Non-indigenous species: Curse and blessing
The Suez Canal as a gateway
The silver-cheeked toadfish (Lagocephalus sceleratus) must be handled with extreme caution. This spotted fish, a member of the pufferfish family, can grow up to a metre in length. It not only bites through fishing nets and metal hooks with ease, but also produces its own venom, called tetrodotoxin, which is found on the fish’s skin as well as in its internal organs. One to two milligrams of this neurotoxin is said to be enough to kill a person, the University of Pula, Croatia, warned in the early summer of 2024. Shortly before the warning was issued, a Croatian fisherman had caught a silver-cheeked toadfish for the first time in the Adriatic Sea and thus in the northern part of the Mediterranean where this fish species does not belong at all.
The silver-cheeked toadfish normally lives in the tropical Indo-West Pacific, in the equatorial waters south of India and the Asian continent. Thousands of kilometres, as the crow flies, lie between its home and the Adriatic Sea. Nevertheless, these squid-eating predators can now be found in large parts of the Mediterranean. In recent decades, this and many other species of flora and fauna from the Red Sea and the Indian Ocean have managed to migrate to the Mediterranean via the Suez Canal.
- 5.1 > The Suez Canal on the eastern edge of the green Nile Delta allows marine organisms from the Red Sea and the Indo-Pacific region to autonomously migrate into the Mediterranean Sea and establish themselves as non-indigenous species.
- The shipping lane in Egypt proved to be an easy gateway. Since its opening in November 1869, the way has been clear for Pacific marine organisms to autonomously spread into the Mediterranean Sea. Even today, more than 155 years after the canal’s official opening, species from the Indo-Pacific are still migrating into the Mediterranean, driven by ocean warming and the widening and deepening of the canal – some for the first time, others repeatedly. Particularly in the eastern part of the Mediterranean, and especially in the coastal waters of Israel and Palestine, marine biologists now recognize more than 450 non-indigenous species of flora and fauna that have migrated through the Suez Canal and settled in the Mediterranean. These include cnidarians such as the nomad jellyfish (Rhopilema nomadica), which occurs in large swarms, more than 160 species of bivalves, and an armada of more than 100 species of fish.
An influx with serious consequences for local biodiversity
Experts have not yet been able to conclusively assess the impact that this invasion has had on native Mediterranean life. However, there is clear evidence that some of the immigrant species are changing native biological communities. These include the rivulated rabbitfish (Siganus rivulatus) and its relative, the squaretail rabbitfish (Siganus luridus). The former was first discovered in Israeli coastal waters in 1924, followed by the latter 31 years later. Since then, the two algae-consuming species have spread to the waters of France and Tunisia.
The two rabbitfish species now make up 95 per cent of the fish biomass in the rocky coastal areas of the Mediterranean. Because of their predilection for certain types of brown algae, they have overgrazed the underwater algal forests in many places. Researchers report that instead of lush macroalgae forests, sponges and bare rocks now dominate the affected coastal areas.
- 5.2 > The jellyfish Rhopilema nomadica is native to the east coast of Africa and the Red Sea. It was introduced into the Mediterranean in the late 1970s via the Suez Canal. Since then it has spread along the Levantine coast from Egypt to Turkey and Greece.
Extra Info Making way: How humans pave the way for non-indigenous species
An important question: Do the newcomers harm the established ecosystem?
The observations from the Mediterranean are one of many examples of how non-indigenous species, also known as neobiota, are changing the structure and functioning of established marine biological communities. The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) therefore lists species of flora and fauna that have been introduced into new habitats by humans or whose autonomous immigration has been facilitated by humans as one of the five direct drivers of global biodiversity decline.
Experts are particularly concerned that the global dispersal of species will increasingly harmonize biological communities in different marine areas. The fear is that a few robust species will become ubiquitous, while specialists will be pushed back. In the long term, this could lead to a global decline in biodiversity. Whether these fears will one day be realized is a matter of debate among scientists. The importance of non-indigenous species in altering established ecosystems and their services is now so great that the IPBES Member States commissioned the IPBES Expert Panel to produce a special report on the subject. The report, published in September 2023, makes a clear distinction between non-indigenous species and invasive non-indigenous species. Non-indigenous species are all organisms that have been introduced into new regions as a result of human activity. The term “invasive”, on the other hand, is restricted to those non-indigenous species that have colonized and spread in their new home and are having a negative impact on local ecosystems, their species and biodiversity.
The proportion of invasive species can be surprisingly low. European waters, for example, are now home to more than 800 neobiota – and according to current knowledge, only a small number of these are harmful to native biological communities and the services they provide.
