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.

　　首先，对于植物来说。你们可以想到，那些烧焦的地方随着时间的发展都会被新的植物覆盖。而且事实上，因为大火给一些本来难以生长的种类得以发芽，黄石的植物因此变得更具多样性。比如，原来长满树木的地方因为火灾遭到破坏，但现在那里长满了需要开阔而阴凉空间的小型植物。另一个例子是，一些植物的种子只有经受过非常高的温度之后才能够发芽。所以，这些植物会在大火之后开始生长。

　　It’s a similar story with the animals. Not only did their populations recover, but the fire also created new opportunities. For instance, the small plants that replaced trees after the fire created an ideal habitat for certain small animals like rabbits and hares. And when rabbits and hares started thriving, so did some predators that depended on them for food. So, certain food chains actually became stronger after the fire than they were before.

　　动物的情况也类似。不但动物的数量会逐步回复，而且大火会给动物的生长带来新的机会。比如，那些在火灾之后取代原有树木的小型植物给一些小型动物提供了极佳的栖息地，比如兔子。当兔子的数量开始变多，那么以它们为食的捕食者的数量也会增加。所以，大火过后，相应的食物链会变得比之前更加稳定。

　　And last, fires like 1988 Yellowstone fire would be a problem for tourism if they happened every year. But they don’t. It was a very unusual combination of factors that year, low rainfall, unusually strong winds, accumulation of dry undergrowth that caused fire to be so massive. This combination has not occurred since and Yellowstone has not seen such a fire since 1988. Visitors came back to the park next year and each year after that.

　　最后，只要1988年的黄石大火不是每年都发生，那么就不会对旅游业造成巨大问题。而事实上，也确实不会每年发生。大火的发生是在那一年诸多因素和偶然的相互作用。比如，降水少、罕见的大风，以及干燥的低层植被的聚集导致了那次大火规模空前。但是这些因素没有再次聚合，而黄石公园自1988年开始也没有出现过这样规模的火灾。游客在大火第二年的时候就会恢复，而且这种趋势会保持下去。