GRE Reading Comprehension: PPO-GRE阅读PPO - 64KN65HYLM_1K7XCS

The cycle of fluctuation in snowshoe hare populations is unusual among animal species in that it is remarkably regular – peaking every eight to eleven years – and broadly synchronized over a vast area. Declines from peak levels are initiated by markedly lower overwinter survival of young hares, sharp decreases in birth rates, and a declining survival rate for adult hares. The onset of population increases is brought about by greatly improved rates of survival and birth. Some biologists hypothesize that the cycle begins when peak snowshoe hare populations exceed their winter food supply; resulting malnutrition triggers a population decline. As hare numbers fall, the ratio of predators to hares increases, as does the impact of predation on the hare population. This extends the decline beyond the period of winter food shortage. Hare scarcity then causes predator population declines, and with fewer predators and more abundant winter food, the hare population begins another cyclic increase. The high mobility of predators responding to local differences in hare abundance contributes to interregional synchrony.