Isle Royale’s two remaining wolves aren’t likely to breed, given their close relatedness and the female’s aggressive display. 

Rolf O. Peterson

Two wolves survive in world’s longest running predator-prey study

For 2 years in a row, a pair of wolves has managed to survive on Isle Royale, Michigan, the last of their kind on the wilderness island. Researchers continue to track the wolves and their moose prey, in the last installments of the world’s longest running predator-prey study.  They report today that although the wolves hunt successfully, they are too few to affect the moose population. Aquatic as well as terrestrial vegetation is taking a hit as moose numbers climb, according to the study’s 59th annual report.

After Canadian wolves colonized the island in 1949, the wolf population peaked at 50 in 1980, and as recently as a decade ago, 30 wolves prowled the island, a U.S. National Park. The island’s now-famous predator-prey study has tracked how wolf and moose numbers have risen and fallen in tandem over the decades, and left their mark on the island’s ecology. 

In contrast to last year’s winter study, when wolf tracks were the only evidence of the predators, wildlife ecologist Rolf Peterson spotted both wolves sitting on lake ice on the January afternoon he arrived on the island.  Weeks later, Peterson and co-investigator John Vucetich, both of Michigan Technological University in Houghton, found the wolves feeding on a freshly-killed moose calf. “We were very lucky,” Peterson says. “There was no mystery left in terms of the wolf population,” or what they were eating.

The researchers also observed that, in what normally would have been the wolves’ breeding period, the 7-year-old female bared her teeth in response to the close interest of the 9-year-old male; he is both her father and half sibling.  Researchers don’t expect the highly inbred pair to reproduce.

The two wolves otherwise appeared healthy and still have all their canines, a key sign of well-being in the carnivores. The pair has already surpassed the average Isle Royale wolf lifespan of 4 years, dodging the main causes of death for their ancestors on the island: other wolves and starvation. “They are swimming in moose,” Vucetich says.

The four wolf-killed carcasses the researchers spotted made little dent in the overall moose population, estimated at 1600 in aerial surveys conducted this past winter.  The 20% increase from last winter is consistent with the population’s growth rate over the past 6 years, as the inbred wolf population dwindled and collapsed.  Both beaver and moose abundance have tripled since 2011, “undoubtedly because of lack of predation,” Vucetich says.

With moose density on the Guam-sized island already five to ten times higher than on the mainland--and with the numbers on track to double in three to four years—browsing on the island’s vegetation is intense. One aquatic plant, floating watershield (Brassenia  schreberi), which was abundant six years ago when moose were at historic lows, now thrives in ponds only where moose are excluded. “It’s the aquatic equivalent of deforestation,” says plant ecologist Eric Hellquist of SUNY Oswego, noting that moose’s effect on aquatic vegetation is not as well studied as that on terrestrial plants. Isle Royale’s ponds are demonstrating “how apex predators can have cascading effects on food webs.” 

As the effects of the missing predators ripple through the island, the Park Service is assessing about 5000 public comments on its proposal to introduce 20 to 30 wolves to the island to establish a new population. The next steps on that plan are expected by the end of this year.