IUCN · Conservation Measurement · Extinction Risk
The Red List
Index
A probabilistic measure of how close the world’s species are moving toward extinction — tracking shifts in threat status across IUCN categories from Least Concern to Extinct in the Wild.
EX Extinct
CR Critically Endangered
EN Endangered
VU Vulnerable
NT Near Threatened
LC Least Concern
0.73
Global RLI for birds, 2022
1.00 = all species LC • 0.00 = all extinct
1.00 = all species LC • 0.00 = all extinct
What Is the Red List Index?
Description & Overview
The Red List Index (RLI) is a statistically derived measure of the aggregate change in extinction risk across a defined set of species over time. Developed by the International Union for Conservation of Nature (IUCN), it uses the globally recognised Red List threat categories as its currency — converting categorical shifts (e.g., a species moving from Vulnerable to Endangered) into a single, comparable index value.
Unlike the Living Planet Index (which tracks abundance), the RLI tracks extinction probability. A species’s position on the IUCN Red List reflects its assessed likelihood of going extinct. When that position worsens — when species systematically move toward higher threat categories — the RLI falls. When conservation actions drive recoveries and categories improve, the RLI rises.
The RLI is bounded between 0 and 1:
- RLI = 1.00 — all species in the assessed group qualify as Least Concern (no extinction threat).
- RLI = 0.00 — all species have gone Extinct in the Wild.
- Values in between reflect the weighted proportion of species “extinct risk” relative to the two extremes.
Core insight: The RLI is a trend indicator. A declining RLI means more species are moving toward extinction than away from it — even if none have actually gone extinct yet. It detects the slow erosion of species safety margins before irreversible loss occurs.
RLI Concept — The Extinction Risk Spectrum
Each species occupies a category; the RLI weights and averages these positions
● RLI = 1.00 (all LC)
→
● RLI = 0.00 (all EX)
Category weights (w) represent the number of steps from LC. The RLI measures how far a group of species has moved up this risk ladder.
Taxonomic coverage: Separate RLIs are calculated for birds, mammals, amphibians, corals, cycads, and other groups. A multi-group RLI is produced for CBD reporting by combining these group-specific indices. The RLI does NOT cover all life — only IUCN-assessed species.
Historical Development
History of the Red List Index
1963 — IUCN Red List Founded
First Red Data Book. The IUCN Species Survival Commission (SSC) published the first Red Data Book, a subjective list of threatened species based on expert opinion. This was the ancestor of the modern Red List, though it lacked standardised, quantitative threat criteria.
1994 — Quantitative Red List Categories
Version 2.3: Standardised Criteria Introduced. The IUCN adopted a new quantitative system of threat categories (CR, EN, VU, NT, LC, EX, EW, DD, NE) with explicit numerical thresholds for population size decline, restricted range, and probability of extinction. This standardisation was the essential prerequisite for converting Red List data into a mathematical index.
2000 — Version 3.1: Refined Criteria
Current Criteria Framework. IUCN Red List Criteria version 3.1 was adopted — the system still in use today. Five criteria (A: population decline; B: geographic range; C: small population; D: very small/restricted population; E: quantitative extinction probability) formalised the evidence base for each category, enabling global comparability.
2004 — RLI Formally Proposed
Butchart et al. (2004). Stuart Butchart and colleagues at BirdLife International published the first formal description of the Red List Index methodology in PLOS Biology. They applied it to birds (for which long-term Red List data existed), demonstrating a statistically significant downward trend in the RLI — proving that species were not just threatened, but getting systematically more threatened over time.
2006 — Multi-Taxon Extension
Butchart et al. (2006) in Science. The methodology was extended to multiple taxonomic groups (mammals, amphibians, corals) and a composite RLI was published, enabling cross-taxon comparison. The paper was the first quantitative global assessment proving that the world was failing to meet the 2010 CBD biodiversity target.
2010 — Convention on Biological Diversity Adoption
Aichi Target Indicator. The CBD formally adopted the RLI as a headline indicator for tracking progress toward the Aichi Biodiversity Targets (2011–2020), specifically Target 12 (prevent extinction of known threatened species). This elevated the RLI from an academic metric to a core international policy instrument.
2016 — RLI and SDGs
Sustainable Development Goal 15.5. The RLI was selected as the official indicator for SDG target 15.5: “Take urgent and significant action to reduce the degradation of natural habitats, halt the loss of biodiversity and, by 2020, protect and prevent the extinction of threatened species.” The RLI thus became part of the UN global monitoring framework, reported to the Secretary-General.
