Unit 9: Biodiversity Decline // Section 5: Categories of Concern: Critically Endangered, Endangered, Vulnerable
What are the criteria for determining whether a species is in danger of extinction? To advise national governments, the multinational World Conservation Union (abbreviated IUCN for its formal title, the International Union for the Conservation of Nature) maintains a database of threatened species and subgroups and publishes the Red List, which catalogues species most at risk. For species in the highest risk groups—Critically Endangered, Endangered, and Vulnerable—IUCN weighs criteria including population size, geographic range, and how individuals are distributed, especially if the population is very small.
IUCN is working to improve its data on species, which currently is biased toward forests and other land ecosystems, emphasizes animals more strongly than plants, and does not cover microbes. Priority areas for the Union include better data on marine species and on arid and semi-arid ecosystems, which are expanding as a result of global climate change. Table 1 summarizes some estimates of threatened species (Critically Endangered, Endangered, and Vulnerable) from the 2006 Red List.
|Number of described species in IUCN database||Number of threatened species in 2006||Number threatened as % of described species|
|Ferns and allies||13,025||139||1.0%|
Other biological inventories cover different sets of organisms and offer different perspectives on which species are most highly threatened. For example, NatureServe (www.natureserve.org) pools data from a network of natural heritage programs and estimates the number of threatened species in the United States to be far greater than the IUCN’s estimates. "There is no single authoritative list of the world's endangered species, because we have yet to count and describe many living species," says Harvard University biologist Anne Pringle.
Scientific evidence is central to identifying endangered species. To determine whether a species is endangered or might become so, scientists collect data to answer questions including:
- Is the population growing, shrinking, or at a steady state, and why? How completely does it occupy its habitat? How is it being affected by competitors, parasites, harvesting, and hybridization with other species?
- Is the species' geographic range expanding or contracting? Is it fragmented into small areas? How many mature (breeding) individuals exist, and where are they located? How are external impacts on its habitat, such as pollution and development, expected to affect the species' range? How much habitat is needed to support a target population level?
- If the population is very small, is it expected to grow or contract? Will the number of mature individuals remain steady or fluctuate? Can they reach each other to breed?
- How biologically distinct is the species from other closely related organisms? Does the target group consist of one single species, or should it be reclassified as several distinct species?
- If a species is recovering from endangered status, what population size and distribution indicate that it no longer needs special protection?
These assessments draw on scientific fields including conservation biology, population ecology, biogeography, and genetics. Captive breeding programs have helped to preserve and reintroduce some species that were extinct in the wild, such as California condors. Recently scientists have successfully cloned several endangered varieties of cows and sheep, and some biologists advocate creating DNA libraries of genetic material from other endangered species. Others counter that cloning fails to address the root causes of the problem, including habitat loss and over-harvesting.