Wolves, Ticks, Send Michigan Moose Numbers Plummeting

John Roach
National Geographic News
June 6, 2005
The moose population on a remote Lake Superior island is down sharply again this year. But the numbers of the moose's only predators—wolves—are holding steady, according to the latest figures from what may be the longest running study of any predator-prey system in the world.

The study on Lake Superior's Isle Royale in the Isle Royale National Park began in 1958. The nearly five decades of data are providing scientists with an unprecedented look at the oscillating relationship between moose and wolves.

"One of the basic fundamental questions we are always interested in is why in some years there're more moose and other years fewer moose. Same for the wolves," said John Vucetich, an assistant research professor in the School of Forest Resources and Environmental Sciences at Michigan Technological University (MTU) in Houghton.

According to the latest population count, the number of moose on the island has dwindled to 540 this year. In the winter of 2002-2003, the moose population was 1,100. A year ago the population had dropped to 740.

Meanwhile, wolves, the moose's only predator, are stable. Their population jumped nearly 50 percent from 19 to 29 between 2003 and 2004. This year the population grew to 30.

Vucetich and his collaborator Rolf Peterson, a professor at MTU, said several factors are likely involved in the moose decline. The factors include increased wolf predation, severe winter weather, and moose ticks.

The ticks have been particularly severe for the past three years. This year the researchers noted for the first time that the ticks may play an important role in the regulation of moose and wolf numbers.

As many as 70,000 ticks may feast on a single moose during one season. "When they are really abundant, they weaken the moose. This is a good deal for the wolves," Vucetich said. Weakened moose are easy prey.

Now that the moose population is sinking toward 500, the researchers expect the smaller food supply will cause the wolf population to decline. Currently, however, there are no signs that the wolves are running out of moose. For example, wolves from the island's three different packs are not trespassing on each other's territory to hunt.

Study History

Moose migrated from Canada to the 132,000-acre (53,400-hectare) island in the early 1900s, likely swimming the 14 miles (23 kilometers) from the mainland.

Prior to the arrival of wolves, moose populations were regulated by food abundance, climate, and—as the researchers are now learning—periodic tick outbreaks. When wolves arrived on the island from Ontario via an ice bridge sometime between 1948 and 1951, the lives of moose were forever changed.

Since the arrival of wolves, the fates of the wolves and moose have been intimately linked. The wolves' arrival set the stage for a natural, long-term study of a single-prey, single-predator system.

In 1958, about a decade after the first wolves arrived, Durwood Allen with Purdue University in West Lafayette, Indiana, launched the study. Peterson took over as project leader in 1975. Vucetich joined Peterson in 1990 and in 2001 was named a co-director.

In the 47-year history of the study, the researchers have observed the interactions between the wolves and moose and noted how their seesawing populations affect the entire island ecosystem.

"If you watch for a long time, you can see patterns, and patterns are what we see and what we make inferences about," Vucetich said.

Healthy Forest

A correlation between an abundant wolf population and vigorous forest growth is one pattern to emerge in the decades of data, according to Vucetich. Trees are the primary food sources for the moose, so when the moose population is high, tree growth is stunted. Conversely, when the moose numbers are low, the trees grow better.

"One idea that has been popular [in conservation biology] is the notion that predators are an important part of the health of the ecosystem," Vucetich said. "The work we've done here points to that."

Indeed, Vucetich said, this year's tree growth is noticeably greater than in recent years. Of course, he added, one good year for the moose and they can wipe out many years of tree growth.

"You can actually see the ebb and flow of the wolf population by counting tree rings and measuring the ring width," Peterson said in an interview with the radio program Pulse of the Planet.

Ticks and Weather

While the moose and wolf populations help keep each other in check, they both grapple with additional challenges such as disease (a virus nearly wiped out the wolves between 1980 and 1982, Vucetich said), a changing climate, and periodic moose tick infestations.

The researchers have only a fuzzy idea as to what spurred the current tick infestation, but suspect it is linked to a series of mild springs.

The ticks latch onto the moose during the winter. Female ticks drop off in the spring to lay eggs. If they land on dry ground, all goes well. But "if spring comes late and there's snow on the ground, they're dead," Vucetich said. In recent winters, spring has arrived early enough to allow the ticks to thrive.

Interestingly, Vucetich added, heartworm, a disease that affects dogs and wolves, recently showed up for the first time in the mainland city od Thunder Bay, Ontario, Canada, about 14 miles (23 kilometers) from the island.

The parasite, which is transferred from mosquitoes to dogs, is usually restricted to warmer southern latitudes but has been creeping north in recent years. The disease has been found in Minnesota wolves. "For it to come [to Isle Royale] many things would have to happen just so, but it gives us pause," Vucetich said.

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