Chain of command: wolves help trees thrive.

These days, ranchers, environmentalists, and policy makers are fiercely debating the pros and cons of reintroducing wolves into U.S. wilderness areas. But if trees had their say, they would vote overwhelmingly in favor of the furry carnivores. At least, that's what new research indicates.

Trees lose fewer leaves if wolves are around to keep large grazing animals in check, two wildlife ecologists report in the Dec. 2 SCIENCE.

In their natural environment, plants and animals align into food chains. Plants fueled by sunlight become fodder for herbivores -- vegetarians of the wild. Then meat-eating predators hunt the herbivores.

While biologists often have no trouble identifying these "links" in the chain, they cannot agree about what causes the number of plants or animals to vary over time.

To make matters worse, few places have intact food chains that include large carnivores, says Stanley I. Dodson of the University of Wisconsin-Madison.

Some researchers assert that the plants at the base of the chain ultimately determine the populations of other organisms. Climate and other physical conditions set the growth rate for plants, thus indirectly influencing the entire food chain. "That's the classical argument, that everything is resource-dependent," explains Rolf O. Peterson, a wildlife ecologist at Michigan Technological University in Houghton. Fewer plants results in fewer herbivores and, consequently, fewer predators.

But a food chain in Isle Royale National Park, a 544-square-kilometer island in Lake Superior, doesn't work that way.

There, wolves prey on moose, which munch extensively on balsam fir foliage. For the past 35 years, biologists have tracked the island's moose and wolves to study the influence of predator and prey populations on each other.

Then Brian E. McLaren, a member of Peterson's team, began adding plants to the equation by studying the tree rings of balsam firs. He discovered that these rings narrowed periodically, an indication of suppressed growth.

When he and Peterson charted the fluctuations in tree-ring width along with the populations of both wolves and moose over time, they discovered that rings narrow only after wolf populations decline and moose numbers increase. The researchers found a lag of a year or two between each link in the chain.

In the early 1980s, the number of wolves declined sharply, most likely because of disease. Fewer aging and newborn moose became meals for these carnivores. As a result, the tree rings indicate, these extra animals grazed very heavily on balsam firs between 1988 and 1991, the two ecologists report.

McLaren and Peterson have therefore ruled out the idea that food availability regulates the abundance of moose and have instead embraced the concept of top-down regulation of food-chain populations.

"There are people who are not going to believe [this result] and other people who will say it proves [top-down regulation]," says Dodson. "I think its significance is in between." He emphasizes that while such a conclusion requires more data, "the important thing is this is all [the data] we have."

"[The study] illustrates the manifold repercussions of a few top-level carnivores in an ecosystem," Peterson emphasizes. "The world is entirely different when you have a top carnivore."