Brian Handwerk, National Geographic News 12 May 08
Greenland's caribou work up quite a hunger during their long migrations. But global warming now has the animals arriving late for dinner—and paying a heavy price.
Fewer caribou calves are being born in the western part of the Danish island, and those that are born have slimmer chances of surviving, a new study reports.
The declines are tied to availability of the willows, sedges, and flowering tundra herbs that caribou and their newborns feed on in spring.
As global warming causes these plants to bloom ahead of schedule, herds are arriving when the bounty is past its peak.
"For these animals, the cafeteria is closing earlier and earlier," said Eric Post, a Pennsylvania State University biologist who co-authored the new study with Mads Forchhammer of the University of Aarhus in Denmark.
Caribou haven't adjusted their arrival time, because they rely not on temperatures but on the steady lengthening of daylight hours to mark the onset of the calving season.
"The problem is twofold: Not only can't you rely on getting the best meal when you're most hungry, you also can't rely on being able to keep up with changes to the cafeteria's opening hours in the future," Post said.
The exact impacts of this developing "trophic mismatch" are still unclear.
"Alone, trophic mismatch might not be the most important factor," Post said, "but it might be the spark that ignites the forest fire when other factors converge."
Fluctuating Populations
Caribou, also known as reindeer, depend on tundra plants. During the Arctic's frigid winter months, they find sustenance mostly from lichen, which they dig from under the snow.
As spring approaches, longer days spur the females to migrate to areas where plants are more abundant so they can give birth.
But temperature climbs of 7 degrees Fahrenheit (4 degrees Celsius) in recent years have caused peak plant availability to occur earlier, impacting caribou birth and survival rates.
Caribou are the first terrestrial mammals to be documented with a trophic mismatch, the study authors say.
Other affected species, such as great tit birds in England, have been able to adjust, and Post and Forchhammer say it's possible caribou could also adapt to shifting plant cycles.
In fact, caribou have survived many climate fluctuations in the past.
Morten Meldgaard, director of the Natural History Museum of Denmark, compiled in the 1980s a comprehensive study of historical population data on Greenland's caribou.
"Over the past 250 years there have been a number of cycles in populations," he said.
By observing climate shifts during that same time period, Meldgaard was able to "pinpoint a relationship between climate change and caribou calving populations."
The data show that athough they didn't always fare well, caribou populations did survive more than two centuries of climate changes.
Juggling Factors
Caribou also survived an ancient period of dramatic warming—the Pleistocene-Holocene transition some 10,000 to 12,000 years ago.
But given today's rapid and unpredictable rate of climate change, Post believes the future of modern caribou is uncertain.
"If this were the only factor caribou had to contend with, they would undoubtedly adjust if the rate of environmental change were constant," he said.
The current rate of change could accelerate, decline, or—worst of all for caribou—fluctuate unpredictably.
"Add on top of this the fact that these caribou are also constrained by other factors like winter conditions, population density, human land use, hunting pressure, parasite loads, and competition with musk oxen," he said.
"Suddenly, there's another ball in the air that the juggler has to keep track of."
The study, to be published this summer in the Philosophical Transactions of the Royal Society of London, was partially funded by the National Geographic Society's Committee for Research and Exploration.
(National Geographic News is part of the National Geographic Society.)
