Endangered Species Handbook

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Vanishing Species

What is Threatening Species? Habitats Under Threat

     Habitat destruction is the foremost threat to wildlife.  In broad terms, more than 85 percent of IUCN-listed birds, mammals and plants are threatened by habitat destruction and degradation (Hilton-Taylor 2000).  The greatest number of threatened species listed in the 2000 IUCN Red List inhabit terrestrial areas.  They total 9,256 highly threatened species.  The largest single terrestrial group is plants, with 5,607 species; birds follow, with 1,144 species; with mammals having only slightly fewer threatened species, 1,111 (Hilton-Taylor 2000).  The vast majority of threatened birds and mammals are terrestrial species.  The destruction of forests is the single most important threat to birds, affecting 75 percent (BI 2000).  At least 900 of the 1,144 threatened birds inhabit tropical rainforests, with almost half of those species restricted to lowland rainforests, and 35 percent in montane rainforest (BI 2000).  Authors of Threatened Birds of the World (BI 2000) found that the vast majority--86 percent--of rainforest birds cannot tolerate much habitat destruction, and 45 percent require near-pristine habitat.  Only 3 percent are highly tolerant of habitat alteration (BI 2000).  Unsustainable selective logging affects 31 percent of threatened birds.  In many tropical forests, logging and forest burning are taking place without any restrictions, totally eliminating habitats of rare rainforest birds.  A total of 4.5 million square kilometers, or 20 percent of the world's forests, were cleared from 1960 to 1990 (BI 2000), and since then, forests--especially tropical forests--have continued their decline.  Although some forests may return to old-growth hundreds or thousands of years in the future, much of the land is being converted to agriculture, grazing land, housing, cities, industry and roads and is unlikely ever to revert to forest.
     Likewise, more than half, or about 57 percent, of threatened mammals inhabit tropical rainforests, 35 percent in lowland, and about 22 percent in montane rainforest (Hilton-Taylor 2000).  Less than 10 percent of threatened birds and mammals are native to temperate mixed forests, coniferous forest and temperate broadleaf forests, according to the IUCN.  A very small number of threatened mammals, about 3 percent, inhabit tropical degraded forest, a sign of the unsuitability of this habitat (Hilton-Taylor 2000). 
     Forests are also home to a wide variety of threatened frogs, salamanders, tree snails and insects, although studies of which type of forest each inhabits have not been done by the IUCN.  Many endangered plants are tropical species, native to islands such as Madagascar and Indonesia, where endemic plants such as orchids and palms abound, and habitat destruction is severe.  The United States leads the world in threatened plant species, with 4,669 identified by the IUCN in its 1997 study (Walter and Gillett 1998) and 7,817 species listed by The Nature Conservancy (Stein et al. 2000).  Many of the latter are in Hawaii, another tropical island with a high percentage of threatened plants.  Lobelias are known for their beautiful flowers, and the ancestor species that arrived in the Hawaiian Islands radiated into 273 species, some of which grew to the heights of small trees.  One-quarter are gone, and another 124 of the surviving species are threatened with extinction or possibly are extinct (Walter and Gillett 1998).  Of the remaining lobelias, only 27 percent now have sizeable enough populations to keep them from extinction; the loss of their pollinators, most of whom were honeycreepers, is a major cause (Buchmann and Nabhan 1996).  The honeycreepers lost their forest habitats to clearance by settlers, and as they faded into extinction--one species after another--lobelia plants they had pollinated disappeared or became endangered.  These ecological webs exist throughout nature.
     The IUCN found that 91 percent of plants identified as threatened were endemic, with their entire distribution restricted to a specific country (Walter and Gillett 1998).  A total of 32,242 threatened or extinct species of plants occur in one country alone, while 2,368 occur in two countries, and only 709 plant species occur in more than two countries.  Only scanty information on threatened plants is available from most countries in South America, Africa and Asia, which are expected to have large percentages of their native plants found to be threatened when assessed (Walter and Gillett 2000).
     Grasslands, shrublands and savannahs are the second most important habitat for threatened birds, home to 383 species, or 32 percent of all listed species (BI 2000).  Two-thirds of these birds inhabit shrublands; 43 percent, grasslands; and 8 percent, savannah (BI 2000).  Three-fourths are tropical birds, whose habitats are threatened by livestock overgrazing, human settlement and farming (BI 2000).  Some 17 percent of threatened mammals inhabit grasslands, while another 8 percent are shrubland species, and about 7 percent are native to desert and semi-desert (Hilton-Taylor 2000).    
     Freshwater habitats, such as rivers, marshes, bogs, streams and ponds are the second most important biome, after terrestrial, for threatened species.  At least 1,946 threatened species, the largest number being fish (627 species) inhabit these areas (Hilton-Taylor 2000).  Freshwater crustaceans (409 species) and mollusks (420 species) are major inhabitants of these aquatic areas, as are 131 amphibians, 111 reptiles, 78 birds and 31 mammals, according to the 2000 IUCN Red List.  The United States, with its extensive water projects--dams, levees, diverted and channelized rivers--harbors large numbers of threatened crustaceans and mollusks, as discussed above. 
    Frogs make up the majority of threatened amphibians and have been in decline for several decades.  Their habitats are being destroyed at an unprecedented rate as wetlands are filled in--half of US wetlands that were present in colonial times--are gone.  Thailand has lost almost all its wetlands, and Southeast Asian lakes and marshes are being drained at an unprecedented rate, threatening frogs and other wildlife.  Frogs have been on Earth for 190 million years, but at the present rate of decline, the majority of the approximately 4,500 species will be gone within decades.  In addition to habitat loss, a variety of threats are eliminating them (see Non-Native Species, Trade and Pollution sections below).

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