为了农田、燃料和木材，雨林被砍伐。森林减失危及许多种植物，并破坏许多动物的栖息环境或自然家园。而在海洋，过度捕鱼和污染已使鱼的供应大大减少了。
3、Humans want to survive just as every other species does .we need food ,shelter ,and a place to rear our young ,So how do our activities endanger other species ?Speciefically ,there are three major ways ,We kill off animals directly in some cases ,We may want their meat ,bones .skins ,tusks .horns,or feathers ;or we may wantto protect
our
crops
and
livestock
from
them
,By
overhunting ,Euro-Americans endangered the buffalo in North America ,and in the nin eteenth century hunters drove to extinction the passenger pigeon ,which was probably the most populous species that have ever lived.
人类和其他物种一样都渴望生存。我们需要食物、住所和哺育后代的地方。那么，我们的行为是怎样危及其他物种的呢？具体说来，主要有三种途径。我们有时直接杀死动物，或许是因为希望得到它们的肉、骨头、皮、牙、角或羽毛；或许是为了保护农作物和牲畜。由于过度狩猎，欧裔美国人使北美的野牛濒于灭绝。19世纪灭绝的旅鸽就是由于过度狩猎所致，而旅鸽曾经可能是生活在地球上的数量最多的鸟类。
4、Another way that we endanger native life-forms is by introducing foreign species into their habitat .A prime example of this was the introduction of European rabbits into Australia . where they multiplied until they endangered the native species of grazing animals by eating all the vegetation .This became a terrible problem that has finally been brought under some control ,though not completely solved.
我们危及当地生态的另一种方式是引进外来物种。一个突出的例子是把欧洲的兔子引入澳洲。在澳洲，它们的数量倍增，以至吃掉所有植被，危及当地其他的吃草动物。这就造成了一个可怕的问题。虽然这一问题最后得到了一定的控制，但并没有完全解决。
7、Many people's answer is thar 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.
许多人的回答是：每个物种都有生存的权利，因而保护它们不需要其它任何理由。而一个更加普遍的理由是物种多样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没有意识到的理由。
8、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 ,\drill accidentally striking a powerline \of Harvard University .\the current sixth great extinction .three species of life-forms are dying out every hour ,or74per day .which equals 2700.each year .Some of these ---and we donot even know which ones ---are undoubtedly keystone species.
理由之一是，每个物种在它所处的由所有动植物和无生命的
气候、土壤、水和空气等组成的生态系统中都占有一个特殊的地位。例如，在森林中，较大的树掉下的残枝在土壤中形成一个水分保持层，供其它植物利用。它们的根可以固定土壤，防止土壤被暴雨冲走。无论是活树还是枯树都可以为动物和鸟提供栖息地，为昆虫提供食物。枯树腐烂以后还可以肥沃森林地里的土壤，得以使其它植物从原处拔地而出。这样的大树就是我们所说的主要物种；如果它们从生态系统中消失，其影响会遍及所有生长在森林里的其它物种。哈佛大学的生物学家爱德华威尔逊说:“一个主要物种的消失就像钻头意外地钻断电线，导致四处断电。” 在当前第六次“大灭绝”时期，每小时平均有三个物种灭绝，也就是每天有74个物种灭绝，每年合计达27000个。其中有些物种（虽然我们不确定是哪些）毫无疑问是主要物种。
9、Natural ecosytems are characterized by biodiversity ,which means that a good variety of plant and animal life are present there ,In many parts of today's world .humans have replaced naturally diverse environments with monoculture ,in which only one species lives --one that we humans value , A prime example in forested regions of the world is the monocultural \farms \have been planted after the original forests have been cut down .The character of these tree farms is very different from that of the original forests.
自然界的生态系统以物种多样性为特征，这意味着种类繁多的动植物在其中生存。当今在世界许多地区，人类用单一环境替换本来自然形成的不同环境。在这些单一的；环境中，只有我们人类看重的某一物种得以生存。在世界上的森林地区就有一个突出的例子：在原森林被砍伐之后，人们在原处建造了“林场”，但只栽种了某单一品种的树木。这样的林场和以前的森林是有很大差别的。
10、In the case of forests .another extre,ely important reason for presserving species is illustrated by the pacific yew tree ,which people used to down and never replant because they thought it had no particular value,But recently medical researches discovered that a substance called taxol .produced naturally in the bark of this tree ,is an
effective
medication
for
treating
certain
kinds
of
cancer ,Suddenly harvesters began flocking to the forests of the North American Pacific Northwest in search of this tree,If it had become extinct before its value had been discovered .many cancer patients would have died needlessly.
谈到森林，还要再提一个及其重要的保护物种的理由。太平洋紫杉的例子就很好的说明了这一点。人们过去认为紫杉没有特殊价值，因而砍伐以后就不再重载。但最近医学研究发现，一种从紫杉树皮中自然产生的叫作紫杉醇的物质是治疗某些癌症的