- 5.4 > After the last ice age, the sand gaper Mya arenaria was only found on the east coast of America. It was then introduced into the North Sea by the Vikings 800 years ago. Juvenile sand gapers in particular have been an important food source for crabs and seabirds there ever since.
5.5 > The Russian cargo ship Iohann Mahmastal is waiting for permission to dock at the Yamal 2 natural gas terminal in western Siberia. The ship is carrying food and construction materials for the new natural gas production facility. The ship had to sail through the Northeast Passage to deliver its cargo. Each such voyage increases the risk of species being introduced into the fragile ecosystems of the Arctic Ocean.Immigration pathways: Marine organisms on the move
Humans intentionally and unintentionally enable marine organisms to cross the boundaries of their traditional range and migrate to new marine regions. For example, the many stowaways that are transported across the world’s oceans clinging to the hulls of boats and ships or floating in ballast water represent unintentional introductions. Once they reach their destination, the organisms fall off or are disposed of with the ballast water in their new home. The Vikings already introduced nonindigenous species such as the North American sand gaper (Mya arenaria) into the waters of the Kattegat 800 years ago. The risk of introducing neobiota by ship increased with the development of modern cargo ships and the globalization of trade. Whereas in 1750 wooden merchant ships could carry an estimated 120 species of marine biofouling and boring organisms, 200 years later international merchant fleets transported more than 10,000 different marine species from one region to another in their ballast tanks. But then, as now, coastal insects and plants were also added to the mix, starting their long journeys hidden among the pebbles, sand and coastal debris forming part of ships’ ballast.
In February 2004, the International Maritime Organization (IMO) adopted the International Convention for the Control and Management of Ships’ Ballast Water and Sediments to reduce the introduction of such unwanted species. The Convention, which came into force on 8 September 2017, stipulates that ballast water can only be discharged if certain limits or requirements are met. For example, ship crews must keep detailed records of when and where they discharged or took on ballast water. Ballast water must also be treated to remove or render harmless any harmful aquatic organisms and pathogens it may contain, or to prevent their ingestion or introduction. For example, one of the regulations states that ballast water may only be discharged into the sea if less than ten viable organisms with a minimum size of 50 micrometres can be detected per cubic metre of water.
5.6 > The polyps of the orange cup coral (Tubastraea coccinea) only extend their tentacles in the dark to catch food. Originally from the Indo-Pacific, this stony coral has spread across the Caribbean to the east coast of Brazil, where it now competes with native Atlantic corals.- Nevertheless, shipping continues to contribute to the spread of species. This applies both to existing transport routes, where traffic is increasing, and to regions such as the Northeast Passage through the Arctic Ocean, where tankers and cargo ships have only recently begun to operate regularly.
Escapes of non-indigenous species from aquaculture cages or other marine breeding facilities are expected, but unintentional. It is estimated that between 1993 and 2012, more than one million Atlantic salmon (Salmo salar) and silver salmon (Oncorhynchus kisutch) escaped into the South Pacific every year from farms in Chile alone. The escapees then mixed with native fish species. The extent to which the farmed salmon compete with the native wild species has not yet been clearly established. This is mainly due to a lack of baseline knowledge as to the species composition and functioning of fish stocks in Chile’s rivers, lakes and fjords. Nevertheless, there has been a human-induced change in species composition with unknown consequences.
In addition to increasing competition for food, farmed fish outbreaks pose two other risks to local biodiversity. Firstly, fish farm escapees can carry viruses, lice and other pathogens that can be transmitted to wild fish and other native marine organisms. Secondly, escaped fish may reproduce locally and establish their own populations. However, for such colonization to be successful, many species-specific environmental conditions must be met. At least in Chile, despite the high numbers of outbreaks in the 1990s and 2000s, there have been no reports to date of escaped Atlantic salmon or silver salmon being able to reproduce in the wild and establish permanent populations.
The situation is different in the Amazon region: Intensive fish farming has contributed significantly to the establishment of 41 non-indigenous fish species in the river and its estuary, more than half of which are predatory or omnivorous. With these research results from 2021, the researchers involved have also disproved the classic assumption that non-indigenous species find it more difficult to colonize species-rich habitats than species-poor ones.
- Observations from European coastal waters highlight the extent to which non-indigenous aquaculture species can carry hitchhikers. Around 60 different pathogens, parasites and fouling species such as tunicates have spread with introduced farmed bivalves such as the Manila clam (Ruditapes philippinarum) and the Pacific oyster (Magallana gigas).
