单选题 .  ①The 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. ②Declines 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. ③The onset of population increases is brought about by greatly improved rates of survival and birth.
    ①Some biologists hypothesize that the cycle begins when peak snowshoe hare populations exceed their winter food supply; resulting malnutrition triggers a population decline. ②As hare numbers fall, the ratio of predators to hares increases, as does the impact of predation on the hare population. ③This extends the decline beyond the period of winter food shortage. ④Hare scarcity then causes predator population declines, and with fewer predators and more abundant winter food, the hare population begins another cyclic increase. ⑤The high mobility of predators responding to local differences in hare abundance contributes to interregional synchrony.33.  The passage suggests that population fluctuations in many other animal species differ from those of the snowshoe hare in that population fluctuations in other species ______