Global trends in bird species survival indicator: data sources and methods, chapter 5

5. Caveats and limitations

• The Red List Index (RLI) requires that the exact same set of species be included in the calculation at each stage in a time series and that the only reason for a species to change from one category to another is a genuine status change. That is, category changes occur solely as a result of improvements or deterioration in the quantitative assessment criteria and not as a result of improvements in knowledge or changes in taxonomy. Both of these conditions are met through the use of a back-casting formula that accommodates the inclusion of new species or taxonomic changes by adjusting earlier RLI values based on current information. Back-casting may be uncertain in some cases.
• The RLI values depend on sufficient knowledge of a species being available to evaluators and consistent application of the Red List categories and criteria by all evaluators. Poor knowledge, incorrect categorization, and inconsistency in the application of criteria are all sources of uncertainty in the RLI values. Annual revisions of the guidelines for the application of Red List categories and criteria, improvements in the RLI calculation, and the existence of an overarching Red List Authority ensure consistency across the assessments for all birds help to reduce some of this uncertainty.
• The RLI value is an index of the proportion of bird species expected to survive in the near future if no conservation measures are undertaken. However, "near future" cannot be precisely quantified because different species in the index have different generation times. It can generally be assumed to be between 10 and 50 years.
• The RLI has moderately low sensitivity. Large changes in population size or distribution may be required to move a species from one Red List category to another. As a result, lack of change in species categorization from one assessment to the next does not necessarily mean the species has no change in risk of extinction over that time period.
• The RLI shows net trends in an aggregated form and, in the RLI calculation, extinction of a species may be offset by conservation efforts directed toward another species. The RLI does not measure the extinction rate as such, nor does it completely capture changes in overall genetic diversity.
• The RLI presented here is representative of trends for the world's birds; it does not include other taxonomic groups, which may be at higher risk of extinction or deteriorating faster. For more information on index development for other taxonomic groups, see Butchart et al. (2005), Vié et al. (2009), Hoffman et al. (2010) and Butchart et al. (2010).
• The RLI does not account for bird species extinctions occurring before 1988.