Rarity-weighted Species Richness for Threatened 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: No (contextual layer; not used in materiality scoring)

What it shows

This layer highlights places that concentrate threatened species with small global ranges. It is a conservation-focused variant of rarity-weighted species richness: rather than considering all species, it is restricted to those classified as threatened on the IUCN Red List. The result emphasises locations where range-restricted, at-risk species are concentrated — the areas where biological impacts would have the greatest conservation consequences.

How it is built

The index weights each species' contribution to local richness by the inverse of its global range size, so narrowly distributed species count for more. Here the calculation is limited to species in the threatened IUCN Red List categories — Vulnerable, Endangered and Critically Endangered. Summed across those species, this produces a continuous global surface that combines geographic rarity with conservation status. The result is delivered as a global grid of values.

How to read it

Higher values indicate a concentration of range-restricted threatened species, meaning high conservation sensitivity and a high potential cost of disturbance. Lower values indicate fewer or more widespread threatened species. As with the broader rarity layer, the score is used here as an inverse proxy for vulnerability to invasive species: the richest areas in rare threatened species are treated as the most sensitive.

Class thresholds

The underlying quantity is a dimensionless biodiversity index — rarity-weighted species richness restricted to threatened species: each species in the threatened IUCN Red List categories (Vulnerable, Endangered, Critically Endangered) contributes the inverse of its global range size, and these contributions are summed, so the score rises where range-restricted, at-risk species are concentrated. It carries no physical unit and 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 break at 0.05:

This is a contextual layer and is not used in materiality scoring, so the 0.05 value acts only as the colour-ramp / High-sensitivity break rather than as a live risk flag. Higher values indicate a concentration of range-restricted threatened species and therefore high conservation sensitivity.

How these thresholds were set. The index follows the established rarity-weighted species richness method (range-size-weighted richness over the IUCN Red List threatened categories). The single 0.05 break used to colour the surface is a Darwin in-house classification — a fixed cut-off applied to the global threatened-rarity surface; it is not a published WWF or IUCN break.

Source

Rarity-Weighted Species Richness restricted to IUCN Red List threatened categories (Vulnerable, Endangered, Critically Endangered). (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 (threatened-species variant) and is aligned with WWF's IUCN Red List range-rarity family. As with the all-species version, Darwin also applies it as an inverse proxy for S13_3 Invasive Species; that use is a proxy mismatch, since WWF's S13_3 measures actual invasive-species presence rather than habitat rarity, and the invasive-species reading should be treated as indicative only.

Legend

Symbolised field: Rarity score

Generated from darwin/layers/layer-rarity-weighted-species-richness-for-threatened-species.toml and risk_indicator_pairs.toml (develop).