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Alien species are non-indigenous organisms introduced into an ecosystem that is not their native habitat either by accident or intentionally. 

While some alien species may have little impact within their new habitat, others can become invasive and pose a serious threat to marine biodiversity, coastal economies, local cultures and livelihoods, and human health. They are therefore used as a Pressure component for many of the goals assessed by the Ocean Health Index.

The threat of alien invasive species continues to grow as global trade, travel, and tourism allow species to be transported over increased distances to areas that were not previously accessible to them. Areas subjected to the worst pollution, intensive fisheries and/or bottom trawling, and major shipping routes are likely to be the most seriously impacted by the invasion of non-native species.

The approximately 3–12 billion tonnes of ballast water transported and discharged throughout the world each year by large ships contains thousands of species and is believed to be a main vector for the spread of invasive aquatic species today. Large numbers of alien species are also transported as ‘hitchikers’ attached to the hulls of ships, on floating objects such as marine trash and accompanying marine plants or animals brought in for mariculture.  

How Do We Measure It?

The Ocean Health Index utilized total counts of all invasive species according to data from the Global Invasive Species Database (GIRD). The database reports the number and type of alien species in each marine ecoregion, with species types categorized as invasive and harmful invasive species.  

Ecoregion based data were re-aggregated to correspond with the Ocean Health Index’s EEZ-based reporting areas. The sum of all invasive species within each reporting area was then rescaled with reference to the maximum global value. It was not possible to predict the full potential impact of alien species, because high-resolution data are not yet available on where these species exist, how far they have spread, and exactly what parts of the ecosystem or food web they affect. Harmful effects would need to be evaluated separately for each goal. That is not yet possible globally, but it might be done for smaller-scale regional assessments where such information could be obtained.

Origins & Pathways of Invasive Marine Species

What Are The Impacts?

Invasive Marine Species - Dispersal Methods


Invasive alien species can harm environments by preying upon or parasitizing native species or competing with them for resources.


Invasive species can introduce harmful microbes and associated bacteria that are transferable to local seafood and bivalve populations, increasing the risk of cholera or other communicable diseases or epidemics.

A South American strain of human cholera bacteria was found in ballast tanks in the port of Mobile, Alabama in 1991. Cholera strains were also found in oyster and fin-fish samples in Mobile Bay, resulting in a public health advisory to avoid handling or eating raw oysters or seafood (Habitattitude 2009).


Eradicating or controlling an invasive alien species is costly and can have significant economic impacts.

In 2000, New Zealand spent $3.5 million to remove a species of invasive seaweed, Undaria pinnatifida, from the fouled hull of a single vessel that sank offshore.

Invasive alien species can decrease the number of visitors to a coastal area and reduce revenue from coastal tourism and recreational activities.

What Has Been Done?

The lionfish, and Indo-Pacific species, was introduced to South Florida waters in the early 1990’s and has been rapidly spreading throughout the Caribbean. Because it is a natural predator to most reef fish in the area, the lionfish  has the potential to significantly decrease populations of ecologically important native species.

To combat the lionfish, Reef Environmental Education Foundation (REEF) hosts numerous lionfish derbies around Florida. Divers who remove lionfish from sanctuary waters are eligible to win cash and prizes.

Mr.KIO/Marine Photobank

Get More Information

The Global Invasive Species Database (GISD) 

The GISD aims to increase awareness and facilitate the prevention and management of invasive alien species.

The International Union for Conservation of Nature (IUCN)  

This booklet provides detailed, accessible information on the marine invasive species issue, and 15 case studies of particularly damaging or costly bioinvasions.  

GloBallast Partnerships: GEF/UNDP/IMO Global Ballast Water Management Programme 

GloBallast works with developing countries to reduce the transfer of invasive species in ballast water and implement the International Maritime Organization (IMO) ballast water guidelines.   

The National Invasive Species Council

Established in the United States by Executive Order 13112, the council ensures that the Federal programs being implemented to prevent and control invasive species are effective.


IUCN. (2009). Marine Menace: alien invasive species in the marine environment. International Union for Conservation of Nature: Gland, Switzerland.

Molnar, J. L., R.L. Gamboa, C.  Revenga and M.D. Spalding. (2008). Assessing the global threat of invasive species to marine biodiversity. Frontiers in Ecology and the Environment 6: 485–492.

Padilla, D. K. and S.L.Williams. (2004). Beyond ballast water: aquarium and ornamental trades as sources of invasive species in aquatic ecosystems. Frontiers in Ecology and Environment 2: 131–138 (2004).