When Predators Become Dinner
When the going gets tough in the north, the tough sometimes resort to dining on one another.
Canadian scientists conducting a study on snowshoe hare population cycles have found that in years where there aren't many hares, predators will prey upon one another rather than starve to death.
In the study, which was described in the March issue of Natural History, scientists from three Canadian universities have been radio tracking lynx and coyote for eight years in a 135 square-mile forested valley in southwest Yukon Territory. Their main mission in the study, dubbed the Kluane Boreal Forest Ecosystem Project, has been to further understand the 10-year population cycle of the snowshoe hare and its effects on predators.
The snowshoe hare, also called the varying hare because its coat changes from brown in summer to white in winter, is one of the most abundant plant-eating mammals in the boreal forests of Alaska and Canada. The hare, which was named for its broad hind feet that leave snowshoe-like tracks in the snow, is a favorite meal of many northern predators, especially lynx, coyotes, great horned owls and goshawks. Hares are not always a dependable food source, however. In the bust period of their 10-year population swings, hares are rare.
The Canadian researchers, among them Field Coordinator Mark O'Donoghue, Predator Biologist Elizabeth Hofer and Raptor Biologist Frank Doyle, determined that the hare population in their study area peaked in the winter of 1989-1990, during which they counted 450 hares per square mile. The hare population took a nose dive during the following two winters, and in the spring of 1992 only about 70 hares per square mile remained.
During the same period, the lynx population dove from about 60 animals to 15. When the hares became harder to find, some lynx moved on in search of better hunting grounds or mating opportunities. Trappers in Alaska, British Columbia, and the Northwest Territories returned radio collars to the researchers, indicating that the lynx would travel up to 500 miles to find food when hares became scarce.
The hare population in the study area crashed in the 1991-1992 season, reaching its 10-year low. At the same time, researchers observed unusual behavior in predators. The scientists began the season monitoring nine lynx. By April, all nine were dead, and signals from their collars showed that none had migrated out of the study area.
The first lynx to die, in late November 1992, was an 11-year old male the researchers had collared a decade before. Assuming the animal had starved to death, the researchers were surprised to track down the collar and find that a wolf had killed the lynx. The researchers chalked it up as a quirk because boreal predators aren't known to eat fellow predators, but a pattern began to emerge.
That winter featured brutal north winds and temperatures as low as 50 degrees below zero. During December and January, two more lynx died. Another shock awaited the researchers after they tracked down the corpse of a female lynx. It was found under the snow, partially eaten. The way the body was cached, along with fresh blood and a second set of lynx tracks, led researchers to deduce that another lynx had killed one of its own species.
Another pair of lynx, a male and a female, moved out of the spruce forest and up into treeless ridges of willow scrub, possibly in an attempt to find ptarmigan. Researchers found the remains of the female three weeks later: a chewed collar, scattered remains and fresh wolverine tracks. The male suffered a similar fate two weeks later, when wolf and wolverine tracks surrounded all that was left of the lynx: the collar, blood and some fur.
With their findings, the researchers have once again shown the importance of the snowshoe hare to the balance of the boreal forest ecosystem, with an interesting twist: predators may need to look over their shoulders a bit more in low hare years, when a tough life gets even tougher.
