Introduction
Marine invasive species are organisms that exist in or very near water bodies, away from where they are meant to be located. Biological invasions such as that of marine invasive species, can pose a great threat to the biodiversity of the local area, and are considered to be one of the most important drivers for biodiversity loss. They can have a great impact on local ecosystems, resulting in both ecological and also economic impacts. The movement of marine species away from where they are meant to be, and as a result marine invasion, has increased in recent times as a result of globalisation, and increases in commercial shipping and transport, as well as the aquarium trade. Organisms are able to cling onto vessels, and they then get transported across the globe to parts of the ocean where they are not meant to be. (Katsanevakis, S. Wallentinus, I et al. 2014) (1)
This literature review aims to set out the main ways in which marine invasive species are transported across the globe, and ways in which this can be prevented and mitigated. It also sets out the main impacts that invasive species have on biodiversity and local ecosystems.
Transport vectors for invasive species
Biofouling, the ballast water from ships, and aquaculture are three of the most significant vectors for invasive species transport. Species are able to be transported on nearly all of the submerged areas on ships, and huge quantities of non-native species are transported daily either through bio fouling or through the ballast water in ships. (Trindade de Castro, Cecilia, M. 2019) (2) ballast water is used to assist boats in keeping stable in the water. Ships take up water and then deliberately release it at a destination in order to counteract the loading or the unloading of cargo. Organism that are in the ballast water of ships have the potential to survive and reproduce once they get out. (Fernandez, Linda 2007) (3)

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Non-native marine species are mostly introduced to new areas through methods of transport such as recreational vessels, or through commercial shipping. Recreational vessels are considered high risk for transporting marine invasive species, due to the fact that they are often moored up and not in use for long periods of the year, therefore allowing a lot of time for organisms and some marine species to colonize parts of the vessel. When the vessels are used, they often Moore in marinas, which contain areas for mooring for many vessels, and are often perfect as they contain many artificial structures that are appropriate for colonisation. Species are able to rapidly colonize and establish populations, and consequently recreational vessels are a significant vector for the spread of marine invasive species.  (Roche, R.C. Monnington, J.M. et al. 2014) (4) aquaculture is also important in the transfer of non-native species. aquaculture and the associated activities are the cause of many introductions of non-native species. The conditions used in aquaculture are often ideal for successful invasions, such as ideal salinity and temperatures. (Mach, M. E., Levings, C. D. et al. 2016). (5)
Graph 1

Impacts of invasive species
Invasive species disrupt the ecosystems of the areas that they have moved too. They compete with the native species of that area for the resources, and as a consequence there are less resources such as food for the native species, resulting in a loss in biodiversity of that area. For example, in China, some invasive species like the zebra mussel are having negative environmental, economic and human health impacts, and are growing quickly in the coastal provinces of Fujian, and Guangdong. In addition to this, some marine invasive species such as toxic algae have been introduced through the ballast water of ships, which can then enter the human food chain and possibly cause harm to human health. (liu, nengye 2013) (6) There is also the possibility that the ballast water of ships can transport both viral and bacterial pathogens, which can be harmful to human health, for example the bacteria that causes cholera. As well as this, vectors such as ballast water are able to transport marine species that are hosts for parasites that affect humans. An example is the Chinese mitten crab. This species of crab has invaded the west coast of the united states and also Europe and is a host for the human liver fluke.  In the majority of cases however, invasive species that have been transported by ballast water or by clinging onto vessels do not survive or are unable able to establish significant populations in the new areas.  The ballast water tanks of ships are usually dark and dirty, and organism do not survive long journeys. Furthermore, the conditions where the ships stop at the final destination can be unsuitable for the organism, and they simply die off and are unable to reproduce. (7) (Bax, N. et al 2003)
Invasive lionfish in the Mediterranean
The invasive lionfish species has invaded the Mediterranean and has spread at a very fast rate. The lionfish are carnivores and are able to eat a great variety of fish and some crustaceans, and consequently have decreased the abundance of more than 40 prey species in the western Atlantic by an average of 65% in the time period of two years. Lionfish are able to spawn year-round, every four days, and as well as this lionfish are able to mature early and reproduce early, meaning that invasive populations can rapidly grow and establish significant populations. Lionfish have very few natural predators because of their spines which are venomous. (kletou, D. Jason, M. Hall-Spencer et al. 2016) (8) In areas where the lionfish are native, the prey species are likely to have evolved so that they are less likely to be caught by predatory species such as Lionfish. They have developed certain traits that lowers the likelihood of predation. In contrast to this, prey species in different areas may not recognise the invasive fish as a predator, and therefore will lack behavioural traits and adaptations that will help prevent them from being caught. This is one of the most significant reasons as to why there are great reductions in prey species. Lionfish also have the potential to affect the entire coral reef ecosystem. If they cause population reductions in certain types of herbivorous fish, then herbivory as a whole will decrease in that coral reef ecosystem. This could enable seaweeds to outcompete and limit reef building corals. (Mark A.Albins 2013) (9)
 
