4.2 Biodiversity, species loss, and ecosystem function  (Page 2/9)

Vertebrates

Scientists know much more about the state of vertebrates—especially mammals, birds, and amphibians—than they do about other forms of animal life. Every one of the 5,488 species of mammals that have been described, for example, has been evaluated for purposes of the Red List. Of them, 76 species have become extinct since 1500, and two, Pere David's deer, which is native to China, and the scimitar oryx from Africa survive only in managed facilities. Another 29 of the mammal species listed as critically endangered are also tagged as "possibly extinct;" they are very likely gone, but the sort of exhaustive surveys required to confirm that fact have not been conducted. Overall, approximately 22% of mammal species worldwide are known to be threatened or extinct. (In the terms of the Red List, the broad designation "threatened" includes three levels of risk for extinction in the wild: Vulnerable [high], Endangered [higher], and Critically Endangered [highest].)

The Red List categorizes a smaller proportion of the world's 9,990 described bird species—14%—as threatened or extinct. But the raw number of species lost since 1500 is at least 134, and four more species persist only in zoos. Another 15 species of birds are considered possibly extinct. The fact that 86% of bird species are categorized as "not threatened" is good news in the context of the Red List.

Among the well-studied vertebrates, amphibians are faring especially poorly. Of the more than 6,000 known species of amphibians, 38 have become extinct worldwide since 1500, and another one, the Wyoming toad, survives only in captivity. Another 120 species are considered possibly extinct. Overall, 2,030, or one-third of the world's amphibian species are known to be threatened or extinct. More troubling still, many amphibian species—42.5%—are reported to be declining, and that number is probably low, since trend information is unavailable for 30.4% of species.

Only small proportions of the world's species of reptiles and fish have been evaluated for purposes of the Red List. Among those, the numbers of species that fall into the threatened category are very high: 1,275 of the 3,481 evaluated species, or 37%, for fish; and 423 of 1,385 evaluated species, or 31%, for reptiles. It should be noted, however, that these percentages are likely overestimates, since species of concern are more likely to be selected for evaluation than others.

Invertebrates

The category "invertebrates" lumps together the vast majority of multi-cellular animals, an estimated 97% of all species. It includes everything from insects and arachnids, to mollusks, crustaceans, corals, and more. Few of these groups have been assessed in a comprehensive way, and so as with fish and reptiles, the Red List percentages of threatened species are skewed high. But assessments within some groups call attention to disturbing, large-scale trends. For example, 27% of the world's reef-building corals are already considered threatened, and many more of them are experiencing rates of decline that move them toward threatened status. The demise of reef-building corals has magnified ecological impacts, since so much other marine life depends on them.