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ISSUE121 物种灭绝和人类的关系
ISSUE121
At various times in the geological past, many species have become extinct as a result of natural, rather than human, processes. Thus, there is no justification for society to make extraordinary efforts, especiaally at a great cost in money and jobs, to save endangered species.
在过去不同的地质时期,许多的物种因为自然作用而非人类行为而灭绝。因此,人类社会为了挽救濒危物种而付出的巨大努力,尤其是以大量的资金和工作机会为代价, 这样做是毫无道理的。
Causes of Endangerment
When discussing the causes of endangerment, it is important to understand that individual species are not the only factors involved in this dilemma. Endangerment is a broad issue, one that involves the habitats and environments where species live and interact with one another. Although some measures are being taken to help specific cases of endangerment, the universal problem cannot be solved until humans protect the natural environments where endangered species dwell.
There are many reasons why a particular species may become endangered. Although these factors can be analyzed and grouped, there are many causes that appear repeatedly. Below are several factors leading to endangerment:
Habitat Destruction
Our planet is continually changing, causing habitats to be altered and modified. Natural changes tend to occur at a gradual pace, usually causing only a slight impact on individual species. However, when changes occur at a fast pace, there is little or no time for individual species to react and adjust to new circumstances. This can create disastrous results, and for this reason, rapid habitat loss is the primary cause of species endangerment. The strongest forces in rapid habitat loss are human beings. Nearly every region of the earth has been affected by human activity, particularly during this past century. The loss of microbes in soils that formerly supported tropical forests, the extinction of fish and various aquatic species in polluted habitats, and changes in global climate brought about by the release of greenhouse gases are all results of human activity.
It can be difficult for an individual to recognize the effects that humans have had on specific species. It is hard to identify or predict human effects on individual species and habitats, especially during a human lifetime. But it is quite apparent that human activity has greatly contributed to species endangerment. For example, although tropical forests may look as though they are lush, they are actually highly susceptible to destruction. This is because the soils in which they grow are lacking in nutrients. It may take Centuries to re-grow a forest that was cut down by humans or destroyed by fire, and many of the world's severely threatened animals and plants live in these forests. If the current rate of forest loss continues, huge quantities of plant and animal species will disappear.
Introduction of Exotic Species
Native species are those plants and animals that are part of a specific geographic area, and have ordinarily been a part of that particular biological landscape for a lengthy period of time. They are well adapted to their local environment and are accustomed to the presence of other native species within the same general habitat. Exotic species, however, are interlopers. These species are introduced into new environments by way of human activities, either intentionally or accidentally. These interlopers are viewed by the native species as foreign elements. They may cause no obvious problems and may eventual be considered as natural as any native species in the habitat. However, exotic species may also seriously disrupt delicate ecological balances and may produce a plethora of unintended yet harmful consequences.
The worst of these unintended yet harmful consequences arise when introduced exotic species put native species in jeopardy by preying on them. This can alter the natural habitat and can cause a greater competition for food. Species have been biologically introduced to environments all over the world, and the most destructive effects have occurred on islands. Introduced insects, rats, pigs, cats, and other foreign species have actually caused the endangerment and extinction of hundreds of species during the past five centuries. Exotic species are certainly a factor leading to endangerment.
Overexploitation
A species that faces overexploitation is one that may become severely endangered or even extinct due to the rate in which the species is being used. Unrestricted whaling during the 20th century is an example of overexploitation, and the whaling industry brought many species of whales to extremely low population sizes. When several whale species were nearly extinct, a number of nations (including the United States) agreed to abide by an international moratorium on whaling. Due to this moratorium, some whale species, such as the grey whale, have made remarkable comebacks, while others remain threatened or endangered.
Due to the trade in animal parts, many species continue to suffer high rates of exploitation. Even today, there are demands for items such as rhino horns and tiger bones in several areas of Asia. It is here that there exists a strong market for traditional medicines made from these animal parts.
