Figure 1: Structure of the Categories
IUCN Red List Categories
I) Introduction
1. The threatened species categories now used in Red Data Books and Red Lists have been in place, with some modification, for almost 30 years. Since their introduction these categories have become widely recognised internationally, and they are now used in a whole range of publications and listings, produced by IUCN as well as by numerous governmental and non-governmental organisations. The Red Data Book categories provide an easily and widely understood method for highlighting those species under higher extinction risk, so as to focus attention on conservation measures designed to protect them.
2. The need to revise the categories has been recognised for some time. In 1984, the SSC held a symposium, ‘The Road to Extinction’ (Fitter & Fitter 1987), which examined the issues in some detail, and at which a number of options were considered for the revised system. However, no single proposal resulted. The current phase of development began in 1989 with a request from the SSC Steering Committee to develop a new approach that would provide the conservation community with useful information for action planning.
In this document, proposals for new definitions for Red List categories are presented. The general aim of the new system is to provide an explicit, objective framework for the classification of species according to their extinction risk.
The revision has several specific aims:
Version 2.0: Mace et al. (1992)
A major revision of Version 1.0, including numerical criteria
appropriate to all organisms and introducing the non-threatened
categories.
Version 2.1: IUCN (1993)
Following an extensive consultation process within SSC, a
number of changes were made to the details of the criteria,
and fuller explanation of basic principles was included. A
more explicit structure clarified the significance of the non-threatened
categories.
Version 2.2: Mace & Stuart (1994)
Following further comments received and additional
validation exercises, some minor changes to the criteria
were made. In addition, the Susceptible category present in
Versions 2.0 and 2.1 was subsumed into the Vulnerable
category. A precautionary application of the system was
emphasized.
Final Version
This final document, which incorporates changes as a
result of comments from IUCN members, was adopted by
the IUCN Council in December 1994.
4. In the rest of this document the proposed system is outlined in several sections. The Preamble presents some basic information about the context and structure of the proposal, and the procedures that are to be followed in applying the definitions to species. This is followed by a section giving definitions of terms used. Finally the definitions are presented, followed by the quantitative criteria used for classification within the threatened categories. It is important for the effective functioning of the new system that all sections are read and understood, and the guidelines followed.
References:
Fitter, R., and M. Fitter, ed. (1987) The Road to Extinction. Gland, Switzerland: IUCN.
IUCN. (1993) Draft IUCN Red List Categories. Gland, Switzerland: IUCN.
Mace, G. M. et al. (1992) “The development of new criteria for listing species on the IUCN Red List.” Species 19: 16–22.
Mace, G. M., and R. Lande. (1991) “Assessing extinction threats: toward a reevaluation of IUCN threatened species categories.” Conserv. Biol. 5.2: 148–157.
Mace, G. M. & S. N. Stuart. (1994) “Draft IUCN Red List Categories, Version 2.2”. Species 21–22: 13–24.
II) Preamble
The following points present important information on the use and interpretation of the categories (= Critically Endangered, Endangered, etc.), criteria (= A to E), and sub-criteria (= a,b etc., i,ii etc.):
1. Taxonomic level and scope of the categorisation process
The criteria can be applied to any taxonomic unit at or below
the species level. The term ‘taxon’ in the following notes,
definitions and criteria is used for convenience, and may
represent species or lower taxonomic levels, including forms
that are not yet formally described. There is a sufficient range
among the different criteria to enable the appropriate listing of
taxa from the complete taxonomic spectrum, with the exception
of micro-organisms. The criteria may also be applied within
any specified geographical or political area although in such
cases special notice should be taken of point 11 below. In
presenting the results of applying the criteria, the taxonomic
unit and area under consideration should be made explicit.
The categorisation process should only be applied to wild
populations inside their natural range, and to populations
resulting from benign introductions (defined in the draft IUCN
Guidelines for Re-introductions as “..an attempt to establish
a species, for the purpose of conservation, outside its recorded
distribution, but within an appropriate habitat and eco-geographical
area”).
2. Nature of the categories
All taxa listed as Critically Endangered qualify for Vulnerable
and Endangered, and all listed as Endangered qualify for
Vulnerable. Together these categories are described as
‘threatened’. The threatened species categories form a part of
the overall scheme. It will be possible to place all taxa into one
of the categories (see Figure 1).
3. Role of the different criteria
For listing as Critically Endangered, Endangered or Vulnerable
there is a range of quantitative criteria; meeting any one of
these criteria qualifies a taxon for listing at that level of threat.
