Rarity-weighted Species Richness
This layer highlights places dominated by range-restricted and endemic species.
Source: Rarity-Weighted Species Richness (IUCN Red List / Map of Life) Year: 2025 Category: Transition risks · Invasive Species Coverage: Global Format: Raster grid Used in risk analysis: Yes — gates Invasive species
What it shows
This layer highlights places dominated by range-restricted and endemic species. Rather than simply counting how many species occur in a location, it gives more weight to species with small global ranges, so areas rich in narrowly distributed species stand out. It is widely used as a proxy for biodiversity sensitivity and conservation value, flagging regions where biological impacts — such as biological invasions or habitat loss — would carry disproportionately high consequences.
How it is built
The index is a global spatial dataset in which each species' contribution to local richness is weighted by the inverse of its geographic range size. Species with very small ranges therefore contribute strongly to the score where they occur, while wide-ranging species contribute little. Summed across all species present, this produces a continuous surface that emphasises endemism and irreplaceability. The result is delivered as a global grid of values.
How to read it
Higher values indicate a concentration of range-restricted and endemic species, meaning greater biodiversity sensitivity and a higher potential cost of disturbance at that location. Lower values indicate areas dominated by more widespread species. In this layer the score is used as an inverse proxy for vulnerability to invasive species: places richest in rare, range-restricted species are treated as most sensitive to disruption.
Class thresholds
The underlying quantity is a dimensionless biodiversity index — rarity-weighted species richness (RWSR): each species present at a location contributes the inverse of its global geographic range size, and these contributions are summed, so the score rises where many narrowly distributed, endemic species co-occur. It is a continuous surface with no physical unit; the layer is symbolised over the range 0 (Low) to 0.05 (High) on the Rarity score field.
Rather than a five-class table, the layer uses a single gate at 0.05:
| Class | Rarity score (RWSR, dimensionless) |
|---|---|
| Low (not flagged) | ≤ 0.05 |
| High (flagged) | > 0.05 |
A site is flagged on the Invasive species dimension when the layer value is above 0.05 (the High end of the colour ramp), combined with a material "Introduction of invasive species" pressure as set out in the Risk analysis table above. Here a high rarity score is used as an inverse proxy for invasive-species vulnerability: the most range-rare, endemic places are treated as the most sensitive.
How these thresholds were set. The index itself follows the established rarity-weighted species richness method (range-size-weighted richness, drawing on the IUCN Red List range family). The single 0.05 cut-off used to gate the layer is a Darwin in-house classification — a fixed threshold applied to the global rarity surface to separate ordinary from highly range-rich locations; it is not a published WWF or IUCN break.
Source
Rarity-Weighted Species Richness (RWSR), a global spatial dataset weighting species by the inverse of their geographic range size. (No explicit attribution provided with the layer.)
Comparison with the WWF Risk Filter Suite
This layer maps to WWF Biodiversity Risk Filter indicator S14_6 Range Rarity, with which it is aligned (both draw on the IUCN Red List range-rarity family). Darwin additionally uses it as an inverse proxy for S13_3 Invasive Species. That second use is a deliberate proxy mismatch: Darwin gates on habitat rarity and vulnerability, whereas WWF's S13_3 measures actual invasive-species presence (from CABI, 2024). The invasive-species reading should therefore be understood as indicative rather than a direct equivalent.
Risk analysis
A site is flagged on a dimension by combining a proximity trigger (this layer) with an activity trigger (the entity's ENCORE pressure/service). Proximity only → Potentially material; proximity and the matching ENCORE pressure/service is material → Very material; neither → Not material.
| Dimension | ENCORE service / pressure | Proximity trigger (this layer) | Activity trigger (entity) |
|---|---|---|---|
| Invasive species | Introduction of invasive species | Layer value above 0.05 | “Introduction of invasive species” pressure ≥ 4 |
Legend
Symbolised field: Rarity score
| Value (Rarity score) | Label | Colour |
|---|---|---|
| 0 | Low | #add8e6 |
| 0.05 | High | #ff0000 |
Generated from darwin/layers/layer-rarity-weighted-species-richness.toml and risk_indicator_pairs.toml (develop).