The IUCN/SSC Shark Specialist Group
IUCN/SSC Red List
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IUCN Red List Categories and Criteria
After six years of research and broad consultation, the new IUCN Red List Categories were adopted
by the IUCN Council in November of 1994. The new IUCN categories and criteria have been developed
to improve the objectivity in assessing the conservation status of species, thus improving consistency
between users. The new system has been designed to incorporate all taxa, with the exception of micro
organisms, and therefore facilitates comparison across widely different taxonomic groups. The new
IUCN categories and criteria are meant to be applied on a global scale to determine the current
status of global biodiversity; however, work is currently being done to adapt the new system so
that it can be applied at different biogeographic and geopolitical scales.
This document outlines the new categories and criteria and provides examples of how the system
should be applied. It is designed to be used as a quick reference guide and to provide a conceptual
framework of the new system, and thus should be used in conjunction with the official IUCN Red
List Categories document.
While the Red List Categories and Criteria will not be revised in the near future, the user guidelines
document will be
reviewed and updated periodically, and input from all users of the IUCN Red List Categories and
Criteria are welcome. Changes to these
guidelines are mostly additions of detail rather than changes in substance.
The user guidelines can be downloaded from the SSC web site by going to the Red List Programme
page (http://www.iucn.org/themes/ssc/red-lists.htm
) and opening the document "Guidelines for using the IUCN Red List Categories and Criteria".
Figure 1. Structure of the Categories illustrates the process that needs to be followed to assess
a taxa in one of the 8 IUCN Categories. Of the 8 categories there are three categories of threat,
Critically Endangered, Endangered, and Vulnerable. Following the Lower Risk category are the three
subcategories for taxa that are not currently threatened.
The Categories
There are eight well-defined categories that enable the classification of almost every species or
subspecies in the world. The eight categories are illustrated in bold in Figure 1. Complete
definitions are on page 3, Table 1. The first two categories in Figure 1 are relatively self
explanatory. Extinct means that there is no reasonable doubt that the last individual
has died. Extinct in the Wild means that the species has been extirpated from its
natural habitat. The following three categories in the diagram are the categories of threat,
each of which have quantitative criteria that are meant to reflect probability of extinction
at different levels of endangerment. These criteria will be discussed further in the next section.
The Lower Risk category has three subcategories:
- Conservation Dependent, taxa that do not qualify for the Vulnerable category because of species-specific or habitat-specific conservation efforts.
- Near Threatened, taxa that are not Conservation Dependent, but almost qualify for one of the Vulnerable criteria.
- Least Concern, taxa that are not in any immediate danger of extinction.
These subcategories help to monitor species and subspecies and may aid in identifying early trends
of endangerment.
The remaining two categories do not reflect the status of taxa. The category Data Deficient
highlights taxa for which sufficient information is lacking to make a sound status assessment.
The category Not Evaluated applies to taxa that have not yet been assessed with the new
categories.
Table 1. The IUCN Red List Categories
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EXTINCT (EX)
A taxon is Extinct when there is no reasonable doubt that the last individual has died.
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EXTINCT IN THE WILD (EW)
A taxon is Extinct in the wild when it is known only to survive in cultivation, in captivity or
as a naturalised population (or populations) well outside the past range. A taxon is presumed
extinct in the wild when exhaustive surveys in known and/or expected habitat, at appropriate times
(diurnal, seasonal, annual), throughout its historic range have failed to record an individual.
Surveys should be over a time frame appropriate to the taxon's life cycle and life form.
- CRITICALLY ENDANGERED (CR)
A taxon is Critically Endangered when it is facing an extremely high risk of extinction in the
wild in the immediate future, as defined by any of the criteria (A to E)
- ENDANGERED (EN)
A taxon is Endangered when it is not Critically Endangered but is facing a very high risk of
extinction in the wild in the near future, as defined by any of the criteria (A to E)
- VULNERABLE (VU)
A taxon is Vulnerable when it is not Critically Endangered or Endangered but is facing a high
risk of extinction in the wild in the medium-term future, as defined by any of the criteria (A to E)
- LOWER RISK (LR)
A taxon is Lower Risk when it has been evaluated and does not satisfy the criteria for any of
the categories Critically Endangered, Endangered or Vulnerable. Taxa included in the Lower Risk category can be separated into three subcategories:
- 1. Conservation Dependent (cd). Taxa which are the focus of a continuing taxon-specific
or habitat specific conservation programme targeted towards the taxon in question, the cessation of which would result in the taxon qualifying for one of the threatened categories above within a period of five years.
- 2. Near Threatened (nt). Taxa which do not qualify for Conservation Dependent,
but which are close to qualifying for Vulnerable.
- 3. Least Concern (lc). Taxa which do not qualify for Conservation Dependent or Near
Threatened.
