Alien Species

Invasive 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.

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 3–5 billion tons of ballast water from large ships transferred throughout the world each year by large ships (Raaymakers, 2002) is believed to be the main vector for the spread of invasive aquatic species today, with an estimated 7,000 species transported each day (Carlton, 2001). Large numbers of alien species are also transported as ‘hitchikers’ attached to the hulls of ships or on floating objects such as marine trash.

Origins & Pathways of Invasive Marine Species
Download Infographic

Which Goals Does This Affect?

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 (GISD). The database reports the number and type of alien species in each marine ecoregion, with species types categorized as invasive and harmful invasive species.

Total count of all invasive species was used. Ecoregion based data were re-aggregated to match the Ocean Health Index’s EEZ-based reporting region. The sum of all invasive species within each reporting unit was then rescaled 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 case-studies where such information could be obtained.

What Are The Impacts?

Invasive Marine Species - Dispersal Methods
Download Infographic

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?

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.

Invasive Species Specialist Group (ISSG)
A network of scientific and policy experts that works to reduce the threats of invasive species by raising awareness and introducing measures to prevent and control alien populations.

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.

Show More


PHOTO(S): © Keith A. Ellenbogen
Follow us