IUCN Red List Guidelines - The IUCN Red List of Threatened Species

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Red List Guidelines 46 VU) apply, which may lead to a down-listing of the taxon that would reflect the fact that it is currently stable or increasing, (2) uncertainty in the data (particularly long-term historical data) if properly incorporated into the assessment may affect the outcome of the listing (see section 3.2). 5.4 Severely depleted populations Some taxa (particularly marine taxa) show persistence at very low fractions of their unexploited equilibrium or carrying capacity. These taxa may not qualify under criterion A1 or A2 because their declines occurred more than three generations ago, and they may be too widespread and abundant to qualify under any other criteria. Nevertheless, they may be more cause for concern because they are more susceptible to unforeseen catastrophic events and marine taxa may be harvested as bycatch in other fisheries. Such taxa are not currently being assessed as threatened under the criteria A1 and A2, although they may still qualify under criteria A3, A4, B, C, D or E. Taxa in this situation may be assessed under criteria A3 or A4 based on projected or suspected population declines in the future, provided there is sufficient evidence for the threats faced by the taxon or the likely decline rate of the taxon to warrant such a listing. The category Near Threatened could also be used if a taxon nearly qualifies as Vulnerable under A3 or A4. It must be remembered however that the IUCN Red List Criteria are designed to identify taxa that exhibit symptoms of endangerment, and not simply depletion or conservation priority. The problem of assessing these taxa is also related to the scaling issues discussed under the definition of area of occupancy (section 4.10), which affects the application of criterion B. If an appropriate taxon-specific scaling factor is used, severely depleted marine taxa may qualify as threatened under criterion B. 5.5 Fisheries Taxa that are the target of fisheries may show a decline in population size due to intentional management action. Under the Red List Criteria, such taxa could be assigned a threatened status under criterion A (declining population). Concern has been expressed that such a listing might not reflect extinction risk, especially if the decline is a consequence of a management plan designed to achieve a goal such as the maximisation of sustainable yield from a fishery. Such listings should not be problematic in the medium- to long-term because, if the fishery is managed effectively, although it currently exhibits symptoms consistent with endangerment, the population will eventually stabilize at a target level and the decline will end, such that the taxon would then no longer qualify for listing. If declines continued, there would be reason for concern; in this case a new assessment, against all five criteria, may indicate that the taxon is still threatened. In addition, fisheries that are being managed sustainably should be assessed under criterion A1, where the higher thresholds of 90% (CR), 70% (EN) and 50% (VU) make it less likely that they will be classified as threatened. 5.6 Trees The generation length of some tree species can exceed 100 years. It is difficult to estimate population declines from a point in time before which the species populations or even the
Red List Guidelines 47 species itself may have been recorded. It is important to emphasize the point that the most significant declines, which are useful to record and which may be possible to reverse, are probably those that have been caused over the last 100 years. 5.7 Relationship between loss of habitat and population reduction Under criterion A, a reduction in population size may be based on a decline in area of occupancy, extent of occurrence and/or quality of habitat. The assumptions made about the relationship between habitat loss and population reduction have an important effect on the outcome of an assessment. In particular, the simplest assumption, that the relationship is linear, is not often true and may lead to over- or under-listing. For example, a bird species may not be reduced by 50% if 50% of its habitat is lost (perhaps because it will colonize new habitats). Or, reduction may happen mostly in lower-density areas, leading to a faster decline in range than in population size. Conversely, if reductions occur predominantly in highdensity areas, population reduction will be faster than can be deducted from range contraction (decrease in EOO) (Rodríguez 2002). Similarly, a coral reef fish may be reduced by more than 50% if 50% of its habitat is lost through fishing with explosives (perhaps because spawning areas have been destroyed). The sensible use of inference and projection is encouraged when estimating population reductions from changes in habitat. For example, if a forest species' extent of occurrence has been 70% clear cut in the last five years it might be justified to infer a 50% decline in the population over the past ten years. The species would therefore qualify as Endangered A2c. In all cases, an understanding of the taxon and its relationship to its habitat, and the threats facing the habitat is central to making the most appropriate assumptions about habitat loss and subsequent population reduction. All assumptions about this relationship, and the information used should be included with the assessment documentation. Available population data may contradict habitat data (e.g., habitat seems to be declining in quality, but population numbers are stable). This can occur because: (1) one set of data is uncertain, biased, or dated, or (2) the population has a lagged response to loss of habitat (likely if generation time is long). In the first case, the assessors must use their judgement to decide which data are more certain. If it is decided that the abundance data are adequate to determine trends, the taxon should be listed under criterion A2. The implications of a possible lagged response in abundance to loss of habitat should, however, be considered when evaluating the taxon under criterion A3. For example, if population reduction in the last three generations is 30% based on abundance data, which are adequate to determine trends, then the species should be listed as VU A2, even if habitat loss in the same period was 60%. However, if a lagged response in abundance to loss of habitat is likely (i.e., the impact of habitat loss at present may lead to a future reduction in the number of mature individuals), then the population may be expected to decline further in the future (even if habitat loss has stopped), so an EN A3 or EN A4 listing should be considered as well, if the 60% loss of habitat is inferred to lead to 50% or more reduction in the number of mature individuals.
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IUCN Red List Guidelines - The IUCN Red List of Threatened Species

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