📕 Red List Index 🦏
📖 Overview
Red List Index (RLI) measures the aggregate extinction risk of groups of species over time, based on genuine changes in IUCN Red List categories. It ranges from 1.0 (all species Least Concern) to 0.0 (all species Extinct).
RLI reveals trends in the status of biodiversity, accounting for species moving between categories due to real improvement or deterioration.
⏳ History & Development
• 2004: First RLI developed by Butchart et al., published in PLoS Biology.
• 2008+: Adopted by CBD as indicator for 2010 target.
• 2015+: Integral to Sustainable Development Goal 15.
• 2022: Used in Kunming-Montreal Global Biodiversity Framework.
Significance: RLI is one of the most robust indicators for global conservation policy, capturing genuine status changes.
🏷️ IUCN Red List Categories & RLI Weights
The RLI uses a numeric weight (Wc) for each category, reflecting extinction risk. These weights are equally spaced from 0 (Least Concern) to 5 (Extinct/Extinct in the Wild).
| Category | Code | Weight (W) | Description |
|---|---|---|---|
| Least Concern | LC | 0 | Widespread and abundant |
| Near Threatened | NT | 1 | Close to qualifying for threatened |
| Vulnerable | VU | 2 | High risk of extinction |
| Endangered | EN | 3 | Very high risk of extinction |
| Critically Endangered | CR | 4 | Extremely high risk |
| Extinct in the Wild | EW | 5 | Survives only in captivity |
| Extinct | EX | 5 | No reasonable doubt of extinction |
Data Deficient (DD) species are excluded; Not Evaluated (NE) not considered.
🔬 Methodology & Equations
The RLI is calculated by multiplying the number of species in each category by its weight, summing these, and scaling relative to the maximum possible score.
Where Wc = weight of category c, Nc = number of species in category c, Wmax = maximum weight (5), Ntotal = total species assessed (excluding DD).
Genuine change: Only reclassifications due to real population changes (not taxonomic revisions or new knowledge) are used to update RLI over time.
🧮 Interactive RLI Calculator
Enter number of species in each IUCN category (or use example). The RLI will be computed automatically.
📐 Worked Example & Interpretation
Scenario: A taxonomic group assessed in 2010 and again in 2025. Numbers below reflect genuine changes only.
| Year | LC | NT | VU | EN | CR | EX/EW | Total | RLI |
|---|---|---|---|---|---|---|---|---|
| 2010 | 50 | 10 | 7 | 4 | 2 | 0 | 73 | 0.882 |
| 2025 | 45 | 12 | 8 | 5 | 3 | 1 | 74 | 0.849 |
Calculation for 2025:
Sum(W×N) = (0×45)+(1×12)+(2×8)+(3×5)+(4×3)+(5×1) = 0+12+16+15+12+5 = 60.
Max possible = 5 × 74 = 370.
RLI = 1 − (60/370) = 1 − 0.1622 = 0.8378 (≈0.838).
🔎 Interpretation: RLI declined from 0.882 to 0.838, indicating a deterioration in extinction risk. More species moved into higher threat categories, signaling biodiversity loss.
🌐 Policy Relevance
The RLI is an official indicator for SDG 15.5, the CBD post-2020 framework, and IPBES assessments. It helps track whether conservation actions are slowing species declines.
of assessed species
threatened (2023)
📘 RLI methodology follows Butchart et al. (2007) and IUCN Red List guidelines.
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