2022 — Kunming-Montreal GBF
Post-2020 Global Biodiversity Framework. The RLI was retained as a core headline indicator (Headline indicator A.4) for the Kunming-Montreal Global Biodiversity Framework, adopted in December 2022. It underpins Target 4 (halt and reverse extinction rates, reducing extinction risk by at least 10-fold by 2050) and Target 5 (ensure sustainable use, reducing overexploitation by 2030).
Why It Matters
Significance & Relevance
The RLI is not merely an academic metric — it is the closest thing the world has to a universal thermometer for biodiversity loss, embedded in international law, conservation science, and national reporting.
UN & CBD Reporting
The only official indicator for SDG 15.5 (halt extinction). Reported annually to the UN Secretary-General and embedded in the Kunming-Montreal GBF monitoring framework.
Early Warning System
Detects worsening extinction risk before extinctions occur — providing time for intervention. Population decline precedes category change by years; the RLI catches both.
Conservation Effectiveness
Downlisting (e.g., EN→VU due to successful conservation) contributes a positive signal. The RLI can quantify how many extinctions have been prevented by conservation action.
Identifies Drivers
RLI disaggregation by threat type (habitat loss, overexploitation, invasive species, climate) enables targeted policy responses by identifying the dominant causes of decline.
Cross-Taxon Comparison
Because the same formula applies to birds, mammals, amphibians, corals, and plants, the RLI enables direct comparison of biodiversity trends across fundamentally different life forms.
National Biodiversity Planning
National RLIs (calculated for species within a country’s borders) allow governments to measure whether domestic conservation policies are working over multi-year periods.
Transparent & Reproducible
The RLI formula, category weights, and species lists are fully published. Any researcher or government can compute their own RLI using IUCN Red List API data and the standard methodology.
Beyond Species Count
A simple count of threatened species is insensitive to change. The RLI measures the direction and magnitude of change across all categories — a far more powerful policy signal.
Key limitation: The RLI only reflects species that have been assessed by the IUCN Red List. As of 2024, fewer than 10% of Earth’s described species have been assessed. The index is therefore most reliable for well-studied groups (birds, mammals) and must be interpreted as a partial, though carefully designed, sample of global biodiversity status.
Foundation Knowledge
IUCN Red List Categories & Weights
The RLI is built on IUCN’s hierarchical threat categories. Understanding these categories — and the weights assigned to them in the RLI calculation — is essential to interpreting the index.
| Category | Code | RLI Weight (w) | Definition & Criterion | Included in RLI? |
|---|---|---|---|---|
| EX Extinct | EX | 5 | No reasonable doubt that the last individual has died. Surveys have failed to find a living specimen. | Yes — maximum weight |
| EW Extinct in the Wild | EW | 5* | Only survives in captivity or as a naturalised population outside its historical range. | Yes — treated as EX in standard RLI* |
| CR Critically Endangered | CR | 4 | Facing an extremely high risk of extinction in the wild (≥50% probability within 10 years or 3 generations). | Yes |
| EN Endangered | EN | 3 | Facing a very high risk of extinction in the wild (≥20% probability within 20 years or 5 generations). | Yes |
| VU Vulnerable | VU | 2 | Facing a high risk of extinction in the wild (≥10% probability within 100 years). | Yes |
| NT Near Threatened | NT | 1 | Close to qualifying for a threatened category or likely to qualify in the near future. | Yes |
| LC Least Concern | LC | 0 | Does not qualify for CR, EN, VU, or NT. Widespread and abundant species. | Yes — baseline (contributes 0 weight) |
| DD Data Deficient | DD | Excluded | Inadequate information to make an assessment of extinction risk. | No — excluded from RLI denominator |
| NE Not Evaluated | NE | Excluded | Has not yet been assessed against the IUCN criteria. | No — excluded from RLI entirely |
* Note on EW: Extinct in the Wild species are treated as EX (weight = 5) in the standard RLI formulation for most taxonomic groups, because they represent complete loss from functional wild populations. Some RLI variants treat EW separately. The key point: EW species moving to EX does not change the RLI value; a species moving from CR to EW does.
Why exclude DD and NE? Including Data Deficient or Not Evaluated species would introduce unknown bias — a DD species could be anywhere from LC to CR. Excluding them maintains the index’s integrity but means the RLI represents only assessed species. The denominator of the RLI formula uses only evaluated (non-DD, non-NE) species.
