At the end of the Pleistocene Epoch, 11,000 years ago, wild horses roamed Alaska. There were steppe bison and musk oxen and wooly rhinos. Saiga antelopes, their bizarre-looking bulbous noses an adaptation to breathing cold dry air, were common here and in Siberia and throughout the Mammoth Steppes, as this type of habitat was called, as, of course were wooly mammoths, the quintessential Ice-Age creature. At times during the Pleistocene, which began 1.8 million years ago, mastodons lived here too, elephant-like creatures covered with long hair. Mastodons are browsers; they use their long trunks to pluck leaves from trees and woody shrubs. Horses and mammoths, on the other hand, eat mostly grass.In fact, of the 31 species of large herbivores that left fossil remains in the far north, 25 were primarily grazers, dependent on grass.
Musk oxen like these roamed the earth during the Ice Ages. they still persist in small pockets in the far north. Why? What can they tell us about the adaptability to climate change?
Today, interior Alaska is a mosaic of spruce and birch forest, not the open, grassy expanse it was in the Pleistocene. "We're hard-pressed to find many grazers here today," said Eric Post, a Penn State ecologist who studies the interactions of climate change and animal populations in Alaska, Greenland, Norway, and Sweden.
During the Pleistocene, temperatures rose and fell several times, and the ice spread and retreated. Yet by the end of the epoch the average temperatures were about five degrees C higher than they had been. In this warmer environment, shrubs grew into and replaced the grasslands. Trees took hold. Humans extended their hunting grounds farther north. By 10,000 years ago, the horses and mammoths were extinct.
But other Pleistocene creatures survived in the far north until today. Not mastodons, of course, but musk oxen, a mixed feeder that eats both grass and leaves, can still be found in isolated pockets. Caribou and moose likewise, along with the Saiga antelope.
The change predicted by the Terrestrial Ecosystem Model, a computer simulation used to predict the productivity of vegetation, is on the order of the change from the Pleistocene to today, assuming a doubling of the amount of carbon dioxide in the atmosphere. "But the warming now is expected to occur much more quickly," said Post, "on the order of one century, not several thousand years."
That warming will likely change the mix of plants and animals that live in the far north. But how? Because plants use carbon dioxide to photosynthesize, it's often suggested that a rise in carbon dioxide will benefit at least some species of plants. As long as the temperature is not high enough to cause the species heat stress, plants will be healthier, greener, and will push out more leaves. Animals that eat those plants should likewise prosper. But Post found, studying caribou in western Greenland, that there was no simple correlation between herd size and rising temperatures.
"Greenland is really a ring of islands separated by an enormous mass of ice called the Inland Ice," he explained. "In historic times, caribou were found throughout Greenland, but they're gone now from most of the places they used to be." Using records of hunting kills to estimate the health of the remaining populations, Post and his colleagues learned that the herds have increased since the 1960s. "Is warming associated with that increase? Not really," Post said. "If you look at the temperature trends overall, in western Greenland there's been cooling. Western Greenland is an interesting anomaly. We see an increase in caribou because caribou is a Pleistocene species adapted to cold dry temperatures."
The musk oxen in northeastern Greenland likewise show a complicated pattern of response to temperature. "This is one of the few places where natural and sustained populations of musk oxen still exist," Post noted. Six distinct herds are separated by tongues of the ice sheet. They do not interbreed because, unlike caribou, Post explained, "musk oxen are not very mobile animals"; they can't cross the ice. Scientists have made yearly counts of each herd. "The counts show no obvious trend," Post said. "There's a lot of moving up and down. When you look at the data in some detail, you see these fluctuations correlate with freeze-thaw cycles." Because musk oxen have relatively short legs and small feet, they have difficulty finding food in deep snow.
For the red deer in Norway, a creature like the North American elk, Post and his colleagues found that the effects of warming, while still complicated, were more clear. "Two years after a warm winter," Post explained, "the female segment of the red deer population increased—but not the male segment." This increase, the researchers determined, correlated with the time that coltsfoot and wood anenome, the earliest flowers, bloomed in the spring. "When they bloom early, the females that are pregnant have access to a very good source of food just when they need it. Their calves are more likely to survive and, if they're females, to reproduce as soon as they're mature, at two years." Elk on Raspberry Island in southwestern Alaska, Post said, showed a similar response.
quoteWhat can we draw from all these examples? Not much, unfortunately," Post remarked. "There are few consistent patterns. We can't really say if warming is good or bad for large mammals in the far north."
Yet by studying these herds of caribou, musk oxen, red deer, or elk, by trying to tease out what effects climate change might have on them, Post began to see that another important question needed to be asked: What effects might the animals themselves have on climate change?
"Large mammals can have profound influences on the ecosystems in which they exist," Post said. "They may be components in how the larger system reacts to climate change."
By preferring to eat some plants over others, herbivores suppress the growth of their favorite species while allowing "inferior competitors" to stay in the system. Moose on Isle Royale in Lake Michigan, for instance, prune balsam fir down to shrubs each winter, while ignoring nearby spruce. In Alaska, when researchers set up high fences to keep grazing animals out of a plot of grassland, they found more small flowering plants inside the protected space than outside. On grass, on the other hand, the effect of grazing can be positive, as the enclosures also demonstrated. "We call it the lawn-mower effect. The grass is adapted to having its top mown off, and it comes in much more lush."
The animals also influence how nutrients cycle through the soil. "This is the fertilizer effect. Where animals feed, the availability of nitrogen is much higher," Post said. Using what he called "very conservative estimates," Post calculated that five million caribou and reindeer directly contribute 264 million pounds of available nitrogen per year to their tundra habitat.
Experiments on vegetation in the far north have shown that tundra vegetation is most limited by nitrogen availability. Plants there only respond to a rise in carbon dioxide if it is accompanied by a rise in temperature. The temperature rise increases the activity of the soil microbes that turn over nitrogen through the processes of rot and decay. Only with the additional nitrogen can the plants make use of additional carbon dioxide.
The Terrestrial Ecosystem Models currently used to predict the extent of climate change contain an estimate of nitrogen flux, yet so far the animal contribution has not been factored into it. Nor are the effects of browsing and grazing figured into how plants might respond to climate change, and those effects could be significant: The same five million caribou eat an estimated 3.3 billion pounds of green matter each year.
"We think of plants as carbon sponges, draining the carbon dioxide out of the atmosphere as we are putting it into the atmosphere," Post said. "If these large animals have such an effect on which plants grow, and if plants have such an effect on global warming, then we need to put the whole picture together, don't we?
"It's a provocative question: Do large animals have the potential to influence climate change? The answer will vary by the kind of vegetation, by the type of herbivore, and by the extent of nitrogen limitation in the system. Field experiments will help us figure this out. But I want the question to be considered."
Eric Post, Ph.D., is assistant professor of biology in the Eberly College of Science. He has served as a consultant to the Greenland Home Rule Government and the Danish Polar Center. Since coming to Penn State in 2000, he has served on the Global Change Committee and Advisory Committee in the Environmental Consortium.