GRE阅读高频机经原文：Snowshoe hare population

　　29. Snowshoe hare population

　　The snowshoe hare is one of the northern mammals that go through periodic cycles of changes in their populations. Over a period of years, their population will increase in numbers and then suddenly crash to very low numbers, only to rise again. Snowshoe hare tend to cycle about once every ten years. The snowshoe hare cycle is one of the best known and fur trappers and traders working in northern Canada first described it. In 1865, traders with the Hudsons Bay Company were flooded by hare pelts brought in by trappers. But, by 1870, only a few hare pelts trickled in. This pattern of high and low abundance of hare pelts repeated itself over and over, roughly every ten years.

　　Snowshoe hares are a dominant herbivore in northern forest ecosystems. In Alaska and western Canada, their populations cycle over 8 to 11 years. The difference in densities and abundance during the cycle are extreme. Densities as high as 4,000 hares per square kilometer have been estimated at the peak of the cycle. These populations can crash to densities of less than one hare per square kilometer within a period of one year. The populations slowly increase to peak densities over the next 6 to 13 years. The densities during peak years, however, are not always the same.

　　Just what is it that drives the snowshoe hare cycle? This question has puzzled people living in northern areas and scientists for many years. On a broad scale, it may be the 22- year sunspot cycle and its effects on boreal forest weather patterns or forest fires. On a smaller scale, the cycles may be affected by over browsing the food supply, predators, shock disease due to stress, parasites, or a combination of these. The primary theory pertaining to the snowshoe hare population cycle describes the mechanics as a combination of predators and browse availability. During the peak of their cycle, hares eat a large proportion of the shrubs available in their area. They can also kill many of the shrubs in their area by chewing through the outer layer of bark. Over-browsing of shrubs results in a food-shortage that causes malnourishment, starvation, and reduced reproduction. This leads to the start of the population crash or decline. Also, as food supplies decrease, the health of hares declines and makes many individuals more vulnerable to predators and disease. In the western Yukon, scientists believe that the hare cycle is caused by changes in food supply and predation. The declines and increases in the population of hares were influenced by the reproductive rates of adult hares and the survival rates of all ages of hares.