450)=450"> 本文选自雅思听力Section 4，难度较大 HINTS El Nino 全文听写，英式拼法 订阅节目更方便
Most coral reefs are in the tropics because natural conditions there are perfect. In their modern form, coral reefs have thrived on earth for over 50 million years. In recent years, however, more than 11% of the world's reefs have been lost, with another 16% severely damaged during the El Nino event in 1998. Up to 32% of coral reefs may be destroyed by human activities in the next 30 years if we do not take action now. Corals and coral reefs are extremely sensitive. Slight changes in the reef environment may have detrimental effects on the health of entire coral colonies. These changes may be due to a variety of factors. One of the greatest threats to coral reefs is human expansion or development. As human population increases, so does the harvest of resources from the sea. Due to overfishing, reef fish populations have been greatly decreased in some areas of the world. The removal of large numbers of reef fish has caused the coral reef ecosystems to become unbalanced.