Red List for British seaweeds: evaluating the IUCN methodology for non-standard marine organisms

Abstract The IUCN Red List of Threatened Species is an authoritative tool in biodiversity conservation. Whilst IUCN criteria have been applied successfully to groups such as birds and mammals, a Red List assessment of British seaweeds in 2021 revealed that the categories to which seaweed species were assigned were dependent on how the criteria were applied. Here, this seaweed assessment is used as a case study with which to evaluate the IUCN methodology for use with ‘non-standard’ groups of organisms. A data-driven assessment of red (Rhodophyta), green (Chlorophyta) and brown (Phaeophyceae) seaweeds, which applied three (A, B and D) of the five IUCN criteria (A–E), categorized 13% of 617 British species as threatened. Following peer review, only 7% of species were categorized as threatened (1% Critically Endangered—CR, 3% Endangered—EN, 3% Vulnerable—VU), and 55% as Data Deficient. This reduction in species categorized as threatened suggests that strict application of the IUCN criteria may, at least for the seaweeds, over-estimate threat. As a result of this assessment, recommendations include the need for a more unified monitoring system and a review of the suitability for/application of the IUCN assessment criteria to some types of organisms. For example, in clonal populations, it is not possible to count individuals, and complex life histories cause additional complications. IUCN criteria must be applicable to a wide range of organisms, including seaweeds.