Fun with Science Club®


			Following hares around leads to hairy discoveries. Carrie Straight 05.03.01 For hundreds of years, people have known that snowshoe hare (rabbit; scientific name Lepus americanus) populations decrease about every 10 years. This population cycle occurs in Canada lynx (Lynx canadensis) about every 10 years too (see Fun Fact on the discovery of cycles in lynx [Elton 1942]). Population changes in animals and plants are explained by four basic factors: births, deaths, immigration (new individuals moving into a population) and emigration (individuals moving out of a population). During the process of many studies, scientists recorded causes of births, deaths, immigration, and emigration. They counted the animals that ate the hares (predators) and the stuff that the hares ate, the number of parasites, - fleas, ticks, and mites - found on them (possibly causing death). The researchers hypothesized that the amount of food available to the hares could determine their population levels. This idea predicted that populations with more food would be larger. Then the researchers did an experiment. First they found a population of hares that was decreasing in number and gave them as much food as they could eat. The number of hares in the experimental area increased, but over the whole region the number of hares continued to decline. The researchers also thought that the number of deaths by predation might be changing the hare population. The researchers put radio collars on the young and adult hares. This allowed them to follow around and find the hares whenever they wanted. They then wrote down the causes of death of the hares that they were following. Hawks, owls, lynx, coyotes and even squirrels ate the hares. These animals definitely affected hare populations. To test this further, in two areas the scientists put up an electric fence to keep out all of the coyotes and lynx. In one of those areas, they also added food. In the areas where coyotes and lynx were excluded, the population of hares did not change nearly as much as those outside the electric fence (controls). From these experiments, the scientists concluded that predation was the main cause of deaths in these hare populations, but food may also cause some deaths.


			Krebs, Charles J., Rudy Boonstra, Stan Boutin, and A.R.E. Sinclair. 2001. What drives the 10-year cycle of snowshoe hares? BioScience 51(1): 25-35.