as some predictions of effects of the increased shrub cover are equivalent to doubling the [atmospheric] CO2 again," Henry said.
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