Mason Inman, for National Geographic News 19 Aug 08;
Grazing musk-oxen and caribou may help protect the fragile Arctic ecosystem from the effects of global warming, according to a new study.
Large grazers could help the region by feasting on woody shrubs and plants that would otherwise take over as temperatures rise and change the way the Arctic looks and functions.
If shrubs dominated, they would darken Arctic lands and absorb more heat from the sun, enhancing warming due to greenhouse gases.
"Careful management and conservation of existing populations of musk-oxen and caribou, as well as other large herbivores, should be a priority in plans to mitigate the effects of climate change on ecosystems," said study leader and Pennsylvania State University researcher Eric Post.
"Until now, these animals seem to have been regarded more as background noise than as an active component of the ecosystem's response to warming," said Post, a National Geographic grantee. (The National Geographic Society owns National Geographic News.)
Grazing Control
As carbon dioxide levels continue to increase, the Arctic is heating up faster than almost anywhere else.
This additional warmth will likely boost the growth of woody shrubs at the expense of grasses in the Arctic, according to the study published online this week in the journal Proceedings of the National Academy of Sciences.
"Keeping [large grazers] in the picture will help maintain other components of the Arctic as we know it, or at least moderate the effects of global warming," Post said.
Post and Christian Pedersen, also at Penn State, used plots in western Greenland to measure the effects of these grazing animals.
Fences kept several patches free of grazers. Some of these patches were also enclosed by clear plastic walls, which raised temperatures inside by 2.7 to 5.4 degrees Fahrenheit (1.5 to 3 degrees Celsius), simulating future warming.
In the plots off-limits to grazers, shrubs such as dwarf birch spread, crowding out grasses, which make up nearly half of the biomass in western Greenland studied by Post.
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.
"Intense" Effects
"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.
