【英语时差8,16】山楂树如何帮助构建生态平衡（在线收听）

 Extinct animals are gone for good, but living species can sometimes tell us about the food chains that existed long ago. Hawthorns are members of the rose family with thorns up to five inches long. Like the thorns of other roses, these long thorns evolved as protection against animals. The theory that thorns keep plants from being eaten makes sense, except that all the animals living today that might eat the hawthorn — such as deer and wild goats — are small enough to reach between the big thorns. A moose might be deterred by the large thorns, but moose and hawthorns don’t live in the same habitats. In fact the hawthorn’s protection isn’t really effective against anything that the hawthorn has to worry about.