subspecies that is represented elsewhere by other abundant and widely 

 distributed subspecies. This often leads the public to believe that the 

 entire species is endangered when only a small population segment is in 

 trouble. The endangered Delmarva fox squirrel, for example, is confined 

 to a small area in eastern Maryland and Virginia. But other kinds of fox 

 squirrel are common to abundant in the southeastern and midwestern 

 United States. 



Forty known kinds of birds and mammals native to the United States 

 and its territories have become extinct: 109 animals, of which 64 are 

 birds and mammals, have been declared endangered by the Secretary of 

 the Interior. 



At face value these figures are alarming. But the situation in North 

 America is not that bleak. Only six of the 32 extinct species and 

 subspecies of birds were native to the North American mainland. Most 

 were native to Hawaii. 



Species and subspecies that have evolved in restricted island habitats 

 are especially vulnerable. They depend for survival upon unique and 

 fragile ecosystems that change dramatically with any intrusion of man. 

 Evolution in isolation has equipped few island species to cope with 

 introduced predators, such as house cats, rats and mongooses, or to 

 resist introduced diseases. 



Civilized man has brought wild and domesticated birds and mammals 

 from his homeland to every island he has settled. Native animals usually 

 are unable to compete with the more aggressive invaders. Grazing and 

 foraging by sheep, cattle, goats and pigs often destroy vital wildlife 

 habitats. Since its discovery, 40 percent of Hawaii's original forms of 

 birds have become extinct, and another 40 percent are endangered, 

 largely because of inadvertent human influences. 



A large majority of the endangered animals and those that have 

 become extinct were island species, literally or figuratively, in that their 

 restricted habitats were surrounded by terrain unsuitable for their surviv- 

 al. Of the eight extinct North American mammals, two were meadow 

 mice or voles. One was confined to a single small island in Long Island 

 Sound; the other occupied an isolated marsh in California. The range of 

 the heath hen was reduced to one island off the coast of Massachusetts 

 soon after the American Revolution. In spite of 50 years of total 

 protection, it became extinct in 1932. The population had been built to 

 nearly 2,000 in the late 1920's when fire swept its nesting grounds at the 

 height of the breeding season. Introduced poultry diseases eliminated 

 the survivors. 



Today, the heath hen's close relative, the Attwater's prairie chicken, 

 occupies a similar vulnerable habitat on a small section of the Gulf Coast 

 of Texas. The natural grasslands that it needs were much reduced and 

 are now surrounded by cultivated lands and heavily grazed pastures. 



Habitat factors even played a role in the fate of species that were 

 destroyed or reduced to remnants principally by direct, deliberate kill- 

 ing. Commercial netting was a major cause of the extinction of the 



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