- DATA DEFICIENT (DD)
A taxon is Data Deficient when there is inadequate information to make a direct, or indirect,
assessment of its risk of extinction based on its distribution and/or population status. A taxon
in this category may be well studied, and its biology well known, but appropriate data on abundance
and/or distribution is lacking. Data Deficient is therefore not a category of threat or Lower Risk.
Listing of taxa in this category indicates that more information is required and acknowledges the
possibility that future research will show that threatened classification is appropriate. It is
important to make positive use of whatever data are available. In many cases great care should be
exercised in choosing between DD and threatened status. If the range of a taxon is suspected to be
relatively circumscribed, if a considerable period of time has elapsed since the last record of the
taxon, threatened status may well be justified.
- NOT EVALUATED (NE)
A taxon is Not Evaluated when it is has not yet been assessed against the criteria.
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The Criteria and Subcriteria for the Categories of Threat
The three categories of threat, Critically Endangered, Endangered, and Vulnerable, are all determined
by quantitative criteria. For each of the categories of threat there are five criteria (A-E),
which are based on biological parameters that generally describe a process of extinction. Most
of the criteria also have subcriteria that are used to justify the listing of a species under a
given category, criteria and subcriteria. Vulnerable: C2a is an example of a category, criteria
and subcriteria. Vulnerable is the category, C the criteria and 2a the subcriteria.
The five criteria are:
- Declining Population (past or projected)
- Small Distribution and Decline or Fluctuation
- Small Population Size and Decline
- Very Small Population or Very Restricted Distribution
- Quantitative Analysis (e.g., Population Viability Analysis)
The criteria aid in assigning a broad range of taxa to one of the categories of threat using
quantitative thresholds. Because the system was designed to encompass species with very different
life histories, not all the criteria will be applicable to every species. For a species to qualify
for either Critically Endangered, Endangered, or Vulnerable, it must only meet one of the five criteria
(A-E) for a specific category of threat. However, taxa should be listed with only as many criteria as
are applicable for the highest (i.e. most severe) category of threat for which it qualifies.
Criteria A-D should be viewed as flags meant to highlight species that are likely to be threatened
with extinction. If a species is identified as threatened under one of he A-D Criteria, but is not
considered threatened when the appropriate SSC Specialist Group (with peer review) applies Criterion
E, (Quantitative Analysis), then Criterion E over rides all other criteria and the species is listed
as Lower Risk.
Although the criteria for each of the categories of threat are based on quantitative thresholds,
the system must remain relatively flexible to ensure that taxa for which there is very little
information can also be assessed within the new framework. This has been done by incorporating
inference and projection into the assessment process. Therefore, the person conducting an
assessment is expected to use the best available information in combination with inference and
projection to test a taxon against the criteria. However, if inference and projection are used,
the assumptions made must be documented. If there is any reasonable concern that a species is
threatened with extinction, it should qualify for the criteria of one of the categories of threat.
Table 2 Summary of New IUCN Categories and Criteria
| Use any of the A-E criteria |
Critically
Endangered |
Endangered |
Vulnerable |
A. Declining Population - population decline
rate at least
using either
1. population reductions observed, estimated, inferred, or suspected in the past or
2. population decline project or suspected in the future based on:
- direct observation
- an index of abundance appropriate for the taxon
- a decline in area of occupancy, extent of occurrence and/or quality of habitat
- actual or potential levels of exploitation
- the effects of introduced taxa, hybridization, pathogens, pollutants, competitors, or parasites
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80% in 10 years
or 3 generations |
50% in 10 years
or 3 generations |
20% in 10 years
or 3 generations |
B. Small Distribution and Decline or Fluctuation
Either extent or occurrence or area of occupancy
and 2 or the following 3:
1. either severely fragmented: (isolated subpopulations
with a reduced probability of recolonization, if one
extinct) or known to exist at a number of locations
2. continuing decline in any of the following:
- extent of occurrence
- area of occupancy
- area, extent and/or quality of habitat
- number of locations or subpopulations
- number of mature individuals
3. fluctuating in any of the following:
- extent of occurrence
- area of occupancy
- number of locations or subpopulations
- number of mature individuals
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< 100 km2
< 10 km2
= 1
any rate
> 1 order/mag.
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< 5,000 km2
< 500 km2
< or = 5
any rate
< 1 order/mag.
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< 20,000 km2
< 2,000 km2
< or = 10
any rate
< 1 order/mag.
