Each decade, snowshoe hares cycle from scarcity to abundance, in prime habitat expanding from as few as one or two hares per square mile to as many as 4,000, or more, per square mile. Predators like lynx and great horned owls proliferate at the expense of numerous hares. At peak cycle, the hares’ impact on plants can transform the landscape. Willows, and a few woody species, may be gnawed down to ground level or killed from girdling. Intense grazing and browsing stimulates new growth in spring. New shoots produce slightly toxic chemicals to repel browsers. Hare populations crash, not because of disease – “plague” – as early scientist thought, but because of a combination of nutritional stress due to over-browsing and relentless predation.
When willows are severely over-browsed, other plants, like balsam poplar, may colonize and crowd out traditional species. Once past their peak cycle, hares, then their predators rapidly decline. One day, it seems, hares and lynx are common, the next gone again. Eight to eleven years later, the whole process repeats itself. In all of the North, nowhere is the interdependence of plants and animals so easily witnessed than through the hare cycle.