More Factors
Disease, pollution, and limited distribution are more factors that threaten various plant and animal species. If a species does not have the natural genetic protection against particular pathogens, an introduced disease can have severe effects on that specie. For example, rabies and canine distemper viruses are presently destroying carnivore populations in East Africa. Domestic animals often transmit the diseases that affect wild populations, demonstrating again how human activities lie at the root of most causes of endangerment. Pollution has seriously affected multiple terrestrial and aquatic species, and limited distributions are frequently a consequence of other threats; populations confined to few small areas due to of habitat loss, for example, may be disastrously affected by random factors. Charles Darwin showed that extinction of a species is part of the evolutionary process. There is no reason at all to be concerned about the disappearance of a species. In fact, a steady rate of extinction is a normal process in the course of evolution, and is called the background rate of extinction. Species have slowly evolved and disappeared throughout geologic time because of climatic changes and the inability to adapt to survive competition and predation. But people who make this argument have overlooked one very important fact: since the 1600s, humans have rapidly accelerated the rate of extinction because of population growth and resource consumption. Today, most of the world's habitats are changing faster than most species can evolve, or adapt to such changes. The current global extinction rate is estimated at about 20,000 species per year, exponentially greater than the background extinction rate. Many biologists believe that we are in the middle of the greatest mass extinction episode since the disappearance of the dinosaurs 65 million years ago. From what is known about present-day populations and from evolutionary theory, the change in either the physical or the biological environment is the key to extinction. But the vulnerability of a species depends on a wide variety of factors, such as its total population size, geographical distribution, reproductive ability, ecological relations with other species, and genetic characteristics. For example, more emphasis is put on the greater vulnerability of species that reproduce slowly as contrasted with those that reproduce rapidly. Other factors, such as food-plant specialization, may make many fast reproducers more vulnerable than species that reproduce more slowly. No matter how fast an insect species that depends on a certain plan can reproduce, it will still go extinct if that plant's habitat is destroyed. It has been estimated that about one half billion species have lived at one time or another, and today's existing species are only 2% of those that have ever evolved. The other 98% have either died out or evolved into something sufficiently different to be called a new species.
Elephant
Elephant, huge mammal characterized by a long muscular snout and two long, curved tusks. Highly intelligent and strong, elephants are the largest land animals and are among the longest-lived, with life spans of 60 years or more. Healthy, full-grown elephants have no natural enemies other than humans.
Throughout history, people have prized elephants for their great size and strength. On the battlefield, soldiers astride elephants have trampled and terrified enemies. Elephants also have been trained to carry heavy supplies through jungles and to haul huge logs from the forests where they once lived.
Elephants have long been revered and honored, and in Thailand, India, and other Southeast Asian countries, beautifully decorated elephants still play a significant role in traditional religious ceremonies. According to Buddhist tradition, the Buddha chose the form of a white elephant as one of his many earthly incarnations, and the rare appearance of a white elephant is still heralded as a manifestation of the gods.
Over the past 40 million years, more than 600 species of elephants have roamed the earth. Today only two species are alive—the African elephant and the Asian elephant. Climate fluctuations over the millennia and resulting vegetation changes caused the extinction of many elephant species, but human impact has also taken its toll. At the turn of the 20th century, elephants numbered from 5 million to 10 million, but widespread hunting and habitat destruction reduced their numbers to an estimated 640,000 by the end of the century. Present-day efforts to save elephants may be inadequate, and biologists are unsure if elephants as a species will survive.
The Elephantoidea group, which evolved 8 million to 10 million years ago, includes the mammoth and Stegolophodon. The mammoth also lived during the Ice Age and was covered with a thick, woolly coat. Unlike the mastodon’s forward-curving tusks, the mammoth’s tusks curved backward. The mammoth displayed a prominent hump on its back. Mammoths roamed North America, Africa, Europe, and Asia, were hunted by early humans, and died out about 8000 years ago. Stegolophodon evolved about the same time as the mammoth and inhabited Europe, Asia, and Africa. Its tusks and other features were intermediate between the mammoth and modern-day elephants. Stegolophodon’s descendants are the African and Asian elephants of today.
