Astrology for snowshoe hares - population cycle linked to sunspots and solar cycle

Throughout the forests of Canada and the northern United States, populations of snowshoe hares are now exploding. But as recently as the winter of 1994, at research sites where only a few years earlier ecologists had recorded many hundreds of hares per square mile, the animals' numbers had dwindled to a few dozen or even none at all. Ecologists, however, didn't panic. They knew that if they waited, the big-footed creatures would soon cram the forests once more. And last summer, the population started booming again as predicted. "We expect the next peak to be about 1998 or 1999 at the latest," reports biologist Charles Krebs of the University of British Columbia.

How can he be so sure? Snowshoe hares go through this same dramatic cycle every 10 years. They are far from the only animals with such swings: Atlantic salmon, muskrats, caribou and lemmings are just a few of the creatures that experience regular, dramatic rises and falls. But exactly what causes the cycle is one of the great questions of ecology. And now answers are coining out in a series of scientific papers, based on 18 years of experiments involving snowshoe hares, from a team of Canadian researchers.

Finding the keys to population cycles, according to biologist Krebs, may be crucial to the future of conservation biology. "Everybody's worried about small populations of endangered animals," he says, "but every 10 years these hares go to ridiculously low numbers, and you think, `Why the hell doesn't the whole system fall apart and go to extinction?'"

The answers start, not so surprisingly, with the complicated choreography among the hares, the plants they eat and the predators that in turn eat the hares. But there's more, having to do with the rise and fall of dark spots on the face of the sun a good 93 million miles away.

During its boom times, the snowshoe hare is a common sight across northern North America. Its range extends from Alaska as far south as California and east all the way to Newfoundland. The hare is active year round, emerging from its den in late afternoon to dine on plants, munching on species like dandelions and wild strawberries in summer and switching in winter to the bark and shoots of shrubs such as willow and birch.

In late September, the hare's brown fur begins to whiten until, by January, the animal is covered in a snowy coat. The hind feet become particularly thick with white hair, which increases the surface area of the feet and allows them to support the hare's weight on snow. The hare uses its powerful legs and camouflaging coat to avoid being caught by predators, which include coyotes, lynx and owls.

The animal's population cycle has long been common knowledge. People have been trapping snowshoe hares for the fur for centuries, and there are good records of the number of skins collected (and therefore of the animals' cycles) dating back more than 150 years.

During the 1960s, the prevailing scientific opinion was that snowshoes were victims of their own success. The theory went like this: The hares breed so rapidly they denude their habitats of winter food within a few years, and the snowshoes begin to starve, weaken and die. By this point, their predators, whose own numbers have swelled from dining on abundant hares, become so numerous that they further drive down the numbers of hares. When the hare populations bottom out, the trees and shrubs they feed on have time to recover and grow. Without prey, the predators die off, giving the hares some time to bounce back.

A team of researchers led by Krebs has been testing this model in the wilds of the Yukon for the past 18 years. "One of our big projects was to put electric fences around one-square-kilometer blocks of the forest to keep out the predators," explains Krebs. "Another was to feed the hares to alleviate winter starvation. Then we watched to see what happens to the system."

If the old model were right, you'd expect that by supplementing the hares' winter food, you could make the cycles disappear, while getting rid of predators would have relatively little effect. In fact, just the reverse occurred, which led Krebs and his coworkers to turn the old model upside-down. The hares don't starve themselves to death ("We find no sign of starvation," says Krebs); instead, the rising tide of predators starts the downward slide as they consume hare prey. Krebs suspects that food does become a problem for the hares, but in a different way than previously thought. Ideally, hares like to feed in forest clearings, where lots of light helps young plants grow quickly - and offer plenty of food. But faced with an onslaught of hungry predators, the snowshoes may not find open spaces such a good place to be. "The hares reassess where they ought to live," says Krebs.

So they opt to spend more time in dense forest where they can more easily escape an attacker - but where they find less food. "They'd rather be safe and alive and hungry than well fed and dead," says Krebs. As the hares become malnourished their reproduction rates fall, causing their population to shrink even faster. Their decline forces their predators' numbers to crash as well, which allows the hares to go back to the clearings to feed and increase their population, initiating the cycle again.