2018年托福作文素材（4）

　　Aesthetic/Recreational

　　Plant and animal species and their ecosystems form the basis of America multi-billion dollar, job-intensive tourism industry. They also supply recreational, spiritual, and quality-of-life values as well.

　　Each year over 108 million people in the United States participate in wildlife-related recreation including observing, feeding, and photographing wildlife. Americans spend over $59 billion annually on travel, lodging, equipment, and food to engage in non-consumptive wildlife recreation. Our national heritage of biological diversity is an invaluable and irreplaceable resource. Our quality of life and that of future generations depends on our preservation of plant and animal species.

　　Source of Information: The National Wildlife Federation抯 Endangered Species Program

　　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.