Marine Mammals
Since there is such an abundance of food in the sea,it is understandable that some of the efficient, highly adaptable, warm-blooded mammals that evolved on land should have returned to the sea.Those that did have flourished. Within about 50 million years -- no time at all, geologically speaking -- one of the four kinds of mammals that has returned to a marine environment has developed into the largest of all animal forms, the whale. A second kind, the seal,has produced what is probably the greatest population of large carnivorous mammals on Earth. This suggests that these "top dogs" of the ocean are prospering and multiplying. However, such has not been the case,at least not for the last 150 years.Trouble has closed in on these mammals in the form of equally warm-blooded and even more efficient and adaptable predators, humans. At sea, as on land, humans have now positioned themselves on the top of the whole great pyramid of life, and they have caused serious problems for the mammals of the sea. There is a simple reason for this. Marine mammals have the misfortune to be swimming aggregates of commodities that humans want: fur, oil and meat. Even so, they might not be so vulnerable to human depredation if they did not,like humans, reproduce so slowly. Every year humans take more than 50 million tons of fish from the oceans without critically depleting the population of any species. But the slow-breeding mammals of the sea have been all but wiped out by humans seeking to satisfy their wants and whims.
既然海洋中有如此豐富的食物，一些原本在陸地生存的高效率的，適應(yīng)性強的暖血型動物返回海洋生存是不難理解的。 那些已經(jīng)返回海洋的哺乳動物種族繁榮。 在大約 5 千萬年間(這在地質(zhì)學(xué)上不過是彈指一揮間)，返回海洋的4 種浦乳動物之一的鯨已成為體積最龐大的動物。 而另一種哺乳動物海豹，它的數(shù)量恐怕是地球上大型食肉哺乳動物中最大的。 這似乎表明這些海洋中的"大哥大"們?nèi)硕∨d旺，一派繁榮景象。但至少在近 150 年以來，事實并非如此。 同樣是暖血動物而且效率更高、適應(yīng)性更強的捕食者，人類，成為這些哺乳動物的日益迫近的威脅。 在陸地和海洋，人類都已處在巨大的生物金字塔的最頂端，成為所有生物的主宰，并給海洋哺乳動物帶來巨大的危脅。 原因很簡單。 這些哺乳動物不幸成為人類所需要的一堆游動著的商品，如皮毛、油和肉。 盡管如此，如果它們不象人類一樣繁殖如此緩慢，它們對人類的掠奪也不會如此無能為力。 每年人類從海洋中捕撈約 5 千萬噸的魚類，但這并未導(dǎo)致任何魚種的滅絕。但繁殖緩慢的海洋哺乳動物卻因為有了一心只想滿足一已私欲的人類而瀕臨。