新GRE阅读机经训练（三）(2)

Steller’s sea cow, a giant sirenian discovered in 1741 and extinct by 1768, is one of the few megafaunal mammal species to have died out during the historical period. The species is traditionally considered to have been exterminated by ‘blitzkrieg’-style direct overharvesting for food, but it has also been proposed that its extinction resulted from a sea urchin population explosion(海胆增多) triggered by extirpation of local sea otter populations(海獭灭绝) that eliminated the shallow water kelps(浅水褐藻灭绝) on which sea cows fed. Hunting records from eighteenth century Russian expeditions to the Commander Islands, in conjunction with life-history data extrapolated from dugongs(儒艮), permit modelling of sea cow extinction dynamics. (用记录model海牛灭绝的过程)Sea cows were massively and wastefully overexploited, being hunted at over seven times the sustainable limit, and suggesting that the initial Bering Island sea cow population must have been higher than suggested by previous researchers to allow the species to survive even until 1768.(作者认为是捕猎导致灭绝的) Environmental changes caused by sea otter declines are unlikely to have contributed to this extinction event.(作者不同意食物链说) This indicates that megafaunal extinctions can be effected by small bands of hunters using pre-industrial technologies, and highlights the catastrophic impact of wastefulness when overexploiting resources mistakenly perceived as ‘infinite’(作者认为：灭绝可以由一小群捕猎者导致，浪费(即过度捕杀原本以为是无尽的资源)会带来灾难性的后果).

　　human hunted sea cows => the author's point of view

　　extirpation of sea otter海獭灭绝 => explosion of sea urchin海胆增多 => eat all the kelps 褐藻灭绝=> no kelp for sea cow to eat海牛饿死 {the alternative explanation that the auhtor disagreed}

　　文章很短，大家看intro就好，后面data神马的不用管..

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