Europe's native oyster ecosystems are collapsed

European native oyster ecosystems have been classified as a ‘collapsed’ species, after an assessment of historical and ecological data revealed it has faced extreme decimation.

The mollusc species has surpassed critically endangered on the International Union for Conservation of Nature’s (IUCN) Red List of Ecosystems, the world’s most comprehensive information source on the global extinction risk, further highlighting the importance of restoration initiatives across Europe.

The evaluation was led by international conservation charity ZSL and the University of Edinburgh, and supported by researchers across Europe including from the University of Portsmouth.

During the analysis, scientists have revealed that thriving oyster reef ecosystems are nothing like those seen in European seas today. Newly compiled historical data shows these complex three-dimensional oyster reefs once grew to the size of a football pitch and collectively covered an area of over 1.7 million hectares, an area larger than Greater London. 

Healthy oyster reefs are vital habitats for many species and have a huge impact on the environment around them. They provide food for people, stabilise shorelines, cycle nutrients, and filter water —a single adult oyster filters up to 200 litres of water daily. Current definitions of the habitat specify a handful of oysters on the seafloor as their defining feature compared to their historical vibrancy.

Therefore, a lack of an ecologically meaningful baseline and accurate definition has hampered current efforts to restore reef ecosystems. The ecosystem red listing, while it delivers bad news about the habitat’s current status, should serve as a catalyst for greater ambition in ecosystem recovery.

Professor Joanne Preston from the University of Portsmouth’s Institute of Marine Sciences and co-founder of the Native Oyster Network UK & Ireland, said: “With the collapse of the native oyster ecosystem across Europe, we’ve also lost an enormous filtration engine in the NE Atlantic due to oysters' role in filtering seawater and removing nutrients. Possibly even more stark is the loss of biodiversity that preyed or lived on these reefs and the organisms that inhabited them.”

Professor Preston co-founded the Native Oyster Network UK in 2017, a joint venture between the University of Portsmouth and the Zoological Society of London, to encourage a national approach and scale of oyster habitat restoration. There are currently 18 native oyster restoration projects underway across the UK and Ireland, all of which are members of the network which collaborates closely with the Native Oyster Restoration Alliance (NORA), a sister network supporting restoration efforts across Europe.

Alison Debney, ZSL Conservation Lead, Wetland Ecosystem Restoration and co-founder of the Native Oyster Network UK & Ireland added: “The ecosystem has been lost from living memory, and the benefits it provided have only just been realised now it is almost too late. The stark contrast between the modern-day flat seabed and historical data must act as a wake-up call for action to restore the once-thriving marine environments.”

Primarily due to historical overexploitation compounded by poor water quality and disease, the seafloor is now a flat, barren expanse of sediment with low diversity. European oyster reef habitats are now so scattered and degraded that, except for a few locations such as Norway and Sweden, oysters are largely found in isolation or in tiny clumps. In Europe, most oyster populations exist in densities of less than one individual per square metre. Where they are found grouped together, these oyster clumps cover less than 0.1ha.

Marcos Valderrábano, Programme Manager Red List of Ecosystems, IUCN said: “This IUCN Red List of Ecosystems assessment of Europe’s native oyster reefs has revealed the true scale of what we have lost. But it also points to solutions”.

“By applying the Red List of Ecosystems criteria, we can reveal the severity of ecosystem degradation, and prioritise restoration efforts. This assessment serves as a wake-up call to protect and restore ecosystems that are not only vital to biodiversity but also provide invaluable services to people and the planet.”

Restoration of native oyster reefs and other coastal habitats can have multiple benefits, including boosting local economies through job creation, increasing the security of coastal livelihoods by improving fish and shellfish stocks, and boosting the tourism and recreation industries. 

The decline of European Native Oyster ecosystems highlights a crucial moment for marine biodiversity. Human activities have drastically altered these habitats, leading to isolated oyster populations. Restoration efforts must recognise oysters' ecological importance and adopt a holistic approach to revitalising marine environments.