The Mediterranean Sea is one of the largest seas in the world and has a great diversity of marine species. Currently, the Mediterranean Sea is facing various threats, with alien species being one of the most important. To address this problem, it is very important to understand how alien species arrive and where they are in the Mediterranean Sea. Furthermore, it is necessary to evaluate whether alien species affect native marine species, and if so, what kind of effects they have. This article will describe our current knowledge about the effects of alien species in the Mediterranean Sea.
Mediterranean Sea and its Marine Species
The Mediterranean Sea is one of the largest seas in the world, surrounded by 22 different countries from three different continents, Europe, Asia and Africa. The Mediterranean Sea is a unique place, with about 17,000 different marine species. Of these species, about 3,500 live only in the Mediterranean Sea and nowhere else in the world.
Despite its uniqueness and the high number of organisms that live there, the Mediterranean Sea is endangered, because of human activities. Some human activities, like fishing or tourism, overuse of marine resources, pollution of the marine environment can cause the direct destruction of marine ecosystems (communities of interacting species and their environment). Some human activities, such as shipping, have resulted in the introduction of species from other seas. We call these species alien species, and they can create serious problems for the native species living in the Mediterranean Sea .
What is the Problem With Alien Species?
Once in their new environment, some alien species do not manage to survive, because the conditions of the new environment might be too different from those of their previous homes. Other alien species, those that are more adaptable, might really like the new conditions and start reproducing, increasing in numbers and expanding throughout the area they occupy. In some cases, alien species are beneficial to their new homes, taking up useful ecological roles and supporting their native neighbors (for example, becoming a source of food for the native species or cleaning up polluted water). But, in many cases, alien species have a negative impact on native ecosystems. For example, alien species may compete for food and space with native species, or may be voracious predators of native species. Often, alien species do not have any enemies in their new environment, which gives them an advantage over native species and, in the end, alien species may cause the extinction of some native ones. When an alien species causes big problems in an ecosystem, it is called an invasive alien species.
But How do Alien Species Arrive in Their New Homes?
A marine species can move into a new area in many different ways, which are called introduction pathways . It is important to identify the introduction pathway that each alien species has used to reach a new area, because this knowledge may help us to stop or control the arrival of new alien species. One way that alien species can reach the Mediterranean Sea is through the Suez Canal, which is a man-made canal connecting the Mediterranean Sea to the Red Sea. The species that reach the Mediterranean through the Suez Canal are called Lessepsian species, from the name of the engineer (Ferdinand de Lesseps) who oversaw the construction of the Suez Canal. Shipping can also assist the introduction of alien species from distant places, as stowaways either within the ballast waters that ships store in special tanks for stability, or attached to the surface of the ship’s hull. Humans have also introduced some species to the Mediterranean Sea on purpose, to breed them in aquaculture facilities, and some alien species escaped or were released from public or private aquariums .
How do we Know That an Alien Species has Arrived in the Mediterranean Sea?
In the Mediterranean Sea, new species are frequently arriving through various introduction pathways. Because of the large number of new alien species and the large size of the Mediterranean Sea, it is very difficult to detect new alien species. Scientists from many countries work hard to find, observe, and monitor the alien species here, and collect as much information as possible. However, not only scientists can spot alien species. Many people visit the Mediterranean Sea for swimming, diving, or fishing (Figure 1). These citizens are also invited to report alien species to the blogs and authorities responsible for dealing with this threat. We call this citizen science—the contribution of citizens to the collection of data, which helps scientists. To bring together all the alien species data from various sources so that it is easy to find, the European Commission created the European Alien Species Information Network (EASIN), where people can share information about alien species across Europe, including the Mediterranean Sea. Access is provided via a website (https://easin.jrc.ec.europa.eu/) that is available to everyone who is interested.
What are the Main Introduction Pathways for Alien Species in the Mediterranean Sea?
In order to answer this question, we used all the records of alien species from the EASIN database up to January 2014. We found 986 alien species in the Mediterranean Sea. Then, we linked each alien species with at least one introduction pathway. Sometimes this was easy, while for some alien species it was very difficult. To discover introduction pathways, first we need to know the native home of the alien species. Then, depending on where we spotted the alien species the first time, we can deduce the introduction pathway. For example, if we find an alien species, whose native home is the Red Sea, in an area close to the opening of the Suez Canal, we can conclude that this species probably entered the Mediterranean Sea by moving from the Red Sea through the Suez Canal. Similarly, if we see an alien species, whose native home is the Atlantic Ocean, in an eastern Mediterranean port, we can conclude that it probably got there on a ship. However, sometimes it is more complicated, and two or more introduction pathways could be responsible for the introduction of an alien species. In our study, we managed to find one introduction pathway for 799 alien species, and two or more possible pathways for 114. We could not make reasonable assumptions about the introduction pathways for 73 alien species.