When humans build artificial waterways, such as the Suez and Panama canals, the subsequent immigration of non-indigenous species is considered unintentional. Experts do not refer to this as active alien species introductions, but rather as the removal of one or more barriers to dispersal.
Interestingly, when new corridors open up between two marine habitats, species always migrate from the species-rich region to the species-poor region, not vice versa. This explains the immigration of Pacific species to the Mediterranean. Almost no species have migrated in the other direction.
When people deliberately release non-indigenous species, there are clear agendas behind their establishment, such as the desire to locally hunt or fish these species. For example, the edible mouthless crab Cardisoma crassum, which is native to Ecuador, ended up in the coastal waters of the Galapagos island of Santa Cruz in this manner.
A profitable fishing industry has developed in Norway’s Barents Sea since the red king crab (Paralithodes camtschaticus) from the North Pacific was deliberately released there in the 1960s. In the absence of natural predators, the large crabs have since spread along the Norwegian north coast – to the delight of local fishers. Today, they export red king crabs worth the equivalent of 100 million Euros per year.
- Humans are partly to blame when species drift into new ocean regions, for example by attaching themselves to plastic debris. The Pacific sun corals Tubastraea coccinea and Tubastraea tagusensis, for instance, have been spreading along the Atlantic coast of Brazil using this pathway. The two Pacific species have extensive defence mechanisms, reproduce rapidly and, due to the lack of predators in the South Atlantic, are well placed to displace native corals such as the reef-building stony coral Mussismilia harttii from their home reefs.
However, experts consider the introduction of non-indigenous species by flotsam and jetsam to be immigration without direct human assistance. Moreover, immigration as a result of climate-induced ocean warming plays a special role. In these cases, organisms autonomously expand their range. They are therefore not considered active introductions of alien species by humans, even if climate change is caused by humans.
How many non-indigenous species are in the ocean?
The first reports of colonisation by non-indigenous marine organisms date back to the early 19th century. Since then, the number of known immigrant or introduced species in the global ocean’s various ecosystems has increased steadily, partly because there has been an increase in the amount of research undertaken on the topic since the 1970s. In 2020, a team of scientists analyzed a large number of existing datasets on non-indigenous aquatic organisms. They found that between 1965 and 2015, an average of 43 primary detection events for new non-indigenous species had been recorded per year in marine and coastal waters around the world and in the Great Lakes of North America.
In total, the researchers counted 1442 introduced or immigrant species in 39 marine and freshwater ecosystems. The newcomers primarily included organisms from the arthropod, bivalve and fish phyla, as well as red algae, annelids, tunicates, and also bryozoans and cnidarians.
- 5.7 > Global shipping is a major cause of the immigration and introduction of non-indigenous species. This fact is highlighted in this graph, which shows the pathways through which newly discovered non-indigenous species in marine habitats and onshore waters were found to have arrived.
- Detailed analysis also showed that the rate of detection of newly established non-indigenous species was relatively stable during 1965 to 1995. On average, 32 non-indigenous species were newly detected each year. In subsequent years the detection rate increased steadily, reaching 51 primary detections annually by the year 2000 and 66 primary detections per year during 2005 to 2010. Possible reasons for this are the increase in shipping on the world’s oceans, the expansion of fish and shellfish farming in aquaculture, and better monitoring programmes in some of the world’s coastal regions.
However, the researchers believe that their data greatly underestimate the true number of neobiota, partly because the datasets they analyzed cover only 73 per cent of the world’s most important aquatic ecosystems. Secondly, in many places it is not at all clear which species have always lived in the region, and which organisms arrived when and how.
As a result, the comparatively young field of marine invasions research still has knowledge gaps at the global level, particularly with regard to standardized methods, comparative time periods and comprehensive observation efforts. For example, there is a lack of information on the extent to which non-indigenous species have invaded the open ocean and water depths greater than 200 metres. Most of the species listed in the study are thought to have entered the ecosystems studied with the help of ships (biofouling, ballast water, etc.). Only a small proportion of the detected introductions were due to escapes from aquaculture installations.
- 5.8 > Researchers have been able to detect many more invasive species on land than in the oceans. However, the number of introduced marine organisms has also been increasing. This is due in part to an increased likelihood of new introductions. At the same time, new monitoring programmes mean that new arrivals are more likely to be detected.