Each species should be evaluated against all the criteria. The
different criteria (A–E) are derived from a wide review aimed
at detecting risk factors across the broad range of organisms
and the diverse life histories they exhibit. Even though some
criteria will be inappropriate for certain taxa (some taxa will
never qualify under these however close to extinction they
come), there should be criteria appropriate for assessing threat
levels for any taxon (other than micro-organisms). The relevant
factor is whether any one criterion is met, not whether all are
appropriate or all are met. Because it will never be clear which
criteria are appropriate for a particular species in advance,
each species should be evaluated against all the criteria, and
any criterion met should be listed.
4. Derivation of quantitative criteria
The quantitative values presented in the various criteria
associated with threatened categories were developed through
wide consultation and they are set at what are generally judged
to be appropriate levels, even if no formal justification for
these values exists. The levels for different criteria within
categories were set independently but against a common
standard. Some broad consistency between them was sought.
However, a given taxon should not be expected to meet all
criteria (A–E) in a category; meeting any one criterion is
sufficient for listing.
5. Implications of listing
Listing in the categories of Not Evaluated and Data Deficient
indicates that no assessment of extinction risk has been made,
though for different reasons. Until such time as an assessment
is made, species listed in these categories should not be treated
as if they were non-threatened, and it may be appropriate
(especially for Data Deficient forms) to give them the same
degree of protection as threatened taxa, at least until their
status can be evaluated.
Extinction is assumed here to be a chance process. Thus, a listing in a higher extinction risk category implies a higher expectation of extinction, and over the time-frames specified more taxa listed in a higher category are expected to go extinct than in a lower one (without effective conservation action). However, the persistence of some taxa in high risk categories does not necessarily mean their initial assessment was inaccurate.
6. Data quality and the importance of inference and projection
The criteria are clearly quantitative in nature. However, the
absence of high quality data should not deter attempts at
applying the criteria, as methods involving estimation, inference
and projection are emphasised to be acceptable throughout.
Inference and projection may be based on extrapolation of
current or potential threats into the future (including their rate
of change), or of factors related to population abundance or
distribution (including dependence on other taxa), so long as
these can reasonably be supported. Suspected or inferred patterns
in either the recent past, present or near future can be based on
any of a series of related factors, and these factors should be
specified.
Taxa at risk from threats posed by future events of low probability but with severe consequences (catastrophes) should be identified by the criteria (e.g. small distributions, few locations). Some threats need to be identified particularly early, and appropriate actions taken, because their effects are irreversible, or nearly so (pathogens, invasive organisms, hybridization).
7. Uncertainty
The criteria should be applied on the basis of the available
evidence on taxon numbers, trend and distribution, making
due allowance for statistical and other uncertainties. Given
that data are rarely available for the whole range or population
of a taxon, it may often be appropriate to use the information
that is available to make intelligent inferences about the overall
status of the taxon in question. In cases where a wide variation
in estimates is found, it is legitimate to apply the precautionary
principle and use the estimate (providing it is credible) that
leads to listing in the category of highest risk.
Where data are insufficient to assign a category (including Lower Risk), the category of ‘Data Deficient’ may be assigned. However, it is important to recognise that this category indicates that data are inadequate to determine the degree of threat faced by a taxon, not necessarily that the taxon is poorly known. In cases where there are evident threats to a taxon through, for example, deterioration of its only known habitat, it is important to attempt threatened listing, even though there may be little direct information on the biological status of the taxon itself. The category ‘Data Deficient’ is not a threatened category, although it indicates a need to obtain more information on a taxon to determine the appropriate listing.
8. Conservation actions in the listing process
The criteria for the threatened categories are to be applied to
a taxon whatever the level of conservation action affecting it.
In cases where it is only conservation action that prevents the
taxon from meeting the threatened criteria, the designation of
‘Conservation Dependent’ is appropriate. It is important to
emphasise here that a taxon require conservation action even
if it is not listed as threatened.
9. Documentation
All taxon lists including categorisation resulting from these
criteria should state the criteria and sub-criteria that were met.
No listing can be accepted as valid unless at least one criterion
is given. If more than one criterion or sub-criterion was met,
then each should be listed. However, failure to mention a
criterion should not necessarily imply that it was not met.
Therefore, if a re-evaluation indicates that the documented
criterion is no longer met, this should not result in automatic
down-listing. Instead, the taxon should be re-evaluated with
respect to all criteria to indicate its status. The factors responsible
for triggering the criteria, especially where inference and
projection are used, should at least be logged by the evaluator,
even if they cannot be included in published lists.
