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题目材料：
It makes sense for large grazing animals to leave the Serengeti Plain in the dry season. The grass there stops growing soon after the rains cease. The wildebeests and zebras then head northwest, to the region of higher rainfall where there is still plenty of grass amid the acacia woodlands. But why do they ever leave those woodlands, especially after the rains have started and the amount of green forage is increasing? Recent studies have shown that grass on the Serengeti Plain contains higher concentrations of protein. This difference could be important to a lactating wildebeest or zebra. Even more striking, however, are the differences in the amounts of calcium and phosphorus available to grazers in the two habitats. Concentrations of calcium in grass samples from the Serengeti Plain are, on average, 40 percent greater than in samples from the northwestern woodlands; phosphorus concentrations are double. The calcium difference may not be very significant, as even the levels found in the woodlands should be adequate. But the concentration of phosphorus in the woodland forage appears to be below the level a lactating female wildebeest requires. In domestic livestock, phosphorus deficiencies have been linked to reduced fertility and milk yield and to other problems; similar problems presumably would affect wild grazers too.
以上解析由 考满分老师提供。