Core Methodology
RLI Methodology — How It Works
Four principles that convert categorical Red List assessments into a robust, comparable trend index
① Species Selection
- Define the taxonomic group (e.g., all birds, all amphibians, all freshwater fishes).
- Exclude DD (Data Deficient) and NE (Not Evaluated) species from all calculations.
- Ensure the same set of species is tracked consistently over time.
- Require at least two genuine IUCN Red List assessments per species at different time points.
② Category Weighting
- Each IUCN category is assigned a numerical weight reflecting steps from LC: LC=0, NT=1, VU=2, EN=3, CR=4, EX=5.
- These equal-interval weights reflect the ordinal nature of threat categories.
- Some authors use alternative weights (e.g., reflecting probability differences), but equal-interval weights are the IUCN standard for RLI.
③ Change Attribution
- Not all category changes count — only genuine status changes are included.
- Genuine change: driven by real biological improvement or deterioration (e.g., population recovery or new threat).
- Non-genuine change: driven by new information, better surveys, or revised taxonomy — these are excluded from RLI computation.
- This attribution requires expert IUCN assessment documentation.
④ Temporal Tracking
- The RLI is computed for each assessment year, producing a time series (e.g., 1988, 1994, 2000, 2004, 2008 …).
- The rate of change between consecutive RLI values shows whether extinction risk is accelerating, stabilising, or improving.
- Confidence intervals are estimated by bootstrapping species assessments.
RLI Computational Pipeline
From species assessments to the published index value
Mathematics
Equations & Formulae
Core Formula — Red List Index
RLI(t) = 1 − [ Σ(wᵢ × Nᵢ (t)) ] ÷ [ wᵗᵐᵡ × Nᵗᵐᵡ(t) ]
RLI(t)Red List Index at time point t. A value between 0 (all extinct) and 1 (all Least Concern).
wᵢWeight assigned to category i (LC=0, NT=1, VU=2, EN=3, CR=4, EX/EW=5).
Nᵢ(t)Number of species in category i at time t.
Σ(wᵢ×Nᵢ)Sum of (weight × number of species) across all categories. This is the actual weighted extinction risk at time t (the numerator, also called the “sum of weights”).
wᵗᵐᵡMaximum possible weight = 5 (weight of EX category). Represents the worst possible status for any species.
Nᵗᵐᵡ(t)Total number of species evaluated (excluding DD and NE) at time t. This is the denominator of the maximum possible sum.
wᵗᵐᵡ×NᵗᵐᵡMaximum possible weighted sum if ALL species were Extinct. Acts as the normalising denominator.
The “1 −” term: Without it, the fraction Σ(wᵢNᵢ) / (5×N) would be 0 when all species are LC (desirable state) and 1 when all are EX (worst state). The subtraction from 1 inverts this so that higher RLI = better state (more intuitive for policy communication).
Equivalent Formulation — Fraction in LC
RLI(t) = 1 − [ (5×Nᶜᵧ + 4×Nᶜᵅ + 3×Nᶜᴳ + 2×Nᶜᵘ + 1×Nᶜᵗᵗ + 0×Nᶜᴸ) ] ÷ (5 × Nᵗᵐᵡ)
This expanded form shows each category’s contribution explicitly. Species in LC add nothing to the numerator (weight 0); species in EX carry maximum weight (5). The formula naturally rewards species in lower-threat categories.
Rate of Change — Comparing Two Time Points
ΔRLI = RLI(t₂) − RLI(t₁)
ΔRLIChange in RLI between two assessment periods. Negative = deterioration; positive = improvement.
RLI(t₂)RLI at the more recent time point
RLI(t₁)RLI at the earlier time point (often the reference/baseline year)
Annual rate: To estimate an annualised rate, divide ΔRLI by the number of years between assessments: Annual ΔRLI = ΔRLI ÷ (t₂ − t₁). This allows comparison across different-length monitoring intervals.
Derived Metric — Extinctions Prevented
PE = Nᵗᵐᵡ × (RLI₁┹⁷₀ − RLI₂₀₂₀) × (1 / 5)
PENumber of species extinctions that conservation has prevented (i.e., species that would have gone EX without intervention, estimated from observed RLI change attributable to conservation actions).
NᵗᵐᵡTotal number of assessed species in the group
1/5Converts category-steps (max 5) to extinction-equivalents. One full category improvement is valued at 1/5 of an extinction prevented.
