“Habitat destruction, pollution or overfishing either kills off wild creatures and plants or leaves them badly weakened，” said Derek Tittensor, an ocean ecologist at the World Conservation Monitoring Centre in Cambridge. “The trouble is that in coming decades, the additional threat of worsening climate change will become more and more common and could then kill off these survivors.”

　　The problem, according to Nature, is worsened because of the huge gaps in scientists' knowledge about the planet's biodiversity. Evaluations of the total number of species of animals and plants alive vary from 2 million to 50 million. In addition, evaluations of current rates of species' disappearances vary from 500 to 36,000 a year. “That is the real problem we face，” added Tittensor. “The scale of uncertainty is huge.”

　　In the end, however, the data indicate that the world is heading cruelly towards a mass extinction—which is defined as one involving a loss of 75% of species or more. This could arrive in less than a hundred years or could take a thousand, depending on extinction rates.

　　文章大意：这是一篇科普文。世界著名科学杂志《自然》作了一个特别分析，用一些惊人的数据说明世界上一些动植物正面临灭绝，同时指出，造成灭绝的直接原因是人类的破坏活动。

　　4．What's the main idea of the first two paragraphs?

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