![fig. 5.8 © after IPBES (2023). Thematic Assessment Report on Invasive Alien Species and their Control of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. [H. E. Roy, A. Pauchard, P. Stoett, T. Renard Truong (eds.)]. IPBES secretariat, Bonn, Germany. doi:10.5281/zenodo.7430682, Figure SPM 4](https://worldoceanreview.com/en/files/2026/01/wor9_k5a_abb_5-8_b950_en-500x286.jpg "fig. 5.8 © after IPBES (2023). Thematic Assessment Report on Invasive Alien Species and their Control of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. [H. E. Roy, A. Pauchard, P. Stoett, T. Renard Truong (eds.)]. IPBES secretariat, Bonn, Germany. doi:10.5281/zenodo.7430682, Figure SPM 4")
![fig. 5.8 © after IPBES (2023). Thematic Assessment Report on Invasive Alien Species and their Control of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. [H. E. Roy, A. Pauchard, P. Stoett, T. Renard Truong (eds.)]. IPBES secretariat, Bonn, Germany. doi:10.5281/zenodo.7430682, Figure SPM 4](https://worldoceanreview.com/en/files/2026/01/wor9_k5a_abb_5-8_b950_en.jpg "fig. 5.8 © after IPBES (2023). Thematic Assessment Report on Invasive Alien Species and their Control of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. [H. E. Roy, A. Pauchard, P. Stoett, T. Renard Truong (eds.)]. IPBES secretariat, Bonn, Germany. doi:10.5281/zenodo.7430682, Figure SPM 4")
- Since the late 1990s, however, scientists have documented an increase in non-indigenous species that have either migrated through new corridor pathways or lived in aquariums until their ill-considered release.
The adverse effects of the release of marine organisms have been demonstrated by the rapid and uncontrolled spread of the Indo-Pacific red lionfish (Pterois volitans) into the tropical regions of the western Atlantic. The fish were probably introduced into Florida’s coastal waters by hobbyist aquarium fishkeepers in the mid-1980s. With no real local enemies, the predatory fish spread across the Gulf of Mexico to the Caribbean within 30 years. These solitary fish are found mainly in coral reefs, where they prey on young reef fish. Scientific experiments in the Bahamas have shown that a red lionfish can significantly reduce the number of juvenile fish in the coral reef it inhabits.
Neither good nor bad
Non-indigenous species not only benefit from the fact that they often do not have to face natural enemies in their new habitats. Successful newcomers frequently have a high physiological tolerance to abiotic factors such as temperature, allowing them to adapt quickly to the conditions in the newly colonized habitat. This permits them to spread rapidly after successful establishment. However, it is too narrow a view to categorize the introduction, establishment and subsequent spread of nonindigenous species as a good or a bad thing as such. Nature knows no such labels.
In nature, change means that some groups of organisms benefit, while others may lose out. For others still, things remain as they are. When non-indigenous organisms invade a habitat, the key question is whether they perform ecological functions similar to native species and whether the range of ecosystem services is maintained. If this is the case, the species inventory changes, but not the ecosystem’s functional diversity.
- 5.9 > Hunting is currently the only way to stop the Indo-Pacific red lionfish (Pterois volitans) from spreading further into the tropical western Atlantic. The first of these invasive fish were probably released by hobbyist aquarium fishkeepers in Florida’s coastal waters in the mid-1980s.
- Compared to terrestrial forests and grasslands, many marine ecosystems are more open beyond their boundaries. As a result, native marine organisms are much better able to avoid introduced species than terrestrial organisms would be. In many instances, native species also come to terms with the immigrants. Studies in the European Wadden Sea, the world’s largest contiguous system of intertidal sand and mud flats, located in the eastern and south-eastern North Sea, show how this coexistence of native and introduced species can actually increase the biodiversity of a marine habitat.
The Wadden Sea as an outdoor laboratory
Researchers are now aware of more than 100 non-indigenous species that have colonized the Wadden Sea, either living on the seabed or as free-swimming organisms in the water column. Twelve per cent of the newcomers arrived directly from remote marine areas. The majority, however, first migrated to neighbouring regions and then made the leap to the Wadden Sea – often because man-made structures such as harbours made it possible for the new organisms to settle.
The Wadden Sea as we know it today is less than 8000 years old. It is therefore a comparatively young habitat that has always been home to many organisms, but only a comparatively small number of species. In other words, its biodiversity is rather low. However, with the increase in shipping since the beginning of the 20th century, the number of introduced species has steadily increased, partly because seawalls and other structures were built along the coast to which stowaways could attach themselves.
Earlier, in the 14th century, the American sand gaper (Mya arenaria) had migrated from the Kattegat. The Vikings had brought it there. Juvenile sand gapers are an important food source for crabs and seabirds in the Wadden Sea to this day. Adults can live for up to 20 years. By then their shells will have grown to a length of up to 15 centimetres, making them one of the largest bivalves in the Wadden Sea.
