在这一系列周期性物种灭绝事件里，白垩纪晚期恐龙和菊石的灭绝仅仅是其中一个比较严重的事件。
The late Cretaceous extinction of the dinosaurs and ammonoids was just one of the more drastic in a whole series of such recurrent extinction episodes.
同样，加利福尼亚沿岸的红杉林也依靠周期性的火灾来减少物种间的竞争，并为植物的新生清理地面和树荫。
Likewise, the giant sequoia redwoods of coastal California rely on periodic fires to thin out the competition and clear the ground and canopy ready for new growth.
西南玻利维亚地区，位于3700米高度（超1200英尺），其物种都要能抵抗周期性发生的冰冻和干旱。
The varieties of this region of southwestern Bolivia -- at 3, 700 meters (over 12, 000 feet) -- are resistant to the freezes and droughts that periodically afflict it.
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