Extinction and its types

Extinction and its types

Extinction is a key concept in conservation. Species extinction is a powerful cultural term in many societies because it can resonate with feelings deep in the human psyche. Avoiding extinction is one of the main aims of the global conservation movement and its study has historically been dominated by ecological thinking about the consequences of small population size and the identification of the proximate causes of population decline.

IUCN (1994, 2001) defines a species as extinct if “there is no reasonable doubt that the last individual has died”. While this statement clearly defines the state of being extinct it falls far short of capturing the implications and meaning of the noun “extinction” primarily based on different degrees of scientific certainty associated with the disappearance of a species and the potential for its re-emergence at some point in the future. Based upon this pretext Ladle & Jepson (2008) have provided a further clarification on the contexts in which the term extinction can is being today used in literature. They find it meaningful to distinguish between the following types of extinction:

1. Linnean extinction

Extinctions inferred from the species-area relationship and estimates of species diversity for a given ecosystem. This enables extinctions of as yet undiscovered and/or undescribed taxa to be extrapolated from data on the rate of new species discovery, the species-area curve, and rates of habitat loss. This “Linnean” meaning of extinction generates startling and worrying figures. For example, the E. O. Wilson’s “conservative” prediction of approximately 27,000 species going extinct every year based on the rate of tropical deforestation (Wilson 1991).

2. Wallacean extinction

It is a term used to describe inadequacies in our knowledge of the global, regional, and even local distribution of many species. Such a lack of information can lead to species erroneously being classed as extinct or threatened with extinction when undiscovered populations are still extant.  It relates to species that have not been documented for many years but in which the population has not been systematically surveyed across its range.

3. Phoenix extinction

These are species that have become extinct under the IUCN definition above, but have been (or have the potential to be) resurrected from the ashes of extinction through human ingenuity, imagination, or both. Phoenix extinctions take at least two forms.

• The first is based on the idea that some species have not become extinct but rather they have been transformed by human action. A fascinating example of this sub-type is happening in Holland where a group of forward thinking government conservationists has successfully managed back-bred domesticated cattle to fashion a wild-acting replicate of the Auroch (Bos primigenius)
• The second form of phoenix extinction is the technological solution of recreating species from genetic material stored in gene banks or extracted from preserved remains. This idea has been around for decades and is already used in agriculture for resurrecting different varieties of domesticated species. However, the advent of new reproductive and genetic technologies has both increased the potential of scientists to recreate extinct species and public interest in this possibility.

4. Ecological extinction

It refers to species that are no longer found in the wild or occur in densities so low that it “no longer interacts significantly with other species” (Estes et al. 1989). Ecological extinction can be alternatively defined as the avoidance of extinction enabled by captive-breeding programmes.

5. Local extinction

It is defined as species that have disappeared from a clearly delimited geographic area (often relating to a geographical or physical boundary) but with extant, free-living populations outside that area.

6. True extinction 

The term is used in reference to those species where there is no reasonable doubt that the last population is extinct and that no captive population or genetic material exists.

It has been differentiated into two types.

• Contemporary extinction: Extinction since the birth of the international conservation movement (mid-19th Century). Last known population has been monitored and surveyed and is now considered globally extinct in the wild. No captive-bred population or genetic material available. E.g. Dodo, Pink headed duck etc
• Historic extinction: Extinction prior to the birth of the international conservation movement. No authenticated record of an extant population. No captive-bred population or genetic material available. E.g. dinosaurs