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8.5: Which Species are at Risk of Extinction?

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    The IUCN has formalised the evaluation of threatened species using an internationally accepted standard of conservation categories describing a taxon’s risk of extinction.

    An important task for conservation biologists is to identify and prioritise those species in greatest danger of extinction. Accomplishing this task requires biologists to collect and review all the information we have on each species. To facilitate this major undertaking, the IUCN has formalised the evaluation and reporting of threatened species assessments using an internationally accepted standard of conservation categories to reflect a taxon’s risk of extinction. These nine categories (Figure 8.7), known as Red List Assessments (IUCN, 2017), are:

    • Extinct (EX). These species are no longer known to exist. As of mid-2019, the IUCN has listed 84 Sub-Saharan African species as Extinct.
    • Extinct in the Wild (EW). These species exist only in cultivation, in captivity, or other human-managed situations. As of mid-2019, the IUCN has listed nine Sub-Saharan African species as Extinct in the Wild.
    • Critically Endangered (CR). These species have an extremely high risk of going extinct in the wild. As of mid-2019, the IUCN has listed 880 Sub-Saharan African species as Critically Endangered. Also included in this category are the 202 Sub-Saharan African species that the IUCN considered possibly Extinct.
    • Endangered (EN). These species have a very high risk of extinction in the wild. As of mid-2019, the IUCN has listed 1,600 Sub-Saharan African species as Endangered.
    • Vulnerable (VU). These species have a moderately high risk of extinction in the wild. As of mid-2019, the IUCN has listed 2,153 Sub-Saharan African species as Vulnerable.
    • Near Threatened (NT). These species are close to qualifying for a threatened category but are not currently considered threatened. As of mid-2019, the IUCN has listed 1,034 Sub-Saharan African species as Near Threatened.
    • Data Deficient (DD). Inadequate information exists to determine the risk of extinction for these species. As of mid-2019, the IUCN has listed 2,441 Sub-Saharan African species as Data Deficient.
    • Least Concern (LC). These species are not considered Near Threatened or threatened. (Widespread and abundant species are included in this category.) As of mid-2019, the IUCN has listed 11,776 Sub-Saharan African species as Least Concern.
    • Not Evaluated (NE). Species that have not yet been evaluated. Most species fall in this category.
    Figure 8.7 Flow diagram illustrating the structure of the IUCN categories of conservation status. An evaluated species can be considered at lower risk of extinction, at high risk of extinction (i.e. threatened), or extinct. A species for which not enough data are available for evaluation is considered Data Deficient (DD). After IUCN, 2017, CC BY 4.0.

    These categories, and the Red List Criteria (Table 8.1) used to classify each species, are broadly based on population viability analysis (Section 9.2), and consider population size, population trends, and habitat availability. Species that are Extinct in the Wild, Critically Endangered, Endangered, and Vulnerable categories are officially considered “threatened with extinction”. The advantage of this system is that it provides a standard protocol by which decisions can be reviewed and evaluated according to widely accepted yet flexible criteria. Consequently, species, subspecies, varieties, populations, and subpopulations can be assessed on a global or regional level, all under a unified set of standards. The resultant threat status assessment forms the basis of Red Data Books and Red Lists: detailed lists of threatened wildlife by group and/or by region compiled by the IUCN and its affiliate organizations. All global (and many regional) Red List assessments are freely available at, with feedback links provided from which anyone can alert the IUCN if they find errors or have suggestions for improvements.

    Table 8.1 The IUCN’s Red List criteria for evaluating a taxon’s threat status. A species that meet any one of criteria A–E could be classified as Critically Endangered.

    Red List criteria A–E

    Summary criteria used to evaluate a taxon as Critically Endangereda

    A. Population size declining

    The population size has declined by 90% (or more) over last 10 years or 3 generations (whichever is longer).

    B. Geographical range declining

    The species is restricted to < 100 km2 and it occurs at a single location and its distribution range is observed/expected to decline.

    C. Small and declining populations

    There are less than 250 mature individuals left and population has declined by 25% (or more) over last 3 years or 1 generation (whichever is longer).

    D. Small populations

    There are less than 50 mature individuals left.

    E. Population viability analysis

    There is a 50% (or greater) chance of extinction within 10 years or 3 generations.

    a Additional criteria for Critically Endangered, as well as criteria for Endangered and Vulnerable listings can be found at

    While nearly 20,000 Sub-Saharan African species have been evaluated as of mid-2019 (IUCN, 2019), these assessments only cover a small proportion of the region’s overall biodiversity. Consider for example that as of mid-2019, just over 4,900 Sub-Saharan Africa’s plants have been listed on the IUCN Red List website. Yet, the Cape Floristic Region alone hosts over 6,200 endemic plant species. The assessment gaps are even more conspicuous for lesser-known taxa; for example, only seven species of bryophytes (a group of non-vascular plants that includes mosses) have been assessed as of mid-2019; some readers of this textbook will have more bryophyte species in their gardens. The reasons for such assessment gaps are many, but most boil down to manpower and funding limitations, which restrict our ability to obtain the data needed for comprehensive assessments. It is thus important to understand that the lack of information on these and other poorly known groups does not mean there is no threat. For example, as of mid-2019, no African abalone (Haliotis spp.) have been assessed, even though these highly valued molluscs are some of Africa’s most heavily exploited (and heavily poached) marine organisms (Minnaar et al., 2018). A lack of information about a species’ threats and populations trends is thus a good argument that more studies are needed, sometimes urgently. Similarly, continued monitoring of species thought to be common is also important, as it can shed light on how new threats may emerge or escalate over time.

    Course-filter assessments

    To fill Red List species assessment gaps, conservation biologists are increasingly relying on broader metrics, or coarse-filter assessments, to identify groups of species that are threatened with extinction. One such method, which reduces the need to evaluate every individual species, is to identify ecosystems that are threatened. This premise rests on the assumption that any threatened ecosystem will contain many threatened species. Hence, protecting and restoring threatened ecosystems will simultaneously allow many populations living in those ecosystems to recover. To facilitate this type of coarse-filter assessment, the IUCN recently established a Red List of Ecosystems (RLE, The RLE assesses ecosystem status against five criteria: (1) distribution declines, (2) distribution restrictions, (3) environmental degradation, (4) disruption of ecological processes and interactions, and (5) quantitative estimates for risk of ecosystem collapse (Keith et al., 2013). While the ecosystem assessment protocol was only recently developed—only three African ecosystems have been assessed as of mid-2019—its holistic strategy promises a more comprehensive accounting of local biodiversity which could be more informative than an accumulation of single species assessments.

    This page titled 8.5: Which Species are at Risk of Extinction? is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by John W. Wilson & Richard B. Primack (Open Book Publishers) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.