We found that the most important introduction pathway into the Mediterranean Sea is the Suez Canal, which was responsible for the introduction of 420 Lessepsian species. Most of these species are now present in the Eastern part of the Mediterranean Sea  (Figure 2). The second most important pathway was shipping, responsible for the introduction of 308 alien species that are scattered all over the Mediterranean, especially close to harbors (Figure 2). Aquaculture was responsible for the introduction of 64 alien species, which are mainly found in two areas with aquaculture facilities, the Thau lagoon (Gulf of Lion, France), and the Venice lagoon (northern Adriatic, Italy) (Figure 2).
Do These Alien Species Affect the Biodiversity of the Mediterranean Sea?
We discovered that more alien fish species are present in certain parts of the Mediterranean, like the Levantine Sea and the southeastern Aegean Sea, than in other regions. Alien invertebrate species (animals without spines) exhibited a similar pattern, but also had a high presence along the French coastline, in the Thau lagoon, along the northern Adriatic and the eastern Sicily. Alien algae showed the opposite pattern, with more alien algae species in the western Mediterranean regions. So, our analysis told us that the distribution of alien species differs according to the introduction pathway used and is dependent on the type of alien species (fish, invertebrates, algae). We also found that alien species composition, the pool of all the alien species present in an area, differed among the different regions. Finally, by analyzing the pool of native fish and invertebrates, we found a pattern that was opposite to the pattern of the alien species. In the western Mediterranean, there are more native species than in the eastern Mediterranean, while the eastern Mediterranean has more alien species than the western Mediterranean (Figure 3). So yes, alien species significantly affect the biodiversity of the Mediterranean Sea.
How can you Help?
Scientists and citizens alike need to chip in to help prevent the continued invasion of alien species! But how can regular people help? First, citizens must adopt responsible behaviors that will reduce the introduction of alien species into the Mediterranean Sea. For example, people with home aquariums must never release their marine organisms into the wild. Fishermen, when traveling long distances for fishing, must thoroughly clean their equipment before using it again, because contaminated equipment can transfer organisms from one place to another.
Also, citizens should become informed about the different Citizen Science projects that exist in their countries, through which they can contribute to the early detection and monitoring of marine alien species. In this way, citizens can meaningfully contribute to the prevention of biological invasions, by informing the scientists and the appropriate authorities.
Biological invasions are one of the biggest threats for the native ecosystems and species of the Mediterranean Sea. During the last three decades, Mediterranean biodiversity has changed rapidly, more than any other marine region in the world, as a result of human activities, such as shipping, aquaculture, and the opening of the Suez Canal. The Suez Canal and the resulting Lessepsian migration is the most important reason for this rapid introduction of alien species into the Mediterranean. Over the last two decades, the water temperature of the Mediterranean has been warming, especially in the eastern regions, and this trend is expected to continue in the future. These higher temperatures favor the establishment and spread of species that like warm waters, such as Lessepsian immigrants, which normally live in warmer seas. These days, alien species are an important part of many ecosystems, and they may change the way ecosystems function. Knowing the number of alien species in every ecosystem is important, both to understand the trend of change and to also monitor the effects of the alien species and the effectiveness of steps taken to prevent new introductions. It is very important to understand the impact that alien species have on ecosystems, because some alien species can have benefits for native species and their habitats, so those alien species do not need to be controlled. However, other alien species are dangerous for ecosystems. Understanding which species help and which harm an ecosystem helps scientists to prioritize alien species for control or eradication.
Alien Species: ↑ Species found out of their native home range, moved to new areas due to human activities.
Native Species: ↑ Species that occur naturally in an area.
Invasive Alien Species: ↑ Species introduced to an area that survive, reproduce, and expand, causing problems in the new ecosystem.
Introduction Pathways: ↑ The modes of introduction of a species to a new location outside its native range.
Biodiversity: ↑ The variety of species living in an area.
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Original Source Article
↑Katsanevakis, S., Coll, M., Piroddi, C., Steenbeek, J., Ben Rais Lasram, F., Zenetos, A., et al. 2014. Invading the Mediterranean Sea: biodiversity patterns shaped by human activities. Front. Mar. Sci. 1:32. doi: 10.3389/fmars.2014.00032
 ↑ Babić, I., Hudina, S., and Bielen, A. 2017. Invasion of the Chinese pond mussels—what makes these harmless-looking animals so dangerous? Front. Young Minds 5:56. doi: 10.3389/frym.2017.00056
 ↑ Katsanevakis, S., Zenetos, A., Belchior, C., and Cardoso, A. C. 2013. Invading European seas: assessing pathways of introduction of marine aliens. Ocean Coast. Manage. 76:64–74. doi: 10.1016/j.ocecoaman.2013.02.024
 ↑ Zenetos, A., Gofas, S., Morri, C., Rosso, A., Violanti, D., Raso, J. E. G., et al. Alien species in the Mediterranean Sea by 2012. A contribution to the application of European Union’s Marine Strategy Framework Directive (MSFD). Part 2. Introduction trends and pathways. Mediterr. Mar. Sci. 13:328–52. doi: 10.12681/mms.327