Climate Change Linked to Population Shifts Among Mammals

13 November 2002 — Scientists have shown, for the first time, that changes in a large-scale climate system can synchronize population fluctuations in multiple mammal species across a continent-scale region. The study, to be published in the 14 November 2002 issue of the journal Nature, compares long-term data on the climate system known as the North Atlantic Oscillation with long-term data from Greenland on the population dynamics of caribou and muskoxen, which are large mammals adapted to breeding in the Arctic.

“The Arctic can provide useful early-warning signals for the rest of the world because the species that live in this sensitive region are expected to be among the first to show the effects of the Earth’s changing climate,” says Eric Post, assistant professor of biology at Penn State University, who is a coauthor of the study along with Mads C. Forchhammer, associate professor of ecology at the University of Copenhagen in Denmark. “Here we have a very simple system with a very clear signal: two species on opposite sides of a continent that never mix, never compete for food, and have no common predators, yet their population dynamics are synchronized. The only thing they have in common is the large-scale climate system that influences weather throughout the northern hemisphere,” Post comments.

Post and Forchhammer studied seven herds of caribou and six herds of muskoxen in Greenland, where the two species live on opposite costs and are separated by an impassable continent-wide ice sheet spanning about 600 miles (1,000 kilometers) at its minimum width. “We chose to study these two species in Greenland because their complete physical and ecological separation rules out the alternative explanations that have confounded previous studies of the role of climate in synchronizing population dynamics, leaving only weather as the controlling factor,” Post explains.

Previous research had focused either on a single species in a habitat that allowed individuals to move from one population to another, or on different species whose lives are somehow intertwined, such as two species that share a common predator. The researchers say their study is the first using both local and global weather data to show cross-species synchrony in species that have no direct contact with each other and that share nothing in common except for the effect on their local weather of a large-scale climate system.

The large-scale system that affects winter weather in Greenland and much of the northern hemisphere is the North Atlantic Oscillation (NAO). “The North Atlantic Oscillation can be pictured as a fluctuating pressure corridor that squeezes and channels the westerly winds between North America and northern Europe, influencing the direction and speed of the winds and affecting temperature and precipitation on both sides of the North Atlantic Ocean,” Post explains. “It also bears some relation to the much larger Arctic Oscillation, which is centered over the North Pole and which seems to exert its greatest influence on spring temperatures in the northern hemisphere,” he adds.

In order to gauge how strongly the North Atlantic Oscillation affects local weather conditions on the coasts of Greenland where the muskoxen and caribou live, the researchers compared records of each herd’s local weather history with the NAO index–a measure of the condition of the North Atlantic Oscillation that has been in use from 1864 to the present. They then sequentially compared each of the caribou herds with each of the muskox herds, looking at the degree of their geographical separation, the timing of their population fluctuations, the weather conditions affecting each herd, and the degree of the North Atlantic Oscillation’s influence on the herd’s population dynamics. “We found that whenever the NAO had an approximately equal effect on the population dynamics of two herds, these fluctuations were more synchronized, even though the herds were on opposite sides of the subcontinent of Greenland,” Post says. Similarly, the researchers found that whenever the North Atlantic Oscillation exerted opposite effects on herds of the two species, their population dynamics were out of phase with each other–when one was thriving the other was declining.

“The physical isolation of the caribou and muskox populations by the continent-wide ice sheet in Greenland, along with their lack of shared competition for food and their lack of shared predators, greatly simplifies the analysis of the role of climate in synchronizing their population dynamics,” Post says. “It is difficult to explain these results by means other than the influence of the North Atlantic Oscillation, which simultaneously affects both of them,” he says.

Because the severity of the climate also is an important factor that can affect herd size, Post and his colleagues have conducted similar studies in areas with climates milder than that in Greenland. Their results have shown influences of the North Atlantic Oscillation on the dynamics of many species of large mammals, birds, and plants throughout the North Atlantic region, including areas with milder weather than in Greenland. “Changes in large-scale climate systems can affect the population dynamics of many species, even where the climate is not as severe as it is in Greenland,” Forchhammer says.

As a result of this research, other scientists may begin to look at the effect of large-scale climate systems on the animals they have been studying. “What does this tell us about the potential ecological consequences of future climate change?” Post asks. “At the very least it should make us wonder whether climate trends might bring into synchrony the ups and downs of populations of species that currently are fluctuating independently,” he says.

This research was sponsored by the U. S. National Science Foundation, the Penn State University Environmental Consortium, and the Danish National Science Research Council. A study by Forchhammer, Post, and colleagues that provides more information about the role of the North Atlantic Oscillation on the muskox and caribou herds in Greenland recently was published in the September 2002 issue of the journal Population Ecology.