Published online 28 October 1999 | Nature | doi:10.1038/news991028-11


Leaders of the pack

What's the chance of a change in air pressure over the North Atlantic causing the balsam fir trees at the west end of an island in the American Great Lakes to grow better? Pretty good, say Eric Post of the University of Oslo, Norway, and colleagues, and they don't have to invoke chaos theory to explain things - just 40 years of careful ecological monitoring. As they explain in the latest issue of Nature1.

Thelink is a change in the hunting behaviour of wolves in very snowy winters. In the protected national park of Isle Royale in Lake Superior, Michigan, wolves are the top predator, preying mainly on moose. In very snowy winters, wolves hunt in larger packs than usual. And the larger packs, coupled with the deep snow, put the moose at a distinct disadvantage. More moose are killed in very snowy winters than normal, especially young calves and old moose, and as a result the wolves thrive.

Balsam fir seedlings at the west end of the island are normally browsed heavily by the moose herd. So, fewer moose mean bigger fir trees the following year.

To make sure they were dealing with a real cause and effect, Post and colleagues checked that the improved tree growth was not simply a direct response to climate by looking elsewhere on the island. They were able to show that similar balsam firs on the east end of the same national park, not habitually browsed by moose, showed no change in growth in response to the climatic variability.

Very snowy winters in the Great Lakes are related to a change in the pattern of air pressure over the North Atlantic known as the North Atlantic Oscillation. This oscillation describes the difference in mean surface atmospheric pressure over Iceland on the one hand and the Azores on the other, between December and March. When the difference is negative, winters are warmer and snowier than usual over northeastern North America; when the difference is positive, they are colder but less snowy.

As Post and colleagues comment, their analysis shows that ecosystems can be shaped by climatic changes in ways that might not be immediately obvious. While ecologists are used to taking likely changes in the numbers of predators and prey into account when estimating possible effects of climate change, this study shows that now they will also have to look for changes in behaviour. 

  • References

    1. Post,E., Peterson, R. O., Stenseth, N. C. & Mclarem, B. E. Ecosystem consequences of wolf behavioural response to climate Nature 401, 905 1999. | Article | ISI | ChemPort |