New species in the Wadden Sea
The Wadden Sea is currently changing faster than it has in millennia. There are already various introduced species of flora and fauna that were never seen in the North Sea 100 years ago. These newcomers hail from distant coastlines and only made their way to the tidal flats of the Wadden Sea with the help of human beings. As a result, the biotic community in and on the sand and mud has been massively transformed – and will continue to do so, since there are on average one or two newly introduced species every year. These new rivals change native diversity and species interactions – though a few of native species are actually benefiting from the developments.
The Wadden Sea – an ecological treasure trove of immeasurable value
With an area of 11,500 square kilometres and measuring 500 kilometres in length, the North Sea coasts of Denmark, Germany and the Netherlands are home to the world’s largest contiguous system of intertidal sand and mud flats. Winds, waves and the constant interplay of low and high tide have created an exceptionally diverse landscape. The comparatively shallow waters of the North Sea just beyond the tidal flats are rich in nutrients. Accordingly, the microscopic algae in the water (phytoplankton), which often reproduce at an astronomic rate, thrive. These microalgae, along with those produced on the floor of the Wadden Sea itself, make the flats one of the most productive landscapes in the world, not to mention an abundant source of food for a biotic community numbering more than 10,000 flora and fauna species. The vast sand flats are covered with microalgal and bacterial colonies. Hidden in the sediment, more than a billion lugworms work to ensure the sand is constantly churned up and enriched with oxygen. Further inland the flats give way to typical salt marshes. Only covered in water during spring floods and storm surges, these marshes contain a range of highly specialised plant species. Its geographic diversity and high primary production make the Wadden Sea the perfect home for fish, crabs, worms and bivalves. At the same time, it is a nursery for common seals, grey seals and porpoises and an important breeding ground and refuge for millions of coastal and migratory birds.
What are neobiota?
The term neobiota – “new life” – refers to species that, through human influence, spread to an area where they are not indigenous. They often originate in distant regions and are unintentionally and intentionally introduced by human beings. Over the past 100 years, intensive global commerce has massively accelerated the spread of neobiota. Within a matter of hours or days, exotic plants and animals – e.g. hidden in the cargo holds of ships and planes – are transported around the globe. In the past, some species were intentionally introduced – e.g. as a form of biological pest control –, most often with serious negative consequences. Another term coined for neobiota is “invasive species”. But the word “invasive”, coming as it does from military contexts, has very negative connotations and implies a rapid, aggressive spread of the new species that always ends in catastrophe. But the introduction of neobiota only rarely – e.g. on remote islands – results in the total extinction of native species. That being said, these new species nearly always mean new competition for similar plants or animals. This can result in changing stocks of certain native species and, in turn, to major shifts in the complex interrelations of biotic communities. Consequently, there are plenty of good reasons to monitor the introduction of non-native species – and if possible, to prevent it.
Facts and Figures
100
species
More than 100 non-native marine species have been detected on the North Sea coast to date and have now permanently established themselves there.
1 or 2
species
Every year 1 or 2 newly introduced species are discovered on the North Sea coast.
16
sampling stations
Along the German North Sea and Baltic Sea coasts, 16 sampling stations are used to analyze biotic communities and record new species on an annual basis.
FAQ
How do new species reach the Wadden Sea?
The main causes for the spread of marine neobiota are maritime trade and aquaculture. Global shipping allows species to be exchanged between distant shores. For example, barnacles and bivalves attach to the steel hulls of container ships, while other species float in the ballast water used to keep the ships stable. Once a ship reaches its destination, the ballast water is unloaded together with the cargo. As a result, those organisms that survived the transoceanic journey in the tanks can now spread into new habitats or have enough time to detach from the ship’s hull and settle in their new home. Another distribution route for new species: aquaculture. The most (in)famous example in the North Sea flats is the Pacific oyster. The tasty shellfish has been imported as a seed oyster since the 1980s and is fattened in nets in the nutrient-rich waters of the Wadden Sea. As a result, the oyster has now successfully settled along Germany’s North Sea coast, and not just at the breeding facilities. But that’s not all: near these farms, non-native species are discovered time and time again, and most likely travelled along as stowaways on the oysters’ shells. In addition, anthropogenic climate change and the resultant rising temperatures in the North Sea are making it easier for these newcomers, which often hail from warmer regions, to adapt to their new home. One thing is certain: all three factors – maritime trade, aquaculture, and climate change – can be traced back to human beings.
What non-native species are there on tidal flats today, and where did they come from?
The majority of the ca. 100 new species in the Wadden Sea come from the Pacific, partly because it has also been a central hub of international maritime trade for the past several years. This is also reflected in the names of the most important newcomers: the Pacific oyster, Japanese wireweed, Pacific shore crab and Australian barnacle. In turn, the American slipper limpet and the American jackknife clam, first found in the German Bight in 1979, both come from the Atlantic Coast of North America.
How have the new species changed the ecosystem?
Especially the non-native Pacific oyster has clearly changed the face of the Wadden Sea. Today, it is forming large reefs in the intertidal zone, which has fundamentally changed the native mussel beds that are actually characteristic of the region. However, the native bivalves have not been completely supplanted, as was initially feared. If you take a closer look, today you’ll find the mussels nestled between the far larger oysters, where they filter their food from the water, safe from predators. These new “mixed” reefs also offer an attractive habitat for other non-native species. In addition to the Pacific rock crab, these include Japanese wireweed, which the oyster reefs near the low water tide line become overgrown with, forming dense underwater forests never before seen in the Wadden Sea. Certain endangered native fish species like the snake pipefish, a relative to the seahorse, and the sea stickleback have even learned to use the wireweed as a new, sheltered habitat, helping their stocks to recover. As such, the spread of the Pacific oyster has created the basis for an entirely new habitat, offering new opportunities for certain non-native and native species alike.
However, that doesn’t mean that the spread of neobiota is harmless, let alone desirable. Granted, so far, the Wadden Sea has continued to perform its important ecological functions. Yet the effects of every new introduced species are unpredictable. And the Pacific oyster also clearly shows a dark side: in 2020, experts from the AWI discovered vast expanses of the alga Vaucheria velutina, previously unknown here, on sand flats near Sylt. This species, which may have reached the North Sea by riding “piggyback” on oysters, now forms vast mats on intertidal sand flats that trap fine particles of silt. This could clog up the burrows made by the constantly digging lugworms – with potentially serious consequences for the entire ecosystem.
How are neobiota in the flats scientifically monitored?
In order to document the establishment and spread of non-native species in the German Wadden Sea, in 2009 a monitoring system was established: every year, fieldwork is conducted in selected ports and aquaculture sites, since that’s where new species are most likely to establish themselves and therefore first be discovered. The resulting data is gathered at a central office for neobiota information. In addition, the data is used to assess the ecological status of the North Sea as part of the European Marine Strategy Framework Directive.
Links
North Sea Office
