how important are they to us? And tiny populations of tropical plants – the world is covered with vegetation, so what difference does it make, how many kinds there are? 但是，那又怎样呢？无论如何，地球上大多数动物物种都是昆虫和小虫，它们对我们到底有多重要呢？热带植物只占很小一部分----世界上到处都覆盖着植被，至于有多少种又有什么有关系呢？
Many people‘s answer is that every life-form has a right to exist, and that no other reason is needed for preserving it. A more common reason is the beauty of many species. Certain species also provide humans with economic value. But scientists identify two additional reasons which may not be obvious to most of us. 许多人回答是：每个生物都有生存的权利，因而保护它们不需要其它任何理由。而是一个更加普遍的理由是物种多化可使地球更加美丽。况且某些物种对人类还有经济价值。然而科学家却给出了另外两个我们大多数人可能都没有意识到的理由。
One of these reasons is that each life-form occupies a special place within its ecosystem – that is, its community of plant and animal life, in combination with the nonliving components of its environment such as the climate, soil, water, and air. For instance, within a forest the larger trees drop off little twigs and debris, making a layer that holds water in the soil for other plants to use. The roots hold the soil and prevent it from washing away in rainstorms. Whether living or dead, the tree provides shelter for animals and birds and food for insects. As the dead tree rots away, it enriches the soil of the forest floor, enabling other plants to spring up in its place. Such large trees are an example of what we call keystone species; if they disappeared from their ecosystem, the consequences would be felt throughout the community of other species living in the forest. ―The loss of a keystone species is like a drill accidentally striking a powerline,‖ says biologist Edwar
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