Alien species sometimes find their way into the Baltic Sea – often completely unnoticed at first. Some manage to establish themselves and spread rapidly, with major consequences for life below the surface. What does it actually mean that a species is alien, and when does it become invasive? BalticWaters explains the difference and how new species can affect our unique and vulnerable inland sea today and in the future.
When a species is described as alien, it means that it has spread outside its natural range, often with human assistance. Aquatic organisms are often spread via shipping, including through ballast water, which is taken in to stabilise the ship and then discharged elsewhere. Species can also grow on a ship’s hull and thus enter new waters. Aquaculture is also a possible route of spread for alien species.
Alien species can sometimes cause changes in the environment, but far from all of them cause damage. So what makes a species classified as invasive? Leon Green, a researcher at the Department of Biology and Environmental Sciences at the University of Gothenburg, explains:
– It is only when an alien species has a negative impact that it is called invasive. This can be in terms of effects on the ecosystem, but also in terms of impacts on, for example, the economy, human health or well-being.
The decisive factor is therefore not how many individuals there are, but what effect the species has – even though the two are often linked.
– Many invasive species quickly become numerous, and the more individuals there are, the greater their impact, adds Leon.
Predicting which species will become invasive is difficult. But they often have certain characteristics in common, such as high reproductive capacity, great adaptability and a lack of natural enemies in the new environment.
The Baltic Sea is particularly sensitive to alien species
New species are not always harmful to an ecosystem, they can also have positive effects. However, when they become invasive, they can cause severe damage. In the worst case, entire ecosystems can be destroyed, which has major negative consequences for both society and the economy. The spread of invasive species can, for example, negatively affect fishing, aquaculture and tourism, cause damage to infrastructure and water facilities, and entail significant costs for control measures.
The Baltic Sea is also particularly vulnerable to invasive species. The ecosystem is species-poor, and in order to survive here, organisms must be able to cope with both fresh and salt water – a physiological challenge that few species can meet.
– The fact that the Baltic Sea has relatively few species means that competition for space and resources is low. There are vacant niches that new species can quickly fill, and once they establish themselves, they can become very numerous, explains Leon Green.
But doesn’t the low salinity mean that fewer species can live here?
– The fact is that many ports, where ships take in ballast water, are located in brackish water environments. This means that the species that accompany ship traffic are often already adapted to a mixture of fresh and salt water and can more easily establish themselves in the Baltic Sea, says Leon Green.
The round goby
The round goby is an invasive species in the Baltic Sea and on the west coast. The species originates from the Black Sea and the Caspian Sea and was first discovered in the Baltic Sea in the mid-1990s, probably after being transported in ships’ ballast water. Since then, it has spread rapidly along the coasts and become one of the Baltic Sea’s most established alien species. It competes with native fish for food and habitat, but has also become an important prey for predatory fish such as cod and perch – demonstrating how complex the effects of invasive species can be.
Source: SLU
Alien species in the Baltic Sea are increasing as shipping traffic grows
However, the appearance of new species in the Baltic Sea is not a new phenomenon. Alien species, such as the Baltic clam, are believed to have reached the Baltic Sea on Viking ships over a thousand years ago. The problem today is rather that the number of alien species in the Baltic Sea has increased significantly – a trend that gained momentum in the latter half of the 20th century and shows no signs of abating. With the rapid growth in maritime traffic, the number is also expected to continue to increase in the future.
A study shows that the number of new alien species in the North Sea has increased from around 3 per year before 2000 to 7 per year in 2022. Forecasts based on future maritime traffic also indicate that between 20 and 40 new species per year could be introduced into northern Europe by 2050. Although the number of new alien species is lower in the Baltic Sea than in the North Sea, the trend is worrying. Species that first establish themselves in the North Sea risk spreading to the Baltic Sea and severely impacting the species-poor ecosystem of the inland sea.
Climate change increases the risk of invasive species in the Baltic Sea
In addition to increased shipping, climate change is also affecting the spread of invasive species in the Baltic Sea. The sea’s unique ecosystem makes it particularly vulnerable, and small changes in temperature and salinity can have major consequences for life below the surface. Although it is difficult to predict exactly how things will develop, it is clear that changes will occur.
According to SMHI, climate change is likely to lead to warmer water, less ice cover and lower salinity in parts of the Baltic Sea. This could have a severe impact on native cold-water species, while allowing new species from warmer seas to establish themselves. When the environment changes, already established alien species may also benefit. Species that previously had no major impact risk becoming invasive when they have better conditions for reproduction and spread.
New challenges for the management of alien invasive species
Increased risks from both climate change and increasing ship traffic place higher demands on the management of alien species. In Sweden, tThe main responsibility for the management of invasive species in the sea lies with the Swedish Agency for Marine and Water Management, while county administrative boards have practical responsibility for control and eradication at regional level.
Anna Dimming, who coordinates work on invasive alien aquatic species at the Västra Götaland County Administrative Board, points to the need for preventive measures.
– It is possible to prevent the spread of a species if it is found at an early stage of establishment and action is taken quickly. But once a species has become established, it is often practically impossible, says Anna.
Despite this, there is currently no targeted monitoring programme for alien species and their effects on the ecosystem. There is a national monitoring programme, but it only covers a few ports and sampling takes place every six years. Species that spread in other ways, for example via ocean currents, are not detected.
In order to meet future challenges, long-term funding is needed to combat invasive alien species. Funding for county administrative boards’ work with alien species in marine and aquatic environments has been significantly reduced over the past year, which, according to Anna, risks undermining previous achievements.
– Unfortunately, funding is a limiting factor. Without resources for both preventive work and continuous control measures, the costs risk becoming significantly higher as the spread of the species increases, says Anna.
At the same time, new methods in environmental DNA offer hope for more effective monitoring in the future, a method that is already being tested in several places. By analysing genetic traces in the water, researchers and authorities can detect alien species more quickly before they have time to spread on a larger scale.
BalticWater’s comment:
Alien invasive species are a threat to the biodiversity of the Baltic Sea. The spread of new species is not always negative, but it can cause great damage – not only to life below the surface, but also to society and the economy.
To address the increased risks associated with growing ship traffic and climate change, it is crucial to strengthen preventive measures – because once a species has become established, it is virtually impossible to eradicate it. At the same time, new species will inevitably find their way into the Baltic Sea. We therefore need to develop effective methods to limit their spread and adapt management to a changing reality.
A healthy ecosystem is more resistant to new species. By reducing eutrophication, limiting environmental toxins and strengthening fish stocks, we can increase the Baltic Sea’s natural resilience and give the sea better conditions to withstand future challenges.
Would you like to know more about how climate change is expected to affect environmental conditions in the Baltic Sea?
Read the article “The inflows that shape the Baltic Sea”, where we take an in-depth look at the inflow dynamics of the Baltic Sea and explain how future changes in water flows may affect the vulnerable ecosystem of our inland sea.
