A new assessment of European Native Oyster Reef Ecosystems yields new definitions and assesses them as Collapsed.

European native oyster (Ostrea edulis) reefs were once widespread throughout European seas and coastal waters. However, overfishing led to extirpation as early as 1703, with widespread declines and local loss accelerating from the mid 1800s to the early 1900s. Because these losses occurred before modern scientific sampling, traditional definitions of oyster reef habitats were based on recent, degraded observations. These metrics  —defined by oyster densities >5 oysters/m² often observed at small spatial extents — reflect a shifted baseline that fails to capture the true conditions of a functioning European Native Oyster Reef Ecosystem.

To derive a comprehensive overview of functioning European Native Oyster Reefs, experts drew on historical literature, data from related species in other regions, and on current records where remnant oyster habitats exist. Their findings indicate that reefs should be present at the hectare scale within networks of reefs spanning km2 in extent. Furthermore, these reefs must support oyster densities exceeding 20 oysters/m2. Based on this definition, no locations in Europe were found to currently support European Native Oyster Reefs at ecosystem scales. The ecosystem was therefore assessed as Collapsed under the IUCN Red List of Ecosystems criteria.

A quantitative definition of European Native Oyster Reef Ecosystems is useful not only for assessing ecosystem status, but also for defining reference models for restoration, informing success criteria, and planning the large-scale recovery efforts, such as those set out within the European Nature Restoration Regulation and the Kunming-Montreal Global Biodiversity Framework.

Figure: An artist’s impression of a healthy European Native Oyster Reef Ecosystem, based on historical records of reef structure and associated species.  Panel A depicts dense reefs, which may be interspersed with lower density areas (Panel B) within the reef system. From zu Ermgassen et al. 2024. Artist: Maria Eggertsen.

 

Written by: Philine S.E. zu Ermgassen, Joanne Preston, Alison Debney and Ruth H. Thurstan.

Style and format: Emy Miyazawa.

Banner photo: Stéphane Pouvreau.