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Local extinction, or extirpation,[clarification needed] is the condition of a species (or other taxon) that ceases to exist in the chosen geographic area of study, though it still exists elsewhere. Local extinctions are contrasted with global extinctions.
Local extinctions may be followed by a replacement of the species taken from other locations; wolf reintroduction is an example of this.
Local extinctions mark a change in the ecology of an area.
The area of study chosen may reflect a natural subpopulation, political boundaries, or both. The Cetacean Specialist Group of the IUCN has assessed the threat of a local extinction of the Black Sea stock of Harbour Porpoise (Phocoena phocoena) that touches six different countries. COSEWIC, by contrast, investigates wildlife only in Canada, so assesses only the risk of a Canadian local extinction even for species that cross into the United States or other countries. Other subpopulations may be naturally divided by political or country boundaries.
Many crocodilian species have experienced localized extinction, particularly the saltwater crocodile (Crocodylus porosus), which has been extirpated from Vietnam, Thailand, Java, and many other areas.
Often a subpopulation of a species will also be a subspecies. For example, the recent disappearance of the Black Rhinoceros (Diceros bicornis) from Cameroon spells not only the local extinction of rhinoceroses in Cameroon, but also the global extinction of the Western Black Rhinoceros (Diceros bicornis longipes).
In at least one case, scientists have found a local extinction useful for research: In the case of the Bay checkerspot butterfly, scientists, including Paul R. Ehrlich, chose not to intervene in a local extinction, using it to study the danger to the world population. However, similar studies are not carried out where a global population is at risk.
IUCN subpopulation and stock assessments
While the World Conservation Union (IUCN) mostly only categorizes whole species or subspecies, assessing the global risk of extinction, in some cases it also assesses the risks to stocks and populations, especially to preserve genetic diversity. In all, 119 stocks or subpopulations across 69 species have been assessed by the IUCN in 2006.
Examples of stocks and populations assessed by the IUCN for the threat of local extinction:
- Marsh deer (three subpopulations assessed)
- Blue whale, North Pacific stock and North Atlantic stock
- Bowhead whale, Balaena mysticetus (five subpopulations assessed), from Critically Endangered to LR/cd
- Lake sturgeon, Acipenser fulvescens, Mississippi & Missouri Basins subpopulation assessed as Vulnerable
- Wild common carp, Cyprinus carpio (River Danube subpopulation)
- Black-flanked rock-wallaby Petrogale lateralis (MacDonnell Ranges subpopulation and Western Kimberly subpopulation)
The IUCN also lists countries where assessed species, subspecies or subpopulations are found, and from which countries they have been extirpated or reintroduced.
The IUCN has only three entries for subpopulations that have become extinctthe Aral Sea stock of Ship sturgeon (Acipenser nudiventris); the Adriatic Sea stock of Beluga (Huso huso); and the Mexican subpopulation of Wolf (Canis lupus), which is extinct in the wild. No plant or fungi subpopulations have been assessed by the IUCN.