Reasons for animals
1. pesticide /?pest?sa?d/ n. 杀虫剂
2. exotic /?ɡ?z?t?k/ adj. 外来的
3. evolve /i?v?lv/ v. 发展;进化
4. evade /??ve?d/ v. 逃避;躲避
5. enforcement /?n?f??rsm?nt/ n. 执行;强制;实施
6. sophisticated /s??f?st?ke?t?d/ adj. 老练的;精密的;复杂的
7. smuggle /?sm?ɡl/ v. 走私;偷运
Today， more and more animal species are on the verge of extinction because of a variety of major factors that cause a species to become endangered， and as you might expect， humans play a role in quite a few of them.
Habitat destruction and pollution
A habitat is not just a residence， it is also where an animal finds food， raises its young and allows the next generation to take over. Unfortunately， humans destroy animal habitats in a number of different ways： building houses， clearing forests to get lumber（木材） and plant crops， draining rivers to bring water to those crops， and paving over meadows to make streets and parking lots.
Besides， animals habitats were polluted by petrol products， pesticides， and other chemicals， which destroys food sources and viable shelters for the creatures and plants of that area. As a result， some species die out outright while others are pushed into areas where they cant find food and shelter. Worse yet， when one animal population suffers it affects many other species in its food web so more than one species population is likely to decline.
Introduction of exotic species destroys delicate food systems
An exotic species is an animal， plant， or insect that is introduced into a place where it did not evolve naturally. Exotic species often have a predatory or competitive advantage over native species， which have been a part of a particular biological environment for centuries， because even though native species are well adapted to their surroundings， they may not be able to deal with species that closely compete with them for food. Basically， native species havent developed natural defenses for an exotic species.
Illegal hunting can endanger species
When hunters ignore rules that regulate the number of animals that should be hunted （a practice known as poaching）， they can reduce populations to the point that species become endangered. Unfortunately， poachers are often hard to catch because they are deliberately trying to evade authorities， and they operate in areas where enforcement is weak.
Furthermore， poachers have developed sophisticated techniques for smuggling animals.Live animals have been sold to people who want exotic pets or medical research subjects. And， animal pelts（毛皮） and other body parts are also secretly smuggled across borders and sold through black market networks of buyers who pay high prices for illegal animal products.
Extinction is a natural part of evolution
Of course， species endangerment and extinction can happen without human interference. Extinction is a natural part of evolution. Fossil records show that long before people came along， factors such as overpopulation， competition， sudden climatic change， and catastrophic events like volcanic eruptions and earthquakes drove the decline of numerous species.
There are a few warning signs that a species could become extinct. If a species has some economic importance， such as the Atlantic salmon， it may be at risk. Surprisingly， large predators， who we might expect to have an advantage over other species， are often at risk as well. This list includes grizzly bears， bald eagles， and gray wolves.
Apart from the reasons mentioned in the text， can you think of any other reasons for animals being endangered？