Wolves, Ticks, Send Michigan Moose Numbers Plummeting

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.

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