Glossary of Red List terms

The IUCN Red List Criteria use a set of terms with highly specific meanings, which can often be different from their normal biological use. You may encounter these terms in species' assessment justifications, which explains how available data on the species meet the criteria requirements for the category it is listed in.

Area of occupancy (AOO)

Area of occupancy is the area in km2 within a species' extent of occurrence (EOO) which is physically occupied by the species. This measure considers the fact that a species will not usually occur throughout the area of its extent of occurrence, which may contain unsuitable or unoccupied habitats. In some cases, for example, irreplaceable colonial nesting sites or crucial feeding sites for migratory species, the area of occupancy is the smallest area essential at any stage to the survival of existing population of a species.

Continuing decline

A continuing decline is a recent, current or projected future population decline that is liable to continue unless remedial actions are taken.

Extent of occurrence (EOO)

Extent of occurrence is the area in km2 that is encompassed by a minimum convex polygon containing all the current sites of occurrence of a species, but excluding vagrancy.

Extreme fluctuations

Extreme fluctuations are large, rapid and frequent variations in population and range size, typically more than one order of magnitude (namely, a tenfold increase or decrease).

Generation length

Generation length is the average age of mature individuals in the population. It is intended to reflect the turnover rate of breeding individuals in the population, and is therefore neither the age of reproductive maturity, nor the age of the oldest breeding individuals, except for species that breed only once.


A location is a geographically or otherwise distinct area where a single threatening event can rapidly affect all individuals of a species present. The size of the location is therefore based on the size of the impact of a threatening event. Location is therefore not the same as subpopulations or 'localities', as it may incorporate a number of subpopulations or just part of a subpopulation, depending on the size of impact. If more than one threat is affecting a species, the most severe threat should be used to define locations. For some threatening processes it may not be possible to determine locations, and in such instances other appropriate criteria should be tested.

Mature individuals

Mature individuals are those individuals that are known, estimated or inferred to be capable of reproduction.


Within the Red List criteria, the term population is used in a very specific sense that is different from its normal biological use. The Red List's definition of population is the total number of individuals of a species.

Population and subpopulation size

The population size, as is used in Criteria C and D, always is a count of only the number of mature individuals in the population or in a subpopulation.

Population reduction

This term, used in Criterion A, refers to a decline in the number of mature individuals in the population as a percentage of the original population size. Therefore, if a population of 10 000 mature individuals declined to 2 000 mature individuals, there has been a population reduction of 80%.

Quantitative analysis

A quantitative analysis is any form of analysis which estimates the probability of extinction of a species based on known life history, habitat requirements, threats and any specified management options.

Severely fragmented

The term severely fragmented refers to a situation where a species' risk of extinction is increased due to the fact that more than half of its individuals occur in small, isolated subpopulations. These small subpopulations may easily become locally extinct due to stochastic processes, with a limited possibility of recolonization of sites of local extinctions. Poor dispersers are more easily severely fragmented than species capable of long-distance dispersal.


Subpopulations are geographically or otherwise distinct groups of individuals within the population between which there is little demographic or genetic exchange.