How to mitigate
Vessel antifouling is essential to the prevention of marine invasive species. In order to inhibit or reduce the amount of hull fouling, ship owners often use anti fouling paint, or anti fouling surfaces that are specifically meant to prevent attachment of unwanted organisms. These antifouling surfaces and paint coatings usually contain biocides, some of which have been banned due to the effects of them on human health and the environment. Biofouling can also be prevented to some extent during the construction of a vessel. For example, the sloping and rounding of corners, as well as flood spaces can be built into a ship to help enable treatment and also cleaning. (10) (Kraska, Rittschof, J. et al. 2015)
Conclusion
In summary, marine invasive species are a major issue and have had and are still having a significant effect on biodiversity and the ecosystems of certain areas. This essay has set out the main ways in which invasive species are transported, which is mostly on shipping vessels through biofouling, followed by the ballast water of ships. It has set out the main impacts that invasive species have, and the impacts on biodiversity can be vast if significant populations are established, including a reduction in the population of certain species as a result of over predation, or due to the invasive species out competing other species for resources.  the majority of organisms are unable to establish significant populations, however. There is also the possibility for human health to be affected as some of the invasive species can act as vectors for parasites that can affect humans. This essay also set out some mitigation strategies and ways in which the transport of invasive species can be prevented, for example by the use of vessel anti fouling paint or surfaces, that can prevent the attachment of unwanted organisms.
 
References
1        Katsanevakis, S. Wallentinus, I et al. (2014) Impacts of invasive alien marine species on ecosystem services and biodiversity: a pan-European review. Aquatic invasions Volume 9, Issue 4: 391–423
2        Trindade de Castro, Cecilia, M. (January 2017) Invasive species in the north eastern and south western Atlantic Ocean: A review university of Plymouth
3        Fernandez, Linda (2007) Maritime trade and migratory species management to               protect biodiversity. Vol. 38 Issue 2, p165-188. 
4        Roche, R.C. Monnington, J.M. et al. (May 2015) Recreational vessels as a vector for marine non-natives: developing biosecurity measures and managing risk through an in-water encapsulation system. Vol. 750 Issue 1, p187-199. 13p.
5        Mach, M. E., Levings, C. D. et al. (2016). Non-native Species in British Columbia Eelgrass Beds Spread via Shellfish Aquaculture and Stay for the Mild Climate. Estuaries and Coasts, 40(1), 187–199.
6        Liu, nengye (2013) prevention of invasive species from ballast water International Journal of Marine and Coastal Law, Vol. 28, Issue 1 (2013), pp. 171-188
7         Bax, N. et al (2003) Marine invasive alien species: a threat to global biodiversity. Marine Policy, 27 (4), 313-323.
8         kletou, D. Jason, M. Hall-Spencer et al. 2016 A lionfish invasion has begun in the          Mediterranean Sea. Vol. 9, p1-7. 7p.
9        Mark A.Albins (2013) effects of invasive pacific red lionfish Pterois volitans versus a native predator on Bahamian coral-reef fish communities. Vol 15 issue 1, p29-43 15p
10    Kraska, J. Rittschof, D. et al. (2015) Toward a global regime of vessel ant-fouling vol.26 issue 1, p53-69. 17p
Bax, N. et al (2003) Marine invasive alien species: a threat to global biodiversity. Marine Policy, 27 (4), 313-323.