Terms & Definitions - Red List Index

Click on any of the following for some background and an explanation of the technical terms used in the species factsheets and additional data tables:

Red List Index

The Red List Index measures change in aggregate extinction risk across groups of species. It is based on genuine changes in the number of species in each category of extinction risk on The IUCN Red List of Threatened Species. The Red List Index value ranges from 1 (all species are categorized as ‘Least Concern’) to 0 (all species are categorized as ‘Extinct’), and so indicates how far the set of species has moved overall towards extinction. Thus, the Red List Index allows comparisons between sets of species in both their overall level of extinction risk (i.e. how threatened they are on average), and in the rate at which this risk changes over time. A downward trend in the Red List Index over time means that the expected rate of future species extinctions is worsening (i.e. the rate of biodiversity loss is increasing). An upward trend means that the expected rate of species extinctions is abating (i.e. the rate of biodiversity loss is decreasing), and a horizontal line means that the expected rate of species extinctions is remaining the same, although in each of these cases it does not mean that biodiversity loss has stopped. A Red List Index value of 1 would indicate that biodiversity loss has been halted.

Threatened species are those listed on The IUCN Red List of Threatened Species in the categories Vulnerable, Endangered, or Critically Endangered (i.e. species that are facing a high, very high, or extremely high risk of extinction in the wild in the medium-term future). Changes over time in the proportion of species threatened with extinction are largely driven by improvements in knowledge and changing taxonomy. The indicator excludes such changes to yield a more informative indicator than the simple proportion of threatened species. It therefore measures change in aggregate extinction risk across groups of species over time, resulting from genuine improvements or deteriorations in the status of individual species. It can be calculated for any representative set of species that have been assessed for The IUCN Red List of Threatened Species at least twice (Butchart et al. 2004, 2005, 2007).

The main limitation of the Red List Index is related to the fact that the Red List Categories are relatively broad measures of status, and thus the Red List Index for any individual taxonomic group can practically be updated at intervals of at least four years. As the overall index is aggregated across multiple taxonomic groups, it can be updated typically annually. In addition, the Red List Index does not capture particularly well the deteriorating status of common species that remain abundant and widespread but are declining slowly.



The Red List Index is calculated at a point in time by first multiplying the number of species in each Red List Category by a weight (ranging from 1 for ‘Near Threatened’ to 5 for ‘Extinct’ and ‘Extinct in the Wild’) and summing these values. This is then divided by a maximum threat score which is the total number of species multiplied by the weight assigned to the ‘Extinct’ category. This final value is subtracted from 1 to give the Red List Index value.

Mathematically this calculation is expressed as:

RLIt = 1 – [(Ss Wc(t,s) / (WEX * N)]

Where Wc(t,s) is the weight for category (c) at time (t) for species (s) (the weight for ‘Critically Endangered’ = 4, ‘Endangered’ = 3, ‘Vulnerable’ = 2, ‘Near Threatened’ = 1, ‘Least Concern’ = 0. ‘Critically Endangered’ species tagged as ‘Possibly Extinct’ or ‘Possibly Extinct in the Wild’ are assigned a weight of 5); WEX = 5, the weight assigned to ‘Extinct’ or ‘Extinct in the Wild’ species; and N is the total number of assessed species, excluding those assessed as Data Deficient in the current time period, and those considered to be ‘Extinct’ in the year the set of species was first assessed.

The formula requires that:

  • Exactly the same set of species is included in all time periods, and
  • The only Red List Category changes are those resulting from genuine improvement or deterioration in status (i.e. excluding changes resulting from improved knowledge or taxonomic revisions), and
  • Data Deficient species are excluded.

In many cases, species lists will change slightly from one assessment to the next (e.g. owing to taxonomic revisions). The conditions can therefore be met by retrospectively adjusting earlier Red List categorizations using current information and taxonomy. This is achieved by assuming that the current Red List Categories for the taxa have applied since the set of species was first assessed for the Red List, unless there is information to the contrary that genuine status changes have occurred. Such information is often contextual (e.g. relating to the known history of habitat loss within the range of the species). If there is insufficient information available for a newly added species, it is not incorporated into the Red List Index until it is assessed for a second time, at which point earlier assessments are retrospectively corrected by extrapolating recent trends in population, range, habitat and threats, supported by additional information. To avoid spurious results from biased selection of species, Red List Indices are typically calculated only for taxonomic groups in which all species worldwide have been assessed for the Red List, or for samples of species that have been systematically or randomly selected.

The methods and scientific basis for the Red List Index were described by Butchart et al. (2004, 2005, 2007, 2010).

Butchart et al. (2010) also described the methods by which Red List Indices for different taxonomic groups are aggregated to produce a single multi-taxon Red List Index. Specifically, aggregated Red List Indices are calculated as the arithmetic mean of modelled Red List Indices. Red List Indices for each taxonomic group are interpolated linearly for years between data points and extrapolated linearly (with a slope equal to that between the two closest assessed points) to align them with years for which Red List Indices for other taxa are available.