Context: Butchart et al. (2006, Science) used this approach to estimate that conservation action prevented approximately 16 bird extinctions and 8 mammal extinctions between 1994 and 2004. It quantifies the positive return on conservation investment.
Worked Example
Step-by-Step RLI Calculation
Scenario: A national wildlife authority tracks 20 freshwater fish species across two Red List assessment periods: Year 2010 and Year 2020. Calculate the RLI for each year, the change in RLI, and interpret the result.
Species Distribution by Category
| Category | Weight (w) | N (Year 2010) | w × N (2010) | N (Year 2020) | w × N (2020) |
|---|---|---|---|---|---|
| EX Extinct | 5 | 0 | 0 × 5 = 0 | 1 | 1 × 5 = 5 |
| CR Critically Endangered | 4 | 2 | 2 × 4 = 8 | 3 | 3 × 4 = 12 |
| EN Endangered | 3 | 4 | 4 × 3 = 12 | 5 | 5 × 3 = 15 |
| VU Vulnerable | 2 | 5 | 5 × 2 = 10 | 4 | 4 × 2 = 8 |
| NT Near Threatened | 1 | 4 | 4 × 1 = 4 | 3 | 3 × 1 = 3 |
| LC Least Concern | 0 | 5 | 5 × 0 = 0 | 4 | 4 × 0 = 0 |
| TOTAL (N_eval = 20) | 20 | Σ = 34 | 20 | Σ = 43 |
1
Compute the weighted sum Σ(wᵢ × Nᵢ) for each year
Multiply each category’s weight by the number of species in that category, then sum across all categories.
Year 2010:
EX: 0 × 5 = 0
CR: 2 × 4 = 8
EN: 4 × 3 = 12
VU: 5 × 2 = 10
NT: 4 × 1 = 4
LC: 5 × 0 = 0
─────────────
Σ(w × N) = 34
Year 2020:
EX: 1 × 5 = 5
CR: 3 × 4 = 12
EN: 5 × 3 = 15
VU: 4 × 2 = 8
NT: 3 × 1 = 3
LC: 4 × 0 = 0
─────────────
Σ(w × N) = 43
2
Compute the maximum possible sum (all species EX)
w_max = 5 (weight of EX category)
N_eval = 20 (total species evaluated, same both years)
Maximum sum = w_max × N_eval = 5 × 20 = 100
3
Compute RLI for Year 2010
RLI(2010) = 1 − [Σ(w × N)] / [w_max × N_eval]
= 1 − 34 / 100
= 1 − 0.34
= 0.66
RLI = 0.66 in 2010: species are on average 34% of the way toward extinction from LC status.
4
Compute RLI for Year 2020
RLI(2020) = 1 − [Σ(w × N)] / [w_max × N_eval]
= 1 − 43 / 100
= 1 − 0.43
= 0.57
RLI = 0.57 in 2020: species are on average 43% of the way toward extinction — a clear deterioration.
5
Compute the change in RLI and annualised rate
ΔRLI = RLI(2020) − RLI(2010)
= 0.57 − 0.66
= −0.09 (a decline of 0.09 over 10 years)
Annual ΔRLI = −0.09 / 10 = −0.009 per year
The RLI fell by 0.09 over a decade, equivalent to a deterioration of −0.009 per year. This negative value confirms that freshwater fish species are, on average, moving toward higher extinction risk at a measurable rate.
6
Interpret what drove the change (category transitions)
Transitions between 2010 and 2020 (genuine changes only):
UPLISTING (worsening — reduce RLI):
• 1 species: LC → NT (+1 category weight, −0.01 RLI contribution)
• 2 species: NT → VU (+1 category weight each)
• 1 species: VU → EN (+1 category weight)
• 1 species: CR → EX (+1 category weight)
Total uplisting weight change = +9
DOWNLISTING (improving — increase RLI):
• 0 genuine downlistings recorded
Net change = +9 − 0 = +9 more weight units
→ ΔRLI = −9/100 = −0.09 ✓ (matches calculated value)
RLI Trend — Freshwater Fishes 2010–2020
Index value: 1.00 = all LC (no extinction threat) • 0.00 = all Extinct
Observed RLI (with CI band)
Projected (counterfactual, no action)
Reading the Results
Interpreting the Red List Index
📒 What RLI Values Mean
- RLI = 1.00 — Every species in the group qualifies as Least Concern. Ideal state; no measurable extinction risk.
- RLI = 0.00 — Every species in the group is Extinct in the Wild. Worst possible state.