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地球:每小时一个物种灭绝
| | 新华社北京电 (记者 王立彬)钟表的时针每走一个数字,就有一个历经千百万年进化的生物从地球上永远消失。
国家濒危物种进出口管理办公室提供的信息显示了这一惊人的生命灭绝过程。
据科学家估计,由于人类活动的强烈干扰,近代物种的丧失速度比自然灭绝速度快1000倍,比形成速度快100万倍,物种的丧失速度由大致每天一个种加快到每小时一个种。目前,世界上已有593种鸟、400多种兽、209种两栖爬行动物以及20000多种高等植物濒于灭绝。这些濒于灭绝的物种大多数与人类的关系十分密切,对人类的生存与发展具有十分重要的意义。造成物种灭绝的原因,除不可抗拒的自然历史及自然灾害因素外,人为活动是其主要原因。特别是由于商业贸易而导致人类对野生动植物资源的掠夺式利用,是造成物种濒危乃至灭绝的重要因素。
野生动植物是宝贵的自然资源,是生态环境的重要组成部分,也是人类生存与发展不可或缺的物质基础。可以说人类社会的发展史,就是人类利用野生动植物资源不断发展的历史。然而,人为活动加剧特别是自工业化革命以来的近200年里,伴随着人口数量膨胀和经济的快速发展,野生动植物的种类和数量以惊人的速度在减少。
《人民日报海外版》 (2001年05月19日第九版)
平均每小时一个物种消失人类活动危害比自然灭绝高1000倍http://www.sina.com.cn 2006年03月24日05:05 浙江日报 3月20日,联合国生物多样性大会秘书长在《全球生物多样性展望》报告中说:“事实上,我们目前正在制造地球历史上的第六次大灭绝。这也是自恐龙在6500万年前消失以来规模最大的灭绝。”
报告中提到,全球人口不断增加,导致了污染、城市扩张、森林遭到砍伐、“外来物种”入侵和全球变暖等诸多问题,从而对动植物的生存环境造成了破坏。估计目前的灭绝 速度是历史上的1000倍。 从珊瑚礁到热带雨林,各种生物都面临着日益严峻的威胁。根据世界自然保护联盟编撰的名单,有844种动植物在过去的500年里灭绝,其中包括渡渡鸟和金蟾蜍。该联盟表示,实际灭绝数量可能远不止844种。
英国皇家学会会员、剑桥大学教授西蒙·莫瑞斯说:“人类活动引起的全球气温变暖与距今约4500万年前地球经历的一次气温变暖十分相似。它们都导致了大量动植物种类灭绝。”
曾经在中国南京参加第四次国际寒武纪大会的多位著名古生物学家也表示,目前物种消失与地球历史上的生物灭绝事件惊人相似,是工业革命拉开了这次生物灭绝的序幕。
科学界普遍认为,地球生命演化史经历过5次大规模的生物灭绝,虽然具体原因众说纷纭,但可以肯定的是,这些都与环境急剧变化有密切关系。除了恐龙消失之外,前五次灭绝的原因可能是小行星撞击、火山爆发或者气候骤然发生变化。最严重的一次生物大灭绝发生在距今约2.5亿年,导致超过95%的地球生物灭绝。
“除了星体撞击、全球性火山爆发等突发事件,我们正在经历的生物灭绝,比地球生命史上其他灭绝事件更为恶劣。”中国西北大学早期生命研究所所长舒德干说。
这位近年来在《科学》和《自然》杂志上发表了10余篇早期生命研究文章的科学家说,以前的生物大灭绝都属于自然灾害,但这一次是人为的。
科学家发现,人类活动造成的物种灭绝比自然灭绝的速度高1000倍,平均每小时就有一个物种灭绝,如夏威夷画眉和康定云杉已经消失。
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[ 本帖最后由 糊涂塌客 于 2007-2-24 05:27 编辑 ] |
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