Herbicide resistance risk (global, Heap 2021)

Source: Heap, International Survey of Herbicide-Resistant Weeds Year: 2021 Category: Physical risks · Mitigating services · Biological hazards Coverage: Global (country-level, with sub-national detail for the US, Australia and Canada) Format: Raster grid (~2 km) Used in risk analysis: Yes — gates Mitigating services

What it shows

This layer maps the risk of herbicide and agrochemical resistance — that is, where weeds have evolved resistance to herbicides. It matters because resistance reduces the effectiveness of agricultural inputs, can drive heavier chemical use, and signals pressure on surrounding ecosystems.

How it is built

The source is the International Survey of Herbicide-Resistant Weeds (Heap), which counts unique cases of herbicide-resistant weeds, each defined by a species combined with a herbicide site-of-action. Counts are compiled at country level for most of the world, and at state or province level for the US, Australia and Canada, where more granular data exists. Following the WWF approach, counts are converted into a five-class scale using fixed thresholds rather than relative ranking: a count of zero is class 1, 1–15 is class 2, 16–28 is class 3, 29–45 is class 4, and more than 45 is class 5. The administrative areas are mapped onto a roughly 2 km grid; oceans carry no data. It is intended as a screening layer.

How to read it

The scale runs from 1 (very low risk) through 2 (low), 3 (moderate), 4 (high) to 5 (very high). Higher classes indicate more documented herbicide-resistance cases in that territory.

Class thresholds

The underlying quantity is a count: the number of unique documented herbicide-resistant weed cases in a territory, where each case is a distinct (weed species × herbicide site-of-action) combination, compiled by the Heap survey at country level (and at state/province level for the US, Australia and Canada). Each territory's count places it in one of five classes:

A site is flagged on the Mitigating services dimension when its territory reaches class 4 (High) or above (i.e. more than 28 unique resistant-weed cases; template: "High herbicide-resistance area (WWF class >= 4, >28 unique resistant weeds)").

How these thresholds were set. The five-class split follows the WWF Biodiversity Risk Filter (BRF) methodology for the Herbicide Resistance indicator (S3_4): it uses fixed count thresholds (0 → 1, 1–15 → 2, 16–28 → 3, 29–45 → 4, >45 → 5) rather than relative ranking, applied to the Heap (2021) case counts. The break values are set in the WWF BRF methodology.

Reference: WWF Biodiversity Risk Filter — riskfilter.org/biodiversity.

Source

Heap, I. (2021), The International Survey of Herbicide-Resistant Weeds (weedscience.org). Administrative boundaries from Natural Earth (public domain).

Comparison with the WWF Risk Filter Suite

This layer maps directly to WWF Biodiversity Risk Filter indicator S3_4 Herbicide / Antimicrobial Resistance. It uses the same Heap source as WWF and is aligned in concept and data.

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.

Legend

Symbolised field: Herbicide resistance risk

Generated from darwin/layers/layer-herbicide-resistance-global.toml and risk_indicator_pairs.toml (develop).