题目

   Coral reefs(珊瑚礁)are not just beautiful to look at, but also very useful. They shelter nearly 25% of ocean life and provide food for billions of people. However, like many other species，this amazing ecosystem(生态系统)is in danger of dying out! There are two main reasons for the destruction—the rising carbon dioxide levels caused by global warming as well as human influences like bottom operation．While not much can be done about the former, the loss caused by the latter can be reduced. Bottom operation involves dropping nets to the ocean bed and then dragging them u如图，ABCD是边长为1的正方形，E、F、G、H分别是AB、BC、CD、DA的中点，则图中阴影部分的面积等于______．