Wolves, Moose and Soil Nutrients: The Unexpected Connection

Isle Royale National Park is unusual among land ecosystems: wolves are the only predator of moose, and moose are nearly the only prey for wolves. Having a single-predator-single-prey structure makes things pretty simple for ecologists studying how species interact. But they still find the unexpected.

Researchers were startled recently to discover "hot spots" of forest fertility on this 50-mile long, eight-mile wide island in the northwest part of Lake Superior—areas rich with nutrients somehow derived from the carcasses of moose—even after the bodies had been picked clean by wolves and scavengers.

 "It was surprising that these moose carcasses led to increased soil nutrients that actually lasted for two or three years," said Joseph K. Bump, assistant professor in Michigan Technological University's School of Forest Resources and Environmental Science and lead author of the study. "If you've ever seen a kill site, not much is left—we predicted nothing was going on in the soil and the plants. And we were wrong."

These unpredictable relationships between wolves, moose and the dynamics of the ecosystem may help policymakers and environmentalists involved in predator management and conservation.

"This is a new link between wolves and ecosystem functions," Bump said. "It's inspiring because it's not intuitive. Wolves and moose and hot spots and dirt are not seemingly inter-related.  Who would have thought wolves were related  to these hot spots in the dirt? Or influenced fertile spots in the earth? Being proved wrong is the best thing in science—I love being wrong."

He and his colleagues studied a 50-year record of more than 3,600 wolf-killed moose carcasses on the island. They examined levels of nitrogen, phosphorus and potassium in the soil, and compared them to spots where there had been no moose bodies. They also analyzed microbes and fungi in the soil and leaf tissue of large-leaf aster, a common native plant that is a frequent part of the moose diet in eastern and central North America.

They discovered that soil at carcass locations had 100 to 600 percent more inorganic nitrogen, phosphorus and potassium than the  control areas used for comparison purposes. Carcass sites also had an average of 38 percent more bacterial and fungal acids, which indicates the growth of bacteria and fungi.

Furthermore, nitrogen levels in plants growing on the carcass sites was 25 to 47 percent higher than those at the control sites. "It definitely showed that plants growing on carcass sites have elevated leaf nitrogen, which is kind of like a leaf protein," Bump said. "It's important for leaf growth—and for anything that eats leaves."

Since large herbivores, such as moose, like to eat nitrogen-rich plants, the carcass sites become foraging areas, further enhancing soil nutrients from the urine and feces of other animals eating there.  "A future research question is to see whether herbivores are preferentially feeding at those kill sites," Bump said. The distribution of moose carcasses over the long term could have an impact on forest dynamics, shifting the "competitive relationship" among plant species. "When plants are competing in the early stages of life, some do better if the nutrients are high, others do better if the nutrients are low," Bump said. "When you change the environment of nutrients, you may favor one species over another."

Rolf O. Peterson, another member of the research team, called it "gratifying" to be able to follow animal-derived nutrients back into plants to enrich them in protein, ready to be eaten again."

Bump has found similar results on the soil and plant life at elk carcass sites in Yellowstone National Park, another area with wolf predators and large herbivores as prey. Also, on the Arctic tundra, where soil nutrients are sparse, other researchers have found a similar impact from muskox carcasses on surrounding vegetation. In that instance, the effects have persisted for at least a decade.

"Predation and nutrient cycling are two of the most important of all ecological process, but they seem just about completely unrelated to each other," said John Vucetich, the third member of the research team. Yet, he added, they are "connected in a most interesting way."

Moose came to Isle Royale in about 1900, probably by swimming. Wolves arrived about 1950, likely walking along an ice bridge from Canada. In a typical year, the island has about two dozen wolves and as many as 1,000 moose. Scientists have been studying the interactions between wolves and moose on Isle Royale since 1958, believed to be the longest-running study of any predator-prey system in the world, according to the researchers.

"On a very fundamental level, this is an understanding you get when you have a healthy wolf population and a healthy moose population," Bump said. "We  understand what we get when those populations are healthy, and we understand what we lose when they aren't healthy."

The study, published in the November issue of Ecology, "illuminates another contribution large predators make to the ecosystem they live in, and illustrates what can be protected or lost when predators are preserved or exterminated," Bump added.

The work was supported by grants from the National Science Foundation and the Environmental Protection Agency.

—By Marlene Cimons, NSF

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