Wolf response to climate change can impact ecosystem

October 26, 1999

HOUGHTON, MI -- The way wolves respond to significant climate changes can have far-reaching consequences for the ecosystems in which they live, according to an article published in the Oct. 28 issue of Nature magazine.

The article details a study conducted by Eric Post and Nils Christian Stenseth of the Department of Biology, University of Oslo, Norway; Rolf Peterson, School of Forestry and Wood Products, Michigan Tech University; and Brian McLaren, Department of Forest Resources, Wildlife Division, St. Johns, Newfoundland, Canada.

The wildlife data used in the study were collected during a long-term and continuing survey directed by Peterson on wolf-moose relationships on Isle Royale National Park in Lake Superior.

Scientists have known that predators at the top of the food chain exert considerable influence on the structure and function of ecosystems that are regulated by top-down interactions with lower level organisms, and that responses to climatic change at the top have the potential to shape responses at lower levels. On Isle Royale, wolves regulate such a system by limiting the productivity of moose, which limit productivity of fir trees.

" We investigated the ecological consequences of apex predatorsÆ responses to climate change by using data collected over 40 years on wolf behavior and how it affected moose and their impact on fir saplings on Isle Royale," explained Peterson.

" Wolves responded to increasingly snowy winters by hunting in larger packs, and consequently, were able to triple the number of moose killed per day compared to less snowy years when they hunted in smaller packs."

Peterson said this greater killing efficiency by wolves brought a decline in the moose population that resulted in less browsing pressure on balsam fir saplings, which showed a noticeable increase one year after snowy winters. " We suggest that this evidence indicates that cascading behavioral responses of apex predators to climatic change may have a substantial impact on ecosystem function," he said.

This type of impact by top-of-the-food-chain predators on lower trophic levels is well known in marine systems, said Peterson, and has been shown to occur in terrestrial systems in the study of invertebrates.

" Considering that numerical responses of apex predators to environmental variability can have profound effects on primary and secondary production, we hypothesized that their behavioral responses to such variability could bring about cascades of similar ecological scale," he said.

Isle Royale is a protected national park that is the focus of the longest ongoing study of an undisturbed, three-trophic-level ecosystem involving gray wolves, moose, and the giant herbivoreÆs primary winter forage--balsam fir.

Peterson said that the mean size of wolf packs on Isle Royale ranged from approximately 4.5 animals during least snowy winters to 12 animals during the most snowy winters. The increase was due primarily to the fact that grown pups who would normally leave the pack to seek mates of their own remained with their family unit during severe winters. Both wolves and moose found it easier to travel along the shoreline of the island park during heavy snow conditions and this increased encounters between the two species and made more old moose and calves available to wolf predation.

The greater food supply for wolves during snowy winters made them healthier and able to reproduce more successfully, according to Peterson.

Michigan Technological University

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