在以往的托福阅读中考过关于人类活动与动物灭绝的话题。托福阅读真题文章讲解了动物的灭绝的原因，文章的理解重点是要把握好解释灭绝的原因，以及相对应所举的例子。按照不同的灭绝的原因梳理文章的结构。接下来针对这个话题，前程百利小编为大家带来相关的背景资料。
相应的背景请参考下文：
As long as species have been evolving species have been going extinct. It is estimated that over 99.9% of all species that ever lived are extinct. The average life-span of a species is 10 million years[citation needed] although this varies widely between taxa. There are a variety of causes that can contribute directly or indirectly to the extinction of a species or group of species. "Just as each species is unique" write Beverly and Stephen C. Stearns "so is each extinction ... the causes for each are varied—some subtle and complex others obvious and simple". Most simply any species that cannot survive and reproduce in its environment and cannot move to a new environment where it can do so dies out and becomes extinct. Extinction of a species may come suddenly when an otherwise healthy species is wiped out completely as when toxic pollution renders its entire habitat unliveable; or may occur gradually over thousands or millions of years such as when a species gradually loses out in competition for food to better adapted competitors. Extinction may occur a long time after the events that set it in motion a phenomenon known as extinction debt.
Habitat degradation
Habitat degradation is currently the main anthropogenic cause of species extinctions. The main cause of habitat degradation worldwide is agriculture with urban sprawl logging mining and some fishing practices close behind. The degradation of a species’ habitat may alter the fitness landscape to such an extent that the species is no longer able to survive and becomes extinct. This may occur by direct effects such as the environment becoming toxic or indirectly by limiting a species#39; ability to compete effectively for diminished resources or against new competitor species.
Habitat degradation through toxicity can kill off a species very rapidly by killing all living members through contamination or sterilizing them. It can also occur over longer periods at lower toxicity levels by affecting life span reproductive capacity or competitiveness.
Habitat degradation can also take the form of a physical destruction of niche habitats. The widespread destruction of tropical rainforests and replacement with open pastureland is widely cited as an example of this; elimination of the dense forest eliminated the infrastructure needed by many species to survive. For example a fern that depends on dense shade for protection from direct sunlight can no longer survive without forest to shelter it. Another example is the destruction of ocean floors by bottom trawling.
Diminished resources or introduction of new competitor species also often accompany habitat degradation. Global warming has allowed some species to expand their range bringing unwelcome competition to other species that previously occupied that area. Sometimes these new competitors are predators and directly affect prey species while at other times they may merely outcompete vulnerable species for limited resources. Vital resources including water and food can also be limited during habitat degradation leading to extinction.
Predation competition and disease
In the natural course of events species become extinct for a number of reasons including but not limited to: extinction of a necessary host prey or pollinator inter-species competition inability to deal with evolving diseases and changing environmental conditions (particularly sudden changes) which can act to introduce novel predators or to remove prey. Recently in geological time humans have become an additional cause of extinction (many people would say premature extinction) of some species either as a new mega-predator or by transporting animals and plants from one part of the world to another. Such introductions have been occurring for thousands of years sometimes intentionally (e.g. livestock released by sailors on islands as a future source of food) and sometimes accidentally (e.g. rats escaping from boats). In most cases the introductions are unsuccessful but when an invasive alien species does become established the consequences can be catastrophic. Invasive alien species can affect native species directly by eating them competing with them and introducing pathogens or parasites that sicken or kill them; or indirectly by destroying or degrading their habitat. Human populations may themselves act as invasive predators. According to the "overkill hypothesis" the swift extinction of the megafauna in areas such as Australia (40000 years before present) North and South America (12000 years before present) Madagascar Hawaii (300-1000 CE) and New Zealand (1300-1500 CE) resulted from the sudden introduction of human beings to environments full of animals that had never seen them before and were therefore completely unadapted to their predation techniques.
Climate change
Extinction as a result of climate change has been confirmed by fossil studies. Particularly the extinction of amphibians during the Carboniferous Rainforest Collapse 305 million years ago. A 2003 review across 14 biodiversity research centers predicted that because of climate change 15–37% of land species would be "committed to extinction" by 2050. The ecologically rich areas that would potentially suffer the heaviest losses include the Cape Floristic Region and the Caribbean Basin. These areas might see a doubling of present carbon dioxide levels and rising temperatures that could eliminate 56000 plant and 3700 animal species.
以上就是今天的托福阅读材料，大家可以在练习托福阅读材料的同时，积累一些相关词汇及句型，以便考试的时候更好地把握文意，夺得托福阅读的高分。前程百利小编预祝大家托福考试取得优异成绩!
