Endangered species

January 16, 2011, 10:10 am
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Endangered Giant pandas, Chengdu, China. Source: C.Michael Hogan

caption Painted hunting dog (Lycaon pictus) in the wild, Botswana. Source: C.Michael Hogan

An endangered species is a biological taxon that is at risk of becoming extinct in a proximate time frame much sooner than the long term horizon in which species typically persist.[1] The application of this term is typically assigned when a species or subspecies population is near its minimum viable population size or is threatened by current or expected environmental alteration. Many governmental units apply the categorization of endangerment in order to create mechanisms of protection for the species involved. The International Union for Conservation of Nature (IUCN) asserts that over one third of all extant organisms are currently endangered based on the sample of species that have been evaluated through the year 2006; however, the IUCN classifies certain other species in their summary statistics that are considered at a somewhat lesser level of threat.

Causes of endangerment

caption Torrey Pine, an endangered tree that is found in only two colonies in coastal California. Source: C.Michael Hogan

There are a variety of reasons for endangerment of species, but virtually all of these are attributable to the presence of humans and the rapidly expanding human population on planet Earth. Most of the species classified as endangered have entered this status sometime during the Holocene Epoch, when human expansion and dominance has been most apparent.[2] Proximate causes of endangerment include loss of habitat, overexploitation, habitat fragmentation, pollution and introduction of alien species. Chief among reasons of habitat loss and fragmentation are conversion of natural habitats to agriculture, with ancillary drivers being deforestation and urbanization. Overexploitation may take the form of overgrazing, overfishing, clearcutting of forests or hunting fauna to a point near the minimum viable population size in a given region.

In the special case of corals, while there are severe threats from parasitical diseases and marine pollution, the status of endangerment was not recognized until relatively recently, due to the widespread geographic distribution of most coral species. The chief stressors of coral reefs are marine sedimentation, ozone depletion in the upper atmosphere and thermal elevation of ocean waters (due to thermal pollution or abrupt seawater warming change). The mechanisms of most of these stressors operate chiefly through the impacts to certain symbiotic species of Zooxanthellae, upon which corals depend for a variety of nutrients.

Relation to biodiversity

The concept of species endangerment is intrinsically related to biodiversity, or the measure of species diversity within a given ecosystem or for the Earth as a whole. To comprehend the full relationship it is necessary to address the number of species that are considered presently endangered and the rate at which species are becoming extinct. It is well known that the disappearance of one species from an ecosystem may cause destabilzation, and in fact endanger other species that depended on the interaction with the extinct taxon. In fact, even a reduction in the robust population of a species as it enters endangered status can have significant impacts on other species by removing a food source, nesting site or other interactive dependence; even removal of a predator can sometimes pose fitness risks to a population, that is accustomed to culling of the least fit of the population. Thus, the concept of endangered species is not only linked to biodiversity, but to the consequences of reduced biodiversity, such as ecosystem services.  

Taxonomic detail

caption Percentage of some common animal groups that are endangered

There is a distinct difference of percentage of endangered species within major fauna and flora groups. It is important to observe that the actual number of species identified as endangered is dramatically lower than the number listed by the IUCN or national agencies. The reason for this under-reportage is due to the facts that: (a) The total number of species on planet Earth is far from being characterized and (b) There is a significant lag between scientific determination of endangerment of a given species and the official listing of that species by a given organization.

Path to extinction

An endangered species can be considered on a path to extinction. Much attention is given to the laudable process of species recovery, or removable of species from endangered status by amplifying population numbers through conservation biology; however, not enough focus is given to the actual peril inherent in the status of an endangered species. The very categorization of a species as endangered is a clear statement that the species is near its minimum viable population size, and at risk of extinction. The concept of extinction vortex has been developed to describe the phenomenology of extinction once the population of an endangered species falls below the minimum viable population size.[3] Four separate models of extinction vortex have been advanced, two of which are driven by genetic factors and two by environmenta factors. 

Categories of endangerment

The International Union  for Conservation of Nature, as well as numerous national and state (or province) govermnents use a variety of categories to denote degrees of risk to a given species. The IUCN categories include:
* Extinct: the last remaining member of the species has died, or is presumed beyond reasonable doubt to have died.  Examples: Javan Tiger, Thylacine, Dodo, Passenger Pigeon, Caribbean monk seal, Dimetrodon, Aurochs, Dusky Seaside Sparrow
* Extinct in the wild: captive individuals survive, but there is no free-living, natural population.  Examples: Alagoas Curassow, Dromedary
* Critically endangered: faces an extremely high risk of extinction in the immediate future. Examples: Mountain Gorilla, Arakan Forest Turtle, Darwin's Fox, Javan Rhino, Brazilian Merganser, Gharial, Gray whale (western subpopulation)
* Endangered: faces a very high risk of extinction in the near future. Examples: Dhole, Blue Whale, Caspian seal, Ethiopian wolf, Giant Panda, Snow Leopard, African Wild Dog, Australian sea lion, Indian Rhinoceros, Yellow-eyed penguin, Crowned Solitary Eagle, Drosera rotundifolia, Hairy-Nosed Otter, Asian elephant, Orangutan, Grevy's zebra, Tasmanian Devil, Harp seal
* Vulnerable: faces a high risk of extinction in the medium-term. Examples: Cheetah, Black-footed penguin, Lion, Sloth Bear, Manatee, Polar Bear, Clouded Leopard, Komodo dragon
* Conservation dependent: The following animals are not severely threatened, but must depend on conservation programs. Examples: Spotted Hyena, Blanford's fox, Leopard Shark, Black Caiman, Killer whale
* Near threatened: may be considered threatened in the near future.  Examples: Blue-billed Duck, Solitary Eagle, Small-clawed Otter, Maned Wolf, Tiger Shark, Okapi
* Least concern: no immediate threat to the survival of the species. Examples: Leather Oak, Florida Black Bear, Subantarctic fur seal, Leopard Cat, Diadem roundleaf bat, California sea lionAmerican kestrel, Chinstrap penguin

