Global trends in bird species survival indicator: data sources and methods, chapter 4

4. Methods

The Red List Index (RLI) is calculated from the Red List categories of the species under consideration. Species categorized as Extinct prior to the 1988 assessment and those classified as Data Deficient are excluded from the analysis; all remaining species are included. No bird species are in the Not Evaluated category. Migratory species and other species with large geographical ranges are included in all biogeographical realms in which they regularly occur.

To calculate the current RLI for each biogeographical realm, each species that occurs in the realm is assigned a score based on its extinction risk category. Species of Least Concern are assigned a score of 0, for Near Threatened a score of 1 is assigned, for Vulnerable a score of 2, for Endangered a score of 3, for Critically Endangered a score of 4, and for Extinct a score of 5. These scores are summed across all species and compared to the maximum possible sum (5 × number of species):

RLI = (maximum possible sum - sum of species scores) / maximum possible sum

This creates an index that ranges from zero (all species are Extinct) to one (all species are Least Concern). That is, higher RLI values indicate a higher expected rate of species survival, and as RLI values decline, the risk of extinction increases.

Back-casting is used to calculate the RLI for previous assessment years while taking into account new information and changes in taxonomic status. Back-casting determines what category would have been assigned to a species had current knowledge been available. For species that occur in more than one realm and that have undergone a category change, the change is assigned to the realm in which the threat has occurred (or where the conservation intervention has been successful). For example, a Canadian neotropical migrant that was threatened by development occurring in its breeding area would have a constant status in the Neotropical realm and a declining status in the Nearctic realm. One that was threatened by logging in its wintering area would have a constant status in the Nearctic realm and a declining status in the Neotropical realm. With these adjustments, the RLI for previous years is calculated in the same way as for the current year.

The methodology only captures changes if any species have been moved from one extinction risk category to another as a result of genuine status changes. Genuine changes are those resulting from an improvement or deterioration in quantitative assessment criteria rather than improved knowledge or changes in taxonomy.

The RLI shows both the level of extinction risk, represented by the RLI value, and the rate at which extinction risk changes over time, represented by the slope of the line connecting two RLI values. For a complete description of RLI calculation and its strengths and limitations, see Butchart et al. (2007).