Red List Glossary

BirdLife International carries out Red List assessments following the Guidelines for Using the IUCN Red List Categories and Criteria. The Guidelines were developed by the Standards and Petitions Committee of the IUCN Species Survival Commission to enable an objective, transparent and consistent assessment of the conservation status of animal, fungi and plant species; they are reviewed and updated periodically. The current version of the Guidelines was published in August 2019.

This Glossary sums up the most important definitions and terms used in an IUCN Red List assessment, in order to facilitate the understanding of the Red List Categories and Criteria. For further detail, please refer to the respective chapters in the Guidelines:

Population (Chapter 4.1)

The population is defined as the total number of individuals of a species throughout its distribution.

Population size and mature individuals (Chapters 4.1 and 4.3)

The population size is measured as the number of mature individuals of a species, i.e. the number of individuals known, estimated or inferred to be capable of reproduction. But note the following:

  • Mature individuals that will never reproduce should not be counted (e.g. when population densities are too low for fertilisation).
  • In the case of populations with biased adult or breeding sex ratios, lower estimates for the number of mature individuals are used (e.g. the estimated effective population size).
  • Where the population is characterised by natural fluctuations, the minimum number is used.
  • Re-introduced individuals must have produced viable offspring before they are counted as mature individuals.

Subpopulations (Chapter 4.2)

Subpopulations are defined as geographically or otherwise distinct groups in the population between which there is little demographic or genetic exchange (typically one successful migrant individual or gamete per year or less).

Generation length (Chapter 4.4)

Generation is defined as the average age of parents in the population. It reflects the turnover rate of breeding individuals in a population.

Reduction (Chapter 4.5)

A reduction is a decline in the number of mature individuals over time. The decline can be caused by a one-time event and does not need to be still continuing.

Continuing decline (Chapter 4.6)

A continuing decline is a recent, current or projected future decline, which may be smooth, irregular or sporadic. The decline is liable to continue, unless remedial measures are taken.

Extreme fluctuations (Chapter 4.7)

Extreme fluctuations occur where population size or distribution area varies widely, rapidly and frequently, typically with a variation greater than one order of magnitude (i.e. a tenfold increase or decrease).

Severely fragmented (Chapter 4.8)

Severely fragmented refers to the situation where increased extinction risks to the species result from the fact that most individuals within a species are found in small and relatively isolated subpopulations, between which dispersal is limited. These small subpopulations may go extinct, with a reduced probability of recolonisation. A fragmentation of habitat can, but does not necessarily have to, indicate a severe fragmentation of the population.

Extent of Occurrence (Chapter 4.9)

The Extent of Occurrence (EOO) is defined as the area contained within the shortest imaginary boundary which can be drawn to encompass all the known, inferred or projected sites of present occurrence of a species. It is measured as a Minimum Convex Polygon around the area of mapped range. The Minimum Convex Polygon is continuous and cannot exclude areas of unsuitable habitat, like for example deforested areas or oceanic areas between islands. The EOO is hence not a measure of the amount of occupied or potential habitat, nor is it a measure of the area of the distribution range. In case of migratory species, the EOO should be based on the minimum of the breeding or non-breeding areas, but not both. Areas where the species is vagrant are not included in the EOO.

Area of Occupancy (Chapter 4.10)

The Area of Occupancy (AOO) represents the area of suitable habitat currently occupied by a species. The AOO is scaled using 2 x 2 km grid cells. The measure reflects the fact that a species will not usually occur throughout the area of its range, which may, for example, contain unsuitable habitats. The AOO is the smallest area essential at any stage to the survival of existing populations of a species (e.g. colonial nesting sites, feeding sites for migratory species).

Location (Chapter 4.11)

Location defines a geographically or ecologically distinct area in which a single threatening event can rapidly (i.e. within one generation) affect all individuals of the species present. The size of the location depends on the area covered by the threatening event and may include part of one or many subpopulations. In case that a species is affected by more than one threatening event, the location should be defined by considering the most serious plausible threat. In the absence of any plausible threat, the term “location” cannot be used and the threshold number for locations will not be met.