Measuring trends in extinction risk: a review of two decades of development and application of the Red List Index

Abstract

The Red List Index (RLI) is an indicator of the average extinction risk of groups of species and reflects trends in this through time. It is calculated from the number of species in each category on the IUCN Red List of Threatened Species, with trends influenced by the number moving between categories when reassessed owing to genuine improvement or deterioration in status. The global RLI is aggregated across multiple taxonomic groups and can be disaggregated to show trends for subsets of species (e.g. migratory species), or driven by particular factors (e.g. international trade). National RLIs have been generated through either repeated assessments of national extinction risk in each country or through disaggregating the global index and weighting each species by the proportion of its range in each country. The RLI has achieved wide policy uptake, including by the Convention on Biological Diversity and the United Nations Sustainable Development Goals. Future priorities include expanding its taxonomic coverage, applying the RLI to the goals and targets of the Kunming-Montreal Global Biodiversity Framework, incorporating uncertainty in the underlying Red List assessments, integrating into national RLIs the impact of a country on species' extinction risk abroad, and improving analysis of the factors driving trends.This article is part of the discussion theme issue 'Bending the curve towards nature recovery: building on Georgina Mace's legacy for a biodiverse future'.

Keywords: IUCN Red List; biodiversity conservation; extinction risk; indicator; multilateral environmental agreement.

Figure 1.

Red List Indices of species survival (a) aggregated across five groups: mammals (5899 species), birds (11 147), amphibians (8011), warm-water reef-building corals (859) and cycads (340); (b) for each of these taxonomic groups individually; (c) disaggregated by realm: terrestrial (23 490 species), freshwater (7032), marine (1840); and (d) disaggregated by region: Africa (5704 species), Americas (10 703), Asia and the Pacific (9707) and Europe and Central Asia (2040). Shading shows confidence intervals.

Figure 2.

Red List Indices of species survival for: (a) bird species recorded in international trade (4318 species), bird (1170) and mammal species (367) known to pollinate plants, migratory bird (2199) and mammal (205) species, and bird (240) and mammal species (58) that are wild relatives of farmed and domesticated species; (b) mammals (5899 species), birds (11 147), amphibians (8011), warm-water reef-building corals (859) and cycads (340), showing trends driven by agriculture or invasive alien species; (c) South Africa, based on repeated assessments of national extinction risk for birds (732 species), mammals (336), amphibians (126), reptiles (397), freshwater fishes (118), dragonflies (163), butterflies (799), a sample of plants (900) and an aggregated index across all these groups (3571); and (d) South Africa, showing the global RLI disaggregated for this country, with each species (312 mammals, 751 birds, 126 amphibians, 97 corals and 38 cycads) weighted by the proportion of its global range within the country. Shading shows confidence intervals.