European native oyster reef ecosystems are universally Collapsed

Oyster reefs are often referred to as the temperate functional equivalent of coral reefs. Yet evidence for this analogy is lacking for the European native species Ostrea edulis and its biogenic habitat. Recently assembled historical data provide a unique opportunity to develop a robust definition for this ecosystem type, confirm that O. edulis are biogenic reef builders, and assess its current conservation status. Today, O. edulis typically occur as scattered individuals or, in a few locations, as dense clumps over a few m2, however, historically O. edulis reef ecosystems persisted at large scales. A key finding is that O. edulis reef ecosystems should therefore be assessed at the ha scale.Using the IUCN Red list of Ecosystems Framework, we conclude the European native oyster reef ecosystem type is Collapsed under three of five criteria (A: reduction in geographic distribution, B: restricted geographic range, and D: disruption of biotic processes and interactions). Criterion C (environmental degradation) was assessed as data deficient and Criterion E (quantitative risk analysis) was not completed as the ecosystem was already deemed collapsed. Our assessment has far reaching implications for conservation policy and action, and shows that the scale of current restoration efforts fall far short of what is necessary for ecosystem recovery.