听力材料：

　　Actually, fires are natural part of ecological cycle and their role is not just destructive but also creative. That is why the “let it burn” policy is fundamentally a good one, even if it sometimes causes fires of the 1988 Yellowstone fire. Let’s look at what happened after 1988 fire.

　　事实上，森林大火是生态系统的组成部分，而且大火的角色不仅仅是破坏性的，同时也是建设性的。这就是为什么“随它燃烧”政策基本上是正确的，尽管它导致了1988年黄石大火。让我们看看1988大火之后发生了什么。

　　First, vegetation. As you might imagine, scorched areas were in time colonized by new plants. As a matter of fact, the plants in Yellowstone became more diverse because the fire created an opportunity for certain plants that could not grow otherwise. For example, areas where the trees have been destroyed by the fire could now be taken over by smaller plants that needed open and shaded space to grow. And another example, seeds of certain plants species won’t germinate unless they’re exposed to very high levels of heat. So, those plants started appearing after the fire as well.