Habitat conservation

The greatest threat to most endangered species is simply the lack of suitable habitat, since humans have appropriated much of the Earth's surface to serve their needs. In many cases the issue is one of identifying and protecting presently suitable habitat; conversely, the restoration of habitat that has been disturbed is sometimes feasible, but often expensive and fraught with administrative obstacles.

Classification and governance

A variety of national, state and province level governments have created their own classification schemes and attribution of endangerment. In addition notable international, national and state non-governmental agencies engage in such categorizations and recommendations for protection. In each case there may be elements of regional political bias that overlay the scientific underpinnings of the case for individual species designation as endangered. Several of the significant classification and governance schemes follow:


The International Union  for Conservation of Nature introduces a number of categories to designate differential threat levels to individual species: Near Threatened, Vulnerable, Endangered and Critically Endangered.[4] Chief problems with the IUCN scheme is that the great majority of Earth's species have not been scientifically evaluated so that the IUCN scheme dramatically underestimates the number of threatened taxa. In addition the IUCN scheme suffers from the general lack of subspecies or regional categorization of endangered species.

United States of America

The USA was one of the first nations to enact comprehensive legislation to identify and protect endangered plants and animals. The United States Endangered Species Act was passed in 1973 and has been strengthened a number of times since.[5] As of June 28, 2009, a total of 1893 species of animals and plants had been listed as either endangered or threatened; 1320 of these taxa occur in the United States and its territories and the remainder only in other countries

International trade agreements

The most significant development in international trade of endangered flora and fauna is found in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) agreement.[6] This agreement provides a framework for international cooperation in the enforcement of bans for exploitation, transport and sale of endangered taxa and their derivative products; while not all nations have expressed the same degree of rigour in the implementation of CITES, the model provides an initial step in protection of endangered taxa within a global context.

Captive breeding

A captive breeding program is the human managed activity of species reproduction in a controlled environments such as a wildlife preserve, zoo or other conservation  facility; sometimes captive breeding may include release of individuals to the wild, when there is sufficient natural habitat to support new individuals or when the threat to the species in the wild is deemed reduced.

This technique has been used with success for some species, with the oldest known instances of captive breeding being attributed to collections of European and Asian potentates, an example of which is Pere David's Deer. The idea was popularized among 20th century conservationists who demonstrated success with a variety of organisms in the 1970s ranging from birds such as the Pink Pigeon, mammals including the Pygmy Hog, reptiles like the Round Island Boa and amphibians (e.g. Poison arrow frog). The California condor became extinct in the wild in 1987, but captive breeding has resulted in re-release into the wild in California, Utah and Arizona.[7] 

Captive breeding can apply to plant as well as animal species; an example of such a species is the rare and endangered Vine Hill Manzanita, that occurs in only two small colonies in the Sonoma County Barrens of California.[8] These preserves are being managed by the California Native Plant Society, and as of 2010 there are signs of a recovery trend. Other examples of captive breeding success were produced by the Flora and Fauna Preservation Society, who conducted re-introduction into the wild of the Arabian Oryx. The Przewalski's horse has also been re-introduced to the wild in its native habitat in Mongolia, .

An inherent obstacle to successful captive breeding is the risk of genetic or population bottleneck, where inbreeding may occur due to an insufficient gene pool, which may lead to the population lacking immunity to diseases and other loss of fitness. Some researchers hold that, over sufficient number of generations, inbred populations can rebuild genetic diversity.[9]


  1. ^ Paul R. Ehrlich and Anne H. Ehrlich. 1981. Extinction: The causes and consequences of the disappearance of species. Random House, New York
  2. ^ E.O. Wilson. 2005. The Future of Life. Alfred A. Knopf. New York
  3. ^ Michael E. Soule and Gary Lease eds. 1995. Reinventing nature?: responses to postmodern deconstruction.
  4. ^ International Union for Conservation of Nature. 2009. The IUCN Red List of Threatened Species 2009.2
  5. ^ CRS Report RL33468, The Endangered Species Act (ESA) in the 109th Congress: Conflicting Values and Difficult Choices.
  6. ^ CRS Report RL32751, The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES): Background and Issues.
  7. ^ Noel F.R.Snyder and Helen Snyder. 2005. Introduction to the California condor. University of California Press. 271 pages
  8. ^ Gren Lucas and Hugh Synge. 1978. The IUCN plant red data book: comprising red data sheets on 250 selected plants threatened on a world scale International Union for Conservation of Nature and Natural Resources. Threatened Plants Committee
  9. ^ Georginam M.Mace. 1986. Genetic management of small populations. International Zoo Yearbook, Vol.24-25, No.1, 1986, pp.167-174


Hogan, C. (2011). Endangered species. Retrieved from


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