10. Threats and priorities
The category of threat is not necessarily sufficient to determine
priorities for conservation action. The category of threat
simply provides an assessment of the likelihood of extinction
under current circumstances, whereas a system for assessing
priorities for action will include numerous other factors
concerning conservation action such as costs, logistics, chances
of success, and even perhaps the taxonomic distinctiveness of
the subject.
11. Use at regional level
The criteria are most appropriately applied to whole taxa at a
global scale, rather than to those units defined by regional or
national boundaries. Regionally or nationally based threat
categories, which are aimed at including taxa that are threatened
at regional or national levels (but not necessarily throughout
their global ranges), are best used with two key pieces of
information: the global status category for the taxon, and the
proportion of the global population or range that occurs
within the region or nation. However, if applied at regional or
national level it must be recognised that a global category of
threat may not be the same as a regional or national category
for a particular taxon. For example, taxa classified as Vulnerable
on the basis of their global declines in numbers or range might
be Lower Risk within a particular region where their
populations are stable. Conversely, taxa classified as Lower
Risk globally might be Critically Endangered within a particular
region where numbers are very small or declining, perhaps
only because they are at the margins of their global range.
IUCN is still in the process of developing guidelines for the use
of national red list categories.
12. Re-evaluation
Evaluation of taxa against the criteria should be carried out at
appropriate intervals. This is especially important for taxa
listed under Near Threatened, or Conservation Dependent,
and for threatened species whose status is known or suspected
to be deteriorating.
13. Transfer between categories
There are rules to govern the movement of taxa between
categories. These are as follows: (A) A taxon may be moved
from a category of higher threat to a category of lower threat
if none of the criteria of the higher category has been met for
five years or more. (B) If the original classification is found to
have been erroneous, the taxon may be transferred to the
appropriate category or removed from the threatened categories
altogether, without delay (but see Section 9). (C) Transfer from
categories of lower to higher risk should be made without
delay.
14. Problems of scale
Classification based on the sizes of geographic ranges or the
patterns of habitat occupancy is complicated by problems of
spatial scale. The finer the scale at which the distributions or
habitats of taxa are mapped, the smaller the area will be that
they are found to occupy. Mapping at finer scales reveals more
areas in which the taxon is unrecorded. It is impossible to
provide any strict but general rules for mapping taxa or habitats;
the most appropriate scale will depend on the taxa in question,
and the origin and comprehensiveness of the distributional
data. However, the thresholds for some criteria (e.g. Critically
Endangered) necessitate mapping at a fine scale.
III) Definitions
1. 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.
2. 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).
3. 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:
• 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.
5. 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.
6. 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.
7. 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).
8. 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.
9. 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).
10. 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 km 2 , and thus to avoid errors in classification, the
area of occupancy should be measured on grid squares (or
equivalents) which are sufficiently small (see Figure 2).
11. 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.
12. 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.
IV) The Categories 1
EXTINCT (EX)
A taxon is Extinct when there is no reasonable doubt that the
last individual has died.
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) on pages 94–95.
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) on
page 95.
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
D) on pages 95 and 96.
LOWER RISK (LR)
A taxon is Lower Risk when it has been evaluated, 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.
V) The Criteria for Critically Endangered, Endangered and Vulnerable
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 following criteria (A to E):
A) Population reduction in the form of either of the following:
E) Quantitative analysis showing the probability of extinction in the wild is at least 50% within 10 years or three generations, whichever is the longer.
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 following criteria
(A to E):
A) Population reduction in the form of either of the following:
2) Continuing decline, inferred, observed or projected, in any of the following:
2) A continuing decline, observed, projected, or inferred, in numbers of mature individuals and population structure in the form of either:
E) Quantitative analysis showing the probability of extinction
in the wild is at least 20% within 20 years or five generations,
whichever is the longer.
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 following
criteria (A to E):
A) Population reduction in the form of either of the following:
2) Continuing decline, inferred, observed or projected, in any of the following:
2) A continuing decline, observed, projected, or inferred, in numbers of mature individuals and population structure in the form of either:
2) Population is characterised by an acute restriction in its area of occupancy (typically less than 100km2) or in the number of locations (typically less than five). Such a taxon would thus be prone to the effects of human activities (or stochastic events whose impact is increased by human activities) within a very short period of time in an unforeseeable future, and is thus capable of becoming Critically Endangered or even Extinct in a very short period.
Note: copies of the IUCN Red List Categories booklet, are available on request from IUCN (address on back cover of this Action Plan)
* Note: As in previous IUCN categories, the abbreviation of each category (in parenthesis) follows the English denominations when translated into other languages.