The destruction of habitats (栖息地) all over the world is the primary reason species are becoming extinct (灭绝) or endangered. Houses, highways, dams, industrial buildings, and ever-spreading farms now dominate （21） formerly occupied by forests, deserts, and wetlands. （22） the beginning of European settlement in America, （23） , over 65, 000, 000 acres of wetlands have been drained. One million acres alone vanished （24） 1985 and 1995. 　　Habitat destruction can be（25） or it can be subtle, occurring over a （26） period of time without being noticed. （27） such as sewage from cities and chemical runoff from farms, can change the （28） and quantity of water in streams and rivers. To （29） living in a delicately balanced habitat, this disturbance can be as （30） as the clear-cutting of a rainforest. 　　（31） remaining habitats are carved into smaller and smaller pockets or islands, remaining species are forced to exist in these （32） areas, which causes further habitat （33） . These species become less adaptable to environmental （34） in fact, they become （35） endangered. Scientists believe that when a habitat is cut by 90%, one-half of its plants, animals and insects will become extinct.