Europe’s native oyster ecosystems have ‘collapsed’

Scientists have revealed that Europe’s native oyster ecosystem has declined to the extent that it has ‘collapsed’, highlighting the importance of ambitious restoration initiatives.

The assessment, led by international conservation charity ZSL and the University of Edinburgh, came after researchers from across Europe reviewed historical and ecological data with the intention of defining the extent of the European native oyster ecosystem.

They discovered that the species has faced such extreme decimation that it must now be classified as ‘collapsed’ according to the IUCN’s Red List of Ecosystems criteria.

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.

They would have hosted a diverse, abundant and flourishing community of fish, crabs, starfish, and birds, such as the distinctive oystercatchers – named for their preference for feeding on oysters.

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 in comparison to their historical vibrancy.

The lack of an ecologically meaningful baseline and accurate definition has hampered current efforts to restore reef ecosystems.

While the ecosystem red listing is bad news for the habitat’s current status, it should serve as a catalyst for greater ambition in ecosystem recovery.

“The rich and diverse seafloor ecosystem described in multiple historical documents is almost unthinkable in the degraded European seas of today. These perspectives open a window to the past and help us imagine what we could gain with greater ambition when conserving and restoring our seas,” said Ruth Thurstan, Associate Professor in Marine and Historical Ecology at the University of Exeter’s Centre for Ecology and Conservation and a co-author on the paper.

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.

“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,” said Joanne Preston, Professor of Marine Biology at the University of Portsmouth and co-founder of the Native Oyster Network UK & Ireland.

“Piecing the historical data together with data from current restoration and recovery efforts, we were able to define the original structure and function of the native oyster reef ecosystem. We were shocked by our findings. We knew from other regions that oyster reefs were important, and we have long known that they are highly degraded in Europe but pulling all this information together in a structured way, as required by the IUCN Ecosystem Red Listing Framework, brought to light the critical role that oyster reefs once served,” said Dr Philine zu Ermgassen of the University of Edinburgh and lead author of the study.

“Given time and space, nature can recover. As a habitat-creating species, restoring native oyster habitat could potentially trigger a cascade of recovery for other species, offering a beacon of hope in the face of collapse. However, it will not be a quick fix as oyster reefs are slow to generate, with layers of new oysters building up on the dead shells of their predecessors,” said Alison Debney, Conservation Lead at the Zoological Society of London.

“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,” said Marcos Valderrábano, Programme Manager Red List of Ecosystems, IUCN.

“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.

Eighteen native oyster restoration projects are currently underway across the UK and Ireland, but the researchers say these projects alone cannot restore native oyster reef ecosystems to the scale required.

“Current recovery efforts will be insufficient. We need to look back in time to work out a true but ambitious baseline for what healthy oyster reef ecosystems looked like to restore them to their vital ecosystem services potential – not base it on modern understanding, which has felt the impact of added pressures of over-fishing and other human impacts,” said Dr Philine zu Ermgassen. “While UK and EU Restoration efforts are a commendable starting point, a cross-sector approach is needed to halt destructive activities. Small-scale restoration projects are valuable, but true ecosystem recovery requires systemic processes and sustainable financing.”

“European Native Oyster Reef Ecosystems Are Universally Collapsed” is published in Conservation Letters.