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C. Small Population Size and Decline
Number of mature individuals and 1 of the following 2:
1. rapid decline rate
2. continuing decline and either
- fragmented or
- all individuals in a single subpopulation
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< 250
25% in 3 years or 1
generation
any rate
all sub-pops - or = 50
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< 2,500
20% in 5 years or 2
generations
any rate
all sub-pops - or = 250
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< 10,000
10% in 10 years or 3
generations
any rate
all sub-pops - or = 1,000
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D. Very Small Restricted
Either
1. number of mature individuals or
2. population is susceptible
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< 50
(not applicable)
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< 250
(not applicable)
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< 1,000
area of occupancy
< 100 km2 or
number of locations
< 5
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E. Quantitative Analysis
Indicating the probability of extinction in the wild to be at least
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50% in 10 years or
3 generations
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20% in 20 years or
5 generations
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10% in 100 years
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Definitions
The IUCN Red List categories and criteria must be clearly understood before the evaluation process is begun.
- Population
Population is defined as the total number of individuals of the taxon.
For functional reasons, primarily owing to differences between life-forms, population numbers are expressed as numbers of mature individuals only. In the case of taxa obligately dependent on other taxa for all or part of their life cycles, biologically appropriate values for the host taxon should be used.
- Subpopulations
Subpopulations are defined as geographically or otherwise distinct groups in the population between which there is little exchange (typically one successful migrant individual or gamete per year or less).
- Mature individuals
The number of mature individuals is defined as the number of individuals known, estimated or inferred to be capable of reproduction. When estimating this quantity the following points should be borne in mind:
- Where the population is characterised by natural fluctuations the minimum number should be used.
- This measure is intended to count individuals capable of reproduction and should therefore exclude individuals that are environmentally, behaviourally or otherwise reproductively suppressed in the wild.
- In the case of populations with biased adult or breeding sex ratios it is appropriate to use lower estimates for the number of mature individuals which take this into account (e.g. the estimated effective population size).
- Reproducing units within a clone should be counted as individuals, except where such units are unable to survive alone (e.g. corals).
- In the case of taxa that naturally lose all or a subset of mature individuals at some point in their life cycle, the estimate should be made at the appropriate time, when mature individuals are available for breeding.
- Generation
Generation may be measured as the average age of parents in the population. This is greater than the age at first breeding, except in taxa where individuals breed only once.
- Continuing decline
A continuing decline is a recent, current or projected future decline
whose causes are not known or not adequately controlled and so is liable to continue unless remedial measures are taken. Natural fluctuations will not normally count as a continuing decline, but an observed decline should not be considered to be part of a natural fluctuation unless there is evidence for this.
- Reduction
A reduction (criterion A) is a decline in the number of mature
individuals of at least the amount (%) stated over the time period (years) specified, although the decline need not still be continuing. A reduction should not be interpreted as part of a natural fluctuation unless there is good evidence for this. Downward trends that are part of natural fluctuations will not normally count as a reduction.
- Extreme fluctuations
Extreme fluctuations occur in a number of taxa 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
Severely fragmented refers to the situation where increased extinction risks to the taxon result from the fact that most individuals within a taxon are found in small and relatively isolated subpopulations. These small subpopulations may go extinct, with a reduced probability of recolonisation.
- Extent of occurrence
Extent of occurrence is defined as the area contained within the shortest continuous imaginary boundary which can be drawn to encompass all the known, inferred or projected sites of present occurrence of a taxon, excluding cases of vagrancy. This measure may exclude discontinuities or disjunctions within the overall distributions of taxa (e.g., large areas of obviously unsuitable habitat) (but see 'area of occupancy'). Extent of occurrence can often be measured by a minimum convex polygon (the smallest polygon in which no internal angle exceeds 180 degrees and which contains all the sites of occurrence).
- Area of occupancy
Area of occupancy is defined as the area within its 'extent of occurrence' (see definition) which is occupied by a taxon, excluding cases of vagrancy. The measure reflects the fact that a taxon will not usually occur throughout the area of its extent of occurrence, which may, for example, contain unsuitable habitats. The area of occupancy is the smallest area essential at any stage to the survival of existing populations of a taxon (e.g. colonial nesting sites, feeding sites for migratory taxa). The size of the area of occupancy will be a function of the scale at which it is measured, and should be at a scale appropriate to relevant biological aspects of the taxon. The criteria include values in km2, and thus to avoid errors in classification, the area of occupancy should be measured on grid squares (or equivalents) which are sufficiently small (see Table 1).
- Location
Location defines a geographically or ecologically distinct area in which a single event (e.g. pollution) will soon affect all individuals of the taxon present. A location usually, but not always, contains all or part of a subpopulation of the taxon, and is typically a small proportion of the taxon's total distribution.
- Quantitative analysis
A quantitative analysis is defined here as the technique of population viability analysis (PVA), or any other quantitative form of analysis, which estimates the extinction probability of a taxon or population based on the known life history and specified management or non-management options. In presenting the results of quantitative analyses the structural equations and the data should be explicit.
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