Caribou Find the Green in a Warming Arctic

Just about anywhere scientists look up north--at shrinking sea ice, at thawing permafrost and at vigorous plant growth--they see a warmer Arctic. Changes in the far north, though often regarded as negative to people and animals, may be helping caribou during the summer.

Brad Griffith, a USGS biologist and the assistant unit leader of the Alaska Cooperative Fish and Wildlife Research Unit, thinks caribou calves have done well recently because the plants they eat have matured earlier during the past decade. Griffith tracks changes in the Porcupine caribou herd, a band of about 128,000 animals that roams the northwest portion of Yukon Territory and the Arctic National Wildlife Refuge in northeast Alaska. Griffith concentrates his research in early summer when caribou calves are born and when much of the North Slope greens up.

After cows give birth about June 4, their energy needs double because they begin producing milk for their calves, Griffith said. When it's time to give birth to calves, the Porcupine caribou migrate to the coastal plain of ANWR or the foothills of the northeastern Brooks Range. Because green-up varies from year to year--sometimes tundra plants along the coast mature first; sometimes those in the hills blossom earlier--caribou gamble on where to hang out when it's time to calve. What surprises Griffith is that they almost always stop in an area where the rate of plant green-up is the greatest, a place from which they won't move far for one or two weeks.

"They somehow pick the best place to drop their calves," he said. "It's a place they'll have access to the greatest amount of new plant growth and digestible forage during lactation."

How caribou know where to calve is still a mystery. Griffith said they may be able to pick up the scent of emerging greenery from beneath shallow snow. Whatever the reason, Griffith said the Porcupine caribou consistently select the areas of high plant productivity for calving, regardless of if these areas are on the coastal plain or in the foothills.

Satellite images of the Arctic taken around June 4 and two weeks after confirm the Arctic is greening up earlier, Griffith said. The images, acquired from 1983 to 1996, have over the years shown an increasing amount of chlorophyll, an indicator of plant life. At calving time during the past decade, caribou of the Porcupine herd have had more food, and they've been able to eat it a few days earlier in most years, Griffith said. With increasing food during calving, more calves have survived the month of June.

More calf survival would seem to equate to a bigger herd, but that's not the case. During the 13 years of the study, the total number of caribou in the Porcupine herd first increased to 178,000, then declined to the present level of 128,000. Griffith thinks the herd may possibly have reached the limits of what the land can provide.