- The Chinese mitten crab (Eriocheir sinensis) is another non-indigenous species. It arrived in the early 20th century when ships started using ballast water for stabilization. It is now the most common crab in the rivers flowing into the Wadden Sea. However, the mitten crab has not yet had any impact on the intertidal ecosystem.
The same cannot be said for the Pacific oyster (Magallana gigas). This fast-growing species spread to the Wadden Sea after fishermen first grew it in oyster farms in France, the Netherlands and the UK. Since 1986, it has also been cultivated as an aquaculture species in net bags off the island of Sylt and thus directly in the Wadden Sea. Oyster larvae only settle on solid ground, which in the Wadden Sea they found in the form of the native blue mussel beds. The juveniles settled on the shells of the mussels and quickly overgrew the beds they form.
However, what initially looked like a hostile takeover followed by displacement turned out over the next 20 years to be a beneficial coexistence. The oysters began to grow only on top of each other, no longer using the mussels as a hard substrate. They quickly formed a new “overstory” on top of the mussel beds.
- 5.10 > This is what a successful coexistence of introduced and native species looks like. In the Wadden Sea, mussels and immigrant Pacific oysters form multi-storey mussel-oyster beds that withstand weather extremes better than pure mussel beds.
- The mussels made their home in the much muddier level below. While nowadays they have less space than in the past and compete with the oysters for food, the top layer of oysters protects them from predators, such as the green shore crab, and from other organisms harmful to the mussels, such as barnacles, which attach themselves to the mussels’ shells. All in all, there are now many more organisms living in the same area, although not all mussel beds are colonized by oysters. Another advantage is that oyster-mussel beds appear to be so robust and stable that they can withstand strong storms much better than mussel-only beds.
And there is another thing the researchers learnt from their observations in the North Sea and the Wadden Sea: the immigration of the same non-indigenous species into different ecosystems can have very different effects. This is shown by the experience with Japanese wireweed (Sargassum muticum). On the rocky coast of the Heligoland archipelago, this macro-algae, which can grow to more than four metres in length, competes with native seaweeds in places. When it wins, it performs the same ecosystem functions as its predecessors, providing a habitat for all the species that used to live in the other algal thickets. In contrast, in the sandy Wadden Sea the Japanese wireweed colonizes the newly formed mixed mussel-oyster beds and forms true algal forests of a size and complexity never before seen in the Wadden Sea. These forests in turn attract other smaller algae, invertebrates and many fish that hide among the kelp. This provides a new habitat and increases the mussel beds’ species diversity.
Such details are known because there are established programmes both in the Wadden Sea World Heritage Site and beyond its borders in which researchers monitor the development of non-indigenous species. Along the German North and Baltic Sea coasts, for example, 16 stations document the occurrence of non-indigenous species and their abundance.
Observations so far only cover short periods of time
Many introduced species exhibit a “boom and bust” development pattern, meaning that after initial mass development, the number of individuals decreases as the species adapts to the new ecosystem. In contrast, other species behave inconspicuously at first and occur in low numbers for a long time. But when climatic or hydrographic conditions change, these “ecological sleepers” may multiply explosively, increasing their impact on the ecosystem. However, all these observations have been limited to periods of only a few decades. As a result, little is known about the long-term effects of non-indigenous species on native ecosystems and biological communities. This is particularly true for the long-term consequences of viruses and diseases that non-indigenous species introduce into new habitats. These have been the subject of little research to date.
For this reason, experts argue that everything possible should be done to prevent new introductions of marine non-indigenous species. Once an invader has established itself in a new habitat, it is almost impossible to push it back, precisely because marine habitats cannot be sealed off. The European Marine Strategy Framework Directive from 2008 therefore includes non-indigenous species as one of the parameters for assessing a marine region’s environmental status. In this context, a quantifiable indicator is being developed to capture the status of a marine area with regard to neobiota.
In its special report from 2019, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) concludes that the introduction of non-indigenous species is a complex process, intertwined with the transport of goods and other components of global change, such as land-use change, climate change and human disturbance. This ecological complexity, the diversity and abundance of non-indigenous species and the difficulty of identifying invaders in new environments have made their prevention and management challenging. However, both are essential to maintaining the natural development of ecosystems.
The IPBES experts conclude that there is now almost nowhere on Earth where non-indigenous species have not migrated or been introduced. They write that research also shows that the number of invasive non-indigenous species is increasing in all regions of the world. At the same time, however, there are major gaps in our knowledge, and valid data are not always available. As a result, the long-term consequences of immigrant species for biodiversity on land and in the oceans are being underestimated.