News & Views | Published:

Conservation biology

Biodiversity barometers

The Red List Index is a new indicator of species' extinction risk. It will make a major contribution in measuring the success of an internationally agreed aim to slow biodiversity loss by 2010.

In April 2002, the 188 parties to the Convention on Biological Diversity agreed to reduce the rate of biodiversity loss significantly by 2010. This commitment covers all of the world's nations except Andorra, Brunei, Iraq, Somalia, Timor-Leste, the United States and the Vatican, and was endorsed by the World Summit on Sustainable Development later that year1. The 2010 target is a ground-breaking advance, in that the world's governments are holding themselves publicly accountable for biodiversity conservation.

The target is also challenging. First, it will be extremely hard to achieve. Second, it is not even obvious how, in 2010, we will be able to determine whether or not it has been reached. Indicators for doing so will effectively require the use of existing data sets, because assessment of a change in the rate of biodiversity loss will require a minimum of three temporal data points. They will also need to have comprehensive geographical extent but fine ecological resolution. Further, they should respond rapidly to changes. Writing in PloS Biology, Butchart et al.2 make a major contribution towards the development of such barometers for biodiversity in proposing a ‘Red List Index’, based on the assessment of species' extinction risk provided by the IUCN Red List3.

Butchart and colleagues' contribution does not come in a vacuum. In February this year, the parties to the convention approved eight indicators for immediate testing4. Two of these measure threats to biodiversity (nitrogen deposition and water quality), two measure responses (protected area coverage and international conservation funding), and one measures cultural context (language diversity). The other three are the most relevant to assess progress towards the 2010 target, being direct measures of the state of biodiversity: habitat indices, measuring trends in extent of selected biomes, ecosystems and habitats; population indices, measuring trends in abundance and distribution of selected species; and the marine trophic index, measuring changes in the mean food-web level of fisheries landings.

Each of these indices has advantages, but also limitations. Habitat and population indices have been proposed as particularly valuable in that they respond quickly to changing biodiversity5. Habitat indices are also geographically representative, but sample a coarse ecological resolution, which does not reflect finer levels of biodiversity organization (such as species) very well6. Global population indices, although they have a fine ecological resolution, are highly biased at present towards better-known (frequently biodiversity-poor) regions. The Living Planet Index7, for example, incorporates few tropical data sets, and those data that it does use may have been collected for populations already known to be declining. Like population indices, the trophic index8 has a fine ecological resolution but is not comprehensive: it is based on data for aquatic ecosystems only, and is specifically a measure of the impact of a particular threat (fisheries).

The Red List Index proposed by Butchart et al.2 fills a portion of ‘biodiversity indicator space’ that complements the indicators already being tested. The index measures changes in overall extinction risk for all species, worldwide, in an entire class of organisms. For this first application it is tested for all birds9. It therefore has both fine ecological resolution and comprehensive geographical representativeness. The cost is a lack of the sharp temporal resolution achieved by habitat, population and trophic indices. This said, the Red List Index is still sensitive enough to reveal a serious increase in extinction risk for birds between 1988 and 2004. Further, it is robust enough for subdivision by biogeographical realm, habitat and taxonomic group. This shows, for example, particularly devastating increases in aggregate extinction risk for Asian species (Fig. 1) (due to deforestation, in Indonesia in particular), and for albatrosses and other pelagic seabirds (due to the increase in longline fisheries).

Figure 1: In decline: a female large green pigeon (Treron capellei), Perak State, Malaysia.


Like many other bird species in Asia, this pigeon is threatened by habitat loss.

The paper introduces several clever innovations. The data underlying the Red List Index are derived through quantitative, transparent and repeatable assessment of extinction risk under the IUCN Red List criteria10. Further, Butchart et al. carefully classify reasons for changes in Red List assessments, and introduce a method of ‘back-casting’ the index. This removes a potential artefact resulting from the many changes to the Red List due to changing knowledge rather than to changing levels of extinction risk11. They also use these back-cast data to calculate error around the current Red List Index value due to genuine but as yet undetected changes in status. Finally, they provide a weighting for the index based on extinction probabilities, through which it becomes largely driven by actual extinctions. Although this gives narrower biodiversity coverage (because the contribution of most species towards this weighted index is negligible), it may provide a better measure of the loss of irretrievable genetic diversity.

As an immediate step for the assessment of progress towards the 2010 target, we suggest that the next meeting (in February 2005) of the Subsidiary Body for Scientific, Technical and Technological Advice to the convention should add the Red List Index to the other eight indicators already recommended for immediate testing. The index then needs expansion in coverage across other taxonomic groups. All mammal and amphibian species are currently being assessed, and Red List Indices for these will be available by 2010. Efforts to assess reptiles, fish and plants are also in progress but will require significant support to produce timely results. Moreover, a sampled Red List Index is under development, with the aim of providing an index that is taxonomically as well as geographically comprehensive. Similarly, the other eight indicators proposed for testing need considerable work to better inform progress towards the 2010 target.

Although we need to be able to measure progress (or lack thereof) towards the 2010 target, the more pressing need is to ensure funding and implementation of those conservation activities needed to actually achieve a significant reduction in the rate of biodiversity loss. Ultimately, an improvement in the Red List Index will require urgent investment in the conservation of species facing a high risk of extinction and of the habitats where they occur.


  1. 1

    Kaiser, J. Science 297, 1785 (2002).

  2. 2

    Butchart, S. H. M. et al. PloS Biol. 2(12), e383 (2004).

  3. 3

  4. 4

  5. 5

    Balmford, A., Green, R. E. & Jenkins, M. Trends Ecol. Evol. 18, 326–330 (2003).

  6. 6

    Araújo, M. B. et al. Ecography 24, 103–110 (2001).

  7. 7

    Loh, J. Living Planet Report 2002 (WWF Int., Gland, Switzerland, 2002).

  8. 8

    Pauly, D., Christensen, V., Dalsgaard, J., Froese, R. & Torres, F. Jr Science 279, 860–863 (1998).

  9. 9

    BirdLife International Threatened Birds of the World 2004 (BirdLife Int., Cambridge, UK, 2004).

  10. 10

    IUCN IUCN Red List Categories and Criteria: Version 3.1 (IUCN — World Conservation Union, Gland, Switzerland, 2001).

  11. 11

    Cuarón, A. D. Nature 366, 118 (1993).

Download references

Author information

Rights and permissions

Reprints and Permissions

About this article

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.