What is IUCN and its Red List?

A lot it is talked about endangered, critically endangered and vulnerable species. In fact, in this blog we have given some examples: Mediterranean monk seal is an example of a critically endangered species and Iberian lynx of an endangered species. But, which is the meaning of these names? Who is responsible of categorising them and how do they do it? Here, there are the answers. 


Have you ever seen this logo in some report or in Internet?

logo iucn uicn
International Union for Conservation of Nature logo (Pictrure: IUCN).

The International Union for Conservation of Nature (IUCN) is a union with more than 1,000 government and NGO member organizations, and about 11,000 scientists from 160 countries.

Its purpose is to preserve nature and, for this reason, it seeks to solve the most urgent environmental problems.

The IUCN is composed by 6 commissions, among which there is the Species Survival Commission (SSC), which have more than 10,000 volunteer experts from all over the world, distributed in different working groups.

The IUCN and the Species Survival Commission are the responsibles of elaborating the Red List. What is a Red List?


The red list of threatened species is an inventory of the the conservation state of plant, animal and fungi species. So, it evaluates the risk of extinction of a species in case no conservation actions are taken.

Lista Roja de la Flora Vascular Española 2008. (Imagen: Jolube).
Red List of Spanish Vascular Flora 2008. (Picture: Jolube).

This list is produced following objective criteria that permit to classify species in 8 threat categories, so that it is easier to compare different taxa. Let’s see them!



To use these criteria, some requirements have to be followed:

  • They can be applied to species and lower taxonomic levels.
  • They can only be applied to wild populations in their natural distribution and to populations resulting from benign introductions.
  • Criteria have to be applied to taxon whatever the level of conservation action affecting it.
  • The conservation status of a species doesn’t have to be necessarily the same in a global scale or a regional or national scale.


From less to more extinction risk, evaluated taxa can be classified in one of the following categories:

  • Least concern (LC): when a taxon cannot be classified in any of the following categories, it is said that there is a least concern for its conservation.
  • Near Threatened (NT): when a taxon cannot be classified in any of the following categories, but is close, it is said that is near threatened because in the near future it will possibly classified in some of them. The dwarf cassowary (Casuarius bennetti), a bird that could be the current velociraptor, has this category.
quandong, cassowary, eating, fruit
Cassowary eating quandongs, one of its favourite fruits (Picture: Christian Ziegler).
  • Vulnerable (VU): when a taxon is considered to be facing a high risk of extinction in the wild, it is classified as vulnerable. Two examples are whale shark and the basking shark, the two biggest fishes in the world and feed on plankton.
  • Endangered (EN): a taxon is endangered when faces a very high risk of extinction in the wild. An example is the Iberian lynx (Lynx pardinus).
  • Critically Endangered (CR): a taxon is critically endangered when faces an extremely high risk of extinction in the wild. An example, it is the Mediterranean monk seal (Monachus monachus).
  • Extinct in the Wild (EW): it means that only survives in cultivation, in captivity or as a naturalized population well outside the past range.
  • Extinct (EX): a taxon is extinct when there is no doubt that the last individual has died. An example is the thylacine.
Un dels pocs llops marsupials que es conserven taxidermitzats en el món. Museo nacional de Ciencias Naturales, Madrid. Foto: Mireia Querol
Thylacine was extinct by humans. Museo nacional de Ciencias Naturales, Madrid (Picture: Mireia Querol).

Vulnerable, Endangered and Critically endangered categories are those that show a threatened condition of the species. So, an endangered and a threatened species are not exactly the same.

Moreover, there are two other categories for those species not evaluated:

  • Data deficient (DD): when there is inadequate information to make an assessment of its risk of extinction.
  • Not evaluated (NE): when it has not yet been evaluated.


5 are the criteria to evaluate the state of conservation of taxa:

  • Criterion A: Reduction in population size. It has to accomplish some of the following subcriteria (A1-A4), evaluating the reduction in the largest period, 10 years or 3 generations.
  Critically endangered Endangered Vulnerable

Reduction in the past where the causes are reversible, understood and have ceased

90% or more 70% or more 50% or more

Reduction in the past where the causes may no have ceased, may no be understood or may not be reversible

80% or more 50% or more 30% or more

Reduction in the future (up to 100 years)


80% or more 50% or more 30% or more

Reduction where the time period mus include both the past and the future (up to 100 years in the future) and where the causes may not have ceased, may not be understood or may not be reversible

80% or more 50% or more 30% or more
  • Criterion B: Geographic range either extent of occurrence or area of occupancy. Let’s see these concepts:
    • Extent of occurrence: when you draw a line that joints all the most external places where there is a taxon (and all the places are included), you find the extent of occurrence.
    • Area of occupancy: it refers to the area inside the extent of occurrence having into account that there is not present in all its extent of occurrence.
Dos ejemplos de la diferencia entre extensión de presencia y área de ocupación. (A) es la distribución espacial de dos especies, (B) es la delimitación de la extensión de presencia y (C) muestra una medida del área de ocupación (Foto: IUCN).
Two examples of the difference between extent of occurrence and area of occupancy. (A) is the spatial distribution of a species, (B) show  the extent of occurrence and (C) shows the area of occupancy (Picture: IUCN).
  Critically endangered Endangered Vulnerable

Extent of occurrence

Less than 100 km2 Less than 5.000 km2 Less than 20.000 km2

Area of occupancy

Less than 10 km2 Less than 500 km2 Les than 2.000 km2
  • Criterion C: Small size of the population and decline.
  Critically endangered Endangered Vulnerable
Number of mature individuals Less than 250

(and C1 and/or C2)

Less than 2.500

(and C1 and/or C2)

Less than 10.000

(and C1 and/or C2)


Continuing decline

At least 25% in 3 years or 1 generation (up to 100 years) At least 20% in 5 years or 2 generations (up to 100 years) At least 10% in 10 years or 3 generations (up to 100 years)

Continuing decline and at least one of the 3 conditions (2a.i, 2a.ii, 2b)


Number of mature individuals in each subpopulation

At least 50 At least 250 At least 1.000

% of individuals in one subpopulation

90-100% 95-100% 100%

Extreme fluctuations in the number of mature individuals

  • Criterion D: Very small population or restricted
  Critically endangered Endangered Vulnerable
Number of mature individuals Less than 250 Less than 2.500 Less than 10.000
  • Criterion E: Quantitative analysis 
  Critically endangered Endangered Vulnerable

Probability of extinction in the wild

At least 50% in 10 years or 3 generations (up to 100 years) At least 20% in 30 years or 5 generations (up to 100 years) At least 10% up to 100 years.

Despite there have to be evaluated all the criteria, with just one of them is enough to classify the species in the category.

The previous criteria are simplified because there are complementary conditions that have to be accomplished. For further details, read the document IUCN Red List Categories and Criteria.



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