- RLI declining — More species are moving toward higher threat categories than are recovering. System is deteriorating.
- RLI stable — Gains and losses are balanced, OR no genuine changes occurred.
- RLI rising — Conservation actions are successfully downlisting species, OR threats are genuinely abating.
⚠ Common Misinterpretations
- NOT extinction rate: A falling RLI does NOT mean species are going extinct — it means threat categories are worsening on average.
- NOT all life: RLI covers only IUCN-assessed species in defined groups — typically <10% of known species.
- NOT comparable across groups directly: RLI(birds) = 0.70 and RLI(amphibians) = 0.68 do not mean amphibians are “better off” — baselines differ.
- NOT about absolute numbers: A group with 5 threatened species of 10 can have the same RLI as one with 50 of 100.
RLI Value Interpretation Guide
0.00 (all EX)
0.25
0.50
0.75
1.00 (all LC)
0.00–0.30Crisis LevelMajority of species in threatened categories (EN/CR/EX). Systemic collapse of the group’s conservation status. Immediate emergency response required.
0.30–0.55Severe DeclineSignificant proportion of species in EN and CR. Our worked example: RLI(2020) = 0.57 — approaching this range. Urgent targeted conservation needed.
0.55–0.70Moderate DeclineMany species in VU/EN. Trend is clearly negative but most species are not yet in extreme categories. Our Year 2010 result (RLI = 0.66) falls here.
0.70–0.85Slight DeclineMost species in lower-threat categories. Some uplisting occurring. Monitoring required; preventive conservation warranted. Typical range for birds globally (~0.73).
0.85–1.00Low / No ThreatMost species at LC or NT. Very few threatened species. Represents a well-conserved or inherently low-risk taxonomic group.
RLI Values for Major Taxonomic Groups — Global
Published values from IUCN / BirdLife / IPBES assessments (approximate)
| Taxonomic Group | RLI (approx.) | Trend | Primary Threat Driver |
|---|---|---|---|
| Birds | 0.73 | ↘ Declining | Agriculture, invasive species, hunting |
| Mammals | 0.68 | ↘ Declining | Hunting/trade, habitat loss, fragmentation |
| Amphibians | 0.65 | ↘ Rapidly declining | Chytridiomycosis fungus, habitat loss, climate change |
| Reef-building corals | 0.61 | ↘ Rapidly declining | Ocean warming, bleaching, acidification |
| Cycads | 0.58 | ↘ Declining | Habitat loss, collection for horticulture |
| Freshwater fishes | <0.65 | ↘ Declining | Water extraction, pollution, dam construction |
RLI vs Other Biodiversity Indicators
| Attribute | Red List Index (RLI) | Living Planet Index (LPI) |
|---|---|---|
| What is measured | Aggregate change in extinction risk (IUCN category shifts) | Aggregate change in population abundance (count trends) |
| Currency | IUCN Red List threat categories (ordinal, expert-assessed) | Population size or density time-series (continuous data) |
| Range | 0 (all extinct) to 1 (all Least Concern) | Anchored at 1970 = 1.0; no upper bound in theory |
| Early warning | Yes — category shifts precede extinction by years/decades | Yes — declining populations precede category uplisting |
| Taxonomic scope | IUCN-assessed species only (strong in birds, mammals) | Vertebrates with monitored population time-series |
| Excludes | DD, NE species; non-genuine category changes | Invertebrates, plants, fungi; species without time-series |
| Conservation signal | Downlisting = success; uplisting = failure or natural decline | Rising population index = recovery; declining = loss |
| UN / CBD use | SDG 15.5 indicator; Kunming-Montreal GBF A.4 | Kunming-Montreal GBF B.4; CBD core indicator |
| Key publication | Butchart et al. (2004) PLOS Biol.; (2006) Science | WWF Living Planet Report (1998–present) |
| Our example | RLI 2010=0.66 → 2020=0.57 (−0.09 in 10 years) | Computed separately from population count data |
Try It Yourself
Interactive RLI Calculator
Enter species counts per IUCN category for two time points. The calculator computes RLI(t₁), RLI(t₂), the change, and provides a full interpretation.
Red List Index Calculator
─ Assign each species to an IUCN category for two assessment periods • DD and NE species are auto-excluded
Note: Enter the number of species in each category. DD (Data Deficient) and NE (Not Evaluated) species are automatically excluded from the RLI computation — do not include them in the counts above.
RLI Year 1
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compute first
RLI Year 2
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compute first
Change (ΔRLI)
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over period
Status
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