Wolves, Moose, and Nutrient Flows at Isle Royale National Park

The coastline of Isle Royale National Park is outlined in two maps. Moose carcasses like the one on which wolves are fedding in the lower map, produce pulses of nutrients that affect soil fertility, decomposition and the nutrition of nearby plants. The yellow to white zones on upper map represent the clustered hotspots of biogeochemical activity on Isle Royale. Photo courtesy of Michigan Technological University.

How close did you pay attention to the landscape the last time you went for a hike in a national park? Did you notice the variety of trees or plants, the birds, the insects? Could you tell where there had been a fire or a flood? More subtly, could you see where the nutrient sinks in the ecosystem were? Most of us probably can't answer that last question, but researchers at Isle Royale National Park have uncovered some intriguing contributions to the park's biodiversity.

If you believe in "eco-systems," with an emphasis on "systems," it's not too difficult to grasp this research. Basically, it revolves around grass and other vegetation, moose that eat this vegetation, wolves that kill and eat the moose, and the remains that rot and are absorbed back into Isle Royale's forests and meadows, which in turn become favored grazing sites.

A trio of Michigan Technological University wildlife biologists -- Joseph Bump, Rolf Peterson and John Vucetich -- has a report in this month's issue of the journal Ecology that explains how the carcasses of moose killed by wolves at Isle Royale National Park enrich the soil in “hot spots” of forest fertility around the kills. These kill sites, the authors point out, fuel rapid microbial and fungal growth that boost nutrients for the surrounding plants.

“This study demonstrates an unforeseen link between the hunting behavior of a top predator—the wolf—and biochemical hot spots on the landscape,” Dr. Bump, an assistant professor in Michigan Tech’s School of Forest Resources and Environmental Science and first author of the research paper, said in a release from the school. “It’s important because it illuminates another contribution large predators make to the ecosystem they live in and illustrates what can be protected or lost when predators are preserved.

The biologists assembled their data, and conclusions, by studying 50 year's worth of records tied to more than 3,600 moose carcasses at Isle Royale. According to the university's release, they measured the nitrogen, phosphorus and potassium levels in the soil at paired sites of wolf-killed moose carcasses and controls. They also analyzed the microbes and fungi in the soil and the leaf tissue of large-leaf aster, a common native plant eaten by moose in eastern and central North America.

The researchers found that soils at carcass sites had 100 to 600 percent more inorganic nitrogen, phosphorus and potassium than soil from surrounding control sites, according to the university release. Carcass sites also had an average of 38 percent more bacterial and fungal fatty acids, evidence of increased growth of bacteria and fungi. The nitrogen levels in plants growing on the carcass sites was from 25 to 47 percent higher than the levels at the control sites. Since large herbivores, like moose, are attracted to nitrogen-rich plants, the carcass sites become foraging sites, further supplementing soil nutrients from the urine and feces of the animals eating there.

“I was initially skeptical that it would be possible to detect something as diffuse in the forest floor as nutrients from dead animals,” said Dr. Peterson, who has been studying the wolves and moose of Isle Royale for decades. “It was gratifying to see Joseph succeed in following animal-derived nutrients back into plants to enrich them in protein, ready to be eaten again.”

Even moose killed in winter and mostly consumed produce substantial nutrient hot spots, Dr. Bump found. “At the landscape scale, long-term carcass deposition patterns could influence forest dynamics by shifting competitive relationships among tree seedlings through changes in the nutrient concentrations in their growth environment," he noted.

Now, understandably you'd still have moose dying and decomposing if wolves weren't the killing mechanism. However, according to Dr. Bump, the wolves in effect are greatly influencing where the deaths occur.

“If you look at patterns where wolves are killing moose, or where moose just die ... there are areas where wolves kill moose 12 times the rate of starvation mortality," the wildlife biologist told the Traveler. “So wolves influence where moose carcasses hit the ground, and we found that they cluster. They cluster largely along water bodies, and shorelines, and the isthmus and peninsulas.”

Dr. Bump has observed similar effects on the soil and plant life at elk carcass sites in Yellowstone National Park, another place where wolves are predators and large herbivores are their prey. And he adds that on the Arctic tundra, where soil nutrients are limited, others have found that the impact of a muskox carcass on surrounding vegetation is dramatic even after 10 years.

In Yellowstone, he said, the effects might be more noticeable than in Isle Royale because of areas of open terrain, such as the Lamar Valley and the Hayden Valley.

“You would notice a fertile spot, often where a carcass has died more noticeably. The effects seem to last two to three growing seasons, maybe three to four, in a grasslands system," Dr. Bump said.

Interestingly, while we're quick to notice other impacts on landscapes, such as those rendered by floods, fires, or particularly powerful storms, we often overlook the subtleties of nature.

"Predation and nutrient cycling are two of the most important of all ecological processes, but they seem just about completely unrelated to one another,” observes Professor Vucetich. Also on the faculty of Michigan Tech’s School of Forest Resources and Environmental Science, Professor Vucetich conducts an annual winter study of the wolves and moose of Isle Royale.

This prey-predator influence on ecosystem nutrient loads, notes Dr. Bump, is just one more aspect that policy makers should take into consideration when they are evaluating predator management plans.

Comments

I'm not sure that "ecosystems" are something to believe in or not. If you believe in conservation of matter (P and K and Ca don't appear or disappear, and N has the slight complication of nitrogen fixation and denitrification back to the atmosphere, which is ~80% N2), then this is simple accounting. Elk take up those elements from what they eat across a wider area; their decomposing carcasses (as well as their feces and the feces of wolves that eat them) make higher-concentration clumps of nutrients on the landscape.

Different species of plants that are better competitors at higher nutrient levels grow in these hot spots, and even plant species found elsewhere tend to have higher tissue nutrient concentrations, and thus are more nutritious and tastier (based on elk feeding preference studies).

The same thing is known for bison in prairies: bison urine patches are several square meters (yards) in size and again provide higher soil nutrient levels that last a couple of years; you can visually detect them months to a couple of years later as patches of greener grass, and bison preferentially eat those patches the following year when they migrate back through.

If anyone wants to see the Isle Royale paper or a couple of the bison papers, reply here and I can put pdfs on a server.

I'm generally considered odd because this is exactly the kind of thing I'd search for and think about on a hike!

I'd love to see the papers, tomp. (I'm partially laid up at the moment and can't get to the library anyway.) The idea that every single minuscule event in a system has some effect on everything else in the system is second nature to me, but I like to collect examples to rattle offer to folks to whom I'm demonstrating the concept.

Thank you!