Environmental Problems, Their Causes, and Sustainability 1

Island species, many of them endemic speciesfound nowhere else on earth, are especially vulnerableto extinction when their habitats are destroyed, degraded,or fragmented.What Is the Role of Habitat Fragmentation?Isolating and Weakening Populationsof SpeciesSpecies are more vulnerable to extinction whentheir habitats are divided into smaller, more isolatedpatches.Habitat fragmentation occurs when a large, continuousarea of habitat is reduced in area and divided intosmaller, more scattered, and isolated patches or “habitatislands.” This divides populations of a species intosmaller and more isolated groups that are more vulnerableto predators, invasion by more competitivespecies, disease, and catastrophic events such as astorm or fire. Also, it creates barriers that can hindersome species from dispersing and colonizing new areas,getting enough to eat, and finding mates.Certain types of species are especially vulnerableto local and regional extinction because of habitat fragmentation.They include species that are rare, thatneed to roam unhindered over large areas, and thatcannot rebuild their population because of a low reproductivecapacity. Also included are species withspecialized niches and species that are sought by peoplefor furs, food, medicines, or other uses.The theory of island biogeography (p. 145) hasbeen used to understand the effects of fragmentationon species extinction and to develop ways to help preventsuch extinction.Case Study: How Do Human ActivitiesAffect Bird Species? A Disturbing Messagefrom the BirdsOur activities are causing serious declines in thepopulations of many bird species.Approximately 70% of the world’s 9,800 known birdspecies are declining in numbers and about one ofevery six bird species is threatened with extinction,mostly because of habitat loss and fragmentation. A2002 National Audubon Society study found that aquarter of all U.S. bird species are declining in numbersor are at risk of disappearing. Figure 12-8 showsthe 10 most threatened U.S. songbird species accordingto a 2002 study by the National Audubon Society.Cerulean warbler Sprague’s pipitBichnell’s thrushBlack-capped vireo Golden-cheeked warblerFlorida scrub jay California gnatcatcher Kirtland’s warbler Henslow’s sparrow Bachman’s warblerFigure 12-8 Threatened natural capital: ten most threatened species of U.S. songbirds according to a 2002study by the National Audubon Society. Most of these species are threatened because of habitat loss and fragmentationfrom human activities. Almost 1,200 species—about 12% of the world’s 9,800 known bird species—may face premature extinction during this century.http://biology.brookscole.com/miller14233