The Red List Indices for each taxonomic group for each year are modelled to take into account various sources of uncertainty:
(i) Data Deficiency: Red List categories (from Least Concern to Extinct) are assigned to all Data Deficient species, with a probability proportional to the number of species in non-Data Deficient categories for that taxonomic group.
(ii) Extrapolation uncertainty: although Red List Indices are extrapolated linearly based on the slope of the closest two assessed point, there is uncertainty about how accurate this slope may be. To incorporate this uncertainty, rather than extrapolating deterministically, the slope used for extrapolation is selected from a normal distribution with a probability equal to the slope of the closest two assessed points, and standard deviation equal to 60% of this slope.
(iii) Temporal variability: the ‘true’ RLI likely changes from year to year, but because assessments are repeated only at multi-year intervals, the precise value for any particular year is uncertain. To make this uncertainty explicit, the Red List Index value for a given taxonomic group in a given year is assigned from a moving window of five years, centered on the focal year. The 95% confidence intervals are calculated from 1,000 replications of a bootstrapping procedure taking into account these various sources of uncertainty. Assessment uncertainty cannot yet be incorporated into the index.

The Red List Index can be downscaled to show national and regional Red List Indices, weighted by the fraction of each species’ distribution occurring within the country or region, building on the method published by Rodrigues et al. (2014) PLoS ONE 9(11): e113934. These show an index of aggregate survival probability (the inverse of extinction risk) for all birds, mammals, amphibians, corals and cycads occurring within the country or region. The index shows how well species are conserved in a country or region to its potential contribution to global species conservation.

The index is calculated as:

RLI(t,u) = 1 – [(Ss(W(t,s) * (rsu/Rs)) / (WEX * Ss (rsu/Rs))

Where t is the year of comprehensive reassessment, u is the spatial unit (i.e. country), W_((t,s)) is the weight of the global Red List category for species s at time t (Least Concern =0, Near Threatened =1, Vulnerable =2, Endangered =3, Critically Endangered =4, Critically Endangered (Possibly Extinct) =5, Critically Endangered (Possibly Extinct in the Wild) =5, Extinct in the Wild =5 and Extinct =5), WEX = 5 is the weight for Extinct species, r_su is the fraction of the total range of species s in unit u, and R_s is the total range size of species s.

The index varies from 1 if the country has contributed the minimum it can to the global RLI (i.e. if the numerator is 0 because all species in the country are LC) to 0 if the country has contributed the maximum it can to the global RLI (i.e. if the numerator equals the denominator because all species in the country are Extinct or Possibly Extinct).

The taxonomic groups included are those in which all species have been assessed for the IUCN Red List more than once. Red List categories for years in which comprehensive assessments (i.e. those in which all species in the taxonomic group have been assessed) were carried out are determined following the approach of Butchart et al. 2007; PLoS ONE 2(1): e140, i.e. they match the current categories except for those taxa that have undergone genuine improvement or deterioration in extinction risk of sufficient magnitude to qualify for a higher or lower Red List category.

The indicator can also be disaggregated by ecosystems, habitats, and other political and geographic divisions (e.g. Han et al. 2014), by taxonomic subsets (e.g. Hoffmann et al. 2011), by suites of species relevant to particular international treaties or legislation (e.g. Croxall et al. 2012), by suites of species exposed to particular threatening processes (e.g. Butchart 2008), and by suites of species that deliver particular ecosystem services, or have particular biological or life-history traits (e.g. Regan et al. 2015). In each case, information can be obtained from The IUCN Red List of Threatened Species to determine which species are relevant to particular subsets (e.g. which occur in particular ecosystems, habitats, and geographic areas of interest).

Full details of the methodology can be found at https://unstats.un.org/sdgs/metadata/files/Metadata-15-05-01.pdf

The RLI was initially designed and tested using data on all bird species (Butchart el al 2004) and then extended to amphibians (Butchart et al. 2005). The methodology was revised and improved in 2007 (Butchart et al. 2007), with methods for aggregating across taxonomic groups and for calculating confidence intervals published in 2010 (Butchart et al. 2010). RLIs for additional groups have been added subsequently. RLIs have been published showing the negative impacts of invasive species (McGeoch et al. 2010) and trade (Butchart 2008), and the positive impacts of conservation action (Hoffmann et al. 2010) and protected areas (Butchart et al. 2012). An RLI to show the impact of a single conservation institution was published by Young et al. (2014). The spatial distribution of the RLI was mapped by Rodrigues et al. (2014). An RLI for pollinators was published by Regan et al. (2015), and for wild relatives of domesticated and farmed species by McGowan et al. (2018). For poorly known, species-rich groups (e.g. insects, fungi, plants, etc), a sampled approach to Red Listing has been developed (Baillie et al. 2008). Once the sample of species in these groups are reassessed, RLIs for will be calculated.



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REGAN, E. et al. (2015). Global trends in the status of bird and mammal pollinators. Conservation Letters. doi: 10.1111/conl.12162. Available from https://conbio.onlinelibrary.wiley.com/doi/full/10.1111/conl.12162

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