That a population crash occurs as a result of natural enemies or disease.

布朗称：“想让小丑瓢虫种群数量骤减，必须有天敌介入或者疾病发生。”

This time, he focused on what he calls the single most risky demographic on the road? Teenage boys? Whose crash rate is twice that of the general population.

这次，他把焦点集中在他所称的——路上最可能出风险的个体：男性多动症青少年，因为他们出事故的概率是普通人群的两倍。

Something similar happened to the region's herring population, which took three years to crash after the Alaska spill and hasn't recovered yet.

类似的情况在青鱼的数量上也体现了出来，阿拉斯加原油泄漏花了三年的时间才溶解，然而该海区尚未恢复。

The future of slender-billed vultures in India may lie with the first of their kind to breed in captivity following a 97% crash in the species population since the 1990s - or it may not.

印度尖嘴秃鹰的未来也许取决于首次尝试的人工繁殖，自从1990年以来，它的数量已经减少了97%。

He was there to witness the crash in population of the common dolphin (Delphinus delphis) from 150 animals in 1996 to almost none in 2007.

他在那里亲眼目睹了海豚群体数量的骤减，海豚的数量从1996年的150头骤减至至2007年的几乎灭绝。