Categories and Criteria – Frequently Asked Questions

Frequently Asked Questions


This section answers commonly-raised questions in workshops and feedback to the Red List regarding the appropriate Red List category in certain circumstances. The below does not cover questions about definitions of key Red List terms, for which you should refer to the presentation in the Categories and Criteria page.


Q: I am aware of evidence that a species proposed as Least Concern is subject to significant threats and appears to be in decline. How was the Least Concern listing derived?


A: The Red List is often described loosely as a measure of a species’ conservation status, however in this context it is important to be explicit that it actually measures a specific, definable property: a species’ risk of extinction in the immediate future. The value of the Red List lies in its ability to apply quantitative criteria with fixed thresholds to extinction risk assessments, and so the short answer is that the species in this case did not meet any of the thresholds for listing in a threatened category. Because we’re measuring the risk of extinction, typically on a global scale, the thresholds for listing a species in a threatened category are rather high (e.g. 30% of the population must have been lost within three generations to qualify for Vulnerable under the A Criterion), and a species will not meet these criteria unless it has a genuine likelihood of becoming globally (or regionally, in the case of regional assessments) extinct in a short timeframe.  As a result a Least Concern listing should in no way be taken to imply that a species is of no conservation concern, or even at no possible risk of extinction. A Least Concern species is, literally, at less risk of extinction than a threatened species, but it may very well still be deserving of conservation attention or undergoing significant declines.


Another possibility in the above case is that these threats have become operational, or the evidence for decline has emerged, since the species was last assessed. If a change in assessment category appears to be warranted on this basis, please contact the relevant IUCN Specialist Group’s Red List Authority Coordinator.


Q: A species is listed as Least Concern on the Red List, however this is nationally protected or subject to other conservation actions because it is perceived to be at risk. How was the Least Concern listing derived?


A: Most Red List assessments are conducted at global scales, and those assessments conducted at regional scales are often at scales larger than individual countries. A species’ situation in a particular country may be very different from its global status (particularly at range margins). Additionally, the Red List is only a component of defining conservation priorities, and conservation measures are often very validly imposed for reasons other than a need to prevent complete extinction, such as ensuring sustainable management or preventing population decline.


Q: Under what circumstances can a species be listed as Data Deficient on the basis of taxonomic uncertainty?


A: Taxonomic uncertainty takes many forms, most of which are not relevant to determining an appropriate assessment category. If a species concept exists, it can be assessed. The concept may be suspected or known to represent a complex, however where the complex is recognized as a single species pending taxonomic action it can be assessed as though it were genuinely a single species. Issues of uncertain allocation of records that affect a species’ distributional extent may affect its threat status, however if the species conservatively defined would fall into the same category as it would under a more liberal definition (e.g. if its range will exceed the threshold for listing under the B Criterion whatever the identity of uncertain records, or if it is not subject to any threats however narrow its range, it will be Least Concern whatever the correct taxonomic concept), it can be assessed. Taxonomic issues in themselves will only justify a Data Deficient listing if there is genuinely not sufficient certainty in what the species concept represents to determine whether a species is Least Concern or threatened.


Q: You propose that a particular species be listed as Data Deficient, however there appears to be quite a lot of information available in its account. Can you explain the reasoning for this?


As with “Least Concern”, the name “Data Deficient” is defined with reference to what the Red List measures, i.e. a species is Data Deficient if the available data cannot be used to determine whether it is at significant risk of extinction. This is commonly the case for very poorly-known taxa, but by no means exclusively so. A species may, for example, be too widespread to qualify for listing on the basis of the geographic range criterion (Criterion B), and thought to be too populous to qualify under Criteria C or D, however it may not be possible to quantify rates of decline or generation length sufficiently to determine whether it is likely to be threatened under Criterion A or to conduct modeling of population trends (Criterion E).  It is important to stress that a large degree of genuine uncertainty over the appropriate category must exist; in the above example it would not be sufficient, for instance, to list that species as Data Deficient simply because there is no data on rates of population decline, rather there must be reason to believe that a realistic possibility exists that – if the data were available – they would show a sufficiently severe decline to warrant listing the species in a threatened category.


Q: When can climate change be invoked as a future threat to list a species as Vulnerable under to Criterion D2?


Climate change is, quite reasonably, increasingly proposed as a threat to species’ persistence as projections of future climate impacts improve. Most climate projections are based on timescales well outside those relevant for Red List assessments of all but the longest-lived animals and some plants. For terrestrial species typically cited threats are from sea level rise (projected to be under a metre over the next century), long-term changes in habitat quality, and upslope displacement (dependent on changes in temperature that represent likely displacement of only tens of metres over the same timescale). While climate change should be mentioned as a future threat in the assessment text where relevant, except for species on mountain summits, those in very low-lying areas at risk from storm surges, or cases where there is evidence that ongoing climate change is associated with declines in a species or its habitat, it cannot be used by itself to justify a threatened category.