| Overview | Details | Data Sources | Links | References |
| Non-native Species | • | How Invasions Occur | • | Ballast Water | • | Examples | • | Minimizing Invasions |
In principle, a species is non-native if it has been introduced by human action into an area where it could not have arived or established without human intervention. A lack of historical evidence makes it sometimes difficult to assess what species are native and which were introduced in the past. Historically, natural barriers such as mountains, oceans, deserts and climatic conditions have provided the isolation essential to allow unique species and ecosystems to evolve, resulting in a much greater species richness across the planet. Voyages of discovery, foundation of colonies and resource exploitation as well as globalisation and growth in trade and tourism have breached these natural barriers. As a result, species have been able to travel vast distances to new habitats where they can become 'invasive alien species' (SSC, 2000).
At an international level there are hundreds of aquatic and terrestrial alien invasive species causing major impacts around the world. The International Maritime Organisation has identified 10 species with the most significant impact in the marine environment (IMO, 2004) while the Invasive Species Specialist Group (ISSG), part of the World Conservation Union (IUCN), has listed the World's top 100 worst invasive species (ISSG, 2004).
How do aquatic invasive species invade?
With the increasing volume and speed of international trade, more and more species are being transported around the world. Not all species introduced survive - some are unsuited to their new environment, while others can be out-competed by native populations. Ships are the most frequent means for introducing aquatic species in ballast water or by hull fouling. The number of species introduced by hull fouling is probably understated. Many alien invasive species, attributed to hull fouling in Ireland, were introduced prior to the usage of tri-butyl-tin (TBT) paint coatings. Now that TBT is banned in favour of less toxic coatings, port regions are likely to receive a wider range of alien invasive species because improvements in water quality will provide more suitable conditions for invasion.
Ballast Water
Ballast provides a means of stabilising ships in the water and is a necessary feature of commercial shipping. Ballast is primarily composed of water and in addition sediment and thousands of living species. Ships take in a certain amount of water for stability and trim before a voyage and once the ship arrives at its destination it may release this ballast water into the sea. International shipping including naval vessels are responsible for the majority of the invasions of alien species of foreign waters. Over 3,000 marine species travel around the world in ships' ballast water on a daily basis (Carlton, 1995).
Examples of aquatic invasive species in Ireland
Zebra Mussel
One of the main species of concern is the zebra mussel
(Dreissena polymorpha). The zebra mussel was
first recorded in the Shannon catchment in the 1990s,
but has since spread into the Erne catchment. Zebra mussels
expanded their range from rivers entering the Black Sea
and Caspian Sea to much of Europe, attached to ships and
boats, following the building of canals. The zebra mussel
was established in Britain by 1830. Due to their ability
to attach to hard surfaces, zebra mussels clog pipes,
drains and screens (as many as 100,000 per square metre).
This causes reduced water delivery to boat engines, power
plants, industries, fish hatcheries and municipal waterworks
resulting in engine overheating, fish mortalities, increased
maintenance costs and sometimes short term closures in
production. In addition, mussels occur in such numbers
that they can change the ecology of some aquatic systems.
Zebra mussels can live out of water for several days in
moist shaded areas. When kept wet, adults can survive
for more than a week, allowing them to be successfully
transported out of water from one region to another. Controlling
the spread of zebra mussels is difficult as it is reliant
on the vigilance of boat-owners. Running the boat engine
at high speed can help detach fouling mussels and hull
waxes or silicone applications can significantly reduce
fouling.Wireweed
 S. muticum found in Farranamanagh, Co. Cork in 2002. |
 Sargassum muticum found in Co. Clare. |
Ways to minimize the introduction of invasive species
| The main method practiced
to reduce the risk from ballast water is by exchanging the water while
in mid-ocean, as recommended by the International Maritime Organisation
(IMO). Although re-ballasting can reduce the risk of introductions,
it will not prevent biota from becoming established on every occasion.
For this reason continued investigations to find a practical method
for sterilisation of ballast water is needed. Other steps to minimize
the introduction of non-native species include: controlling which
species to import/export (especially for mariculture), proper disposal
of aquarium species and the use of non-toxic anti-fouling coatings
on ships hulls. 'Biosecurity' aimed at preventing the introduction
of non-native species is increasingly becoming a major activity in
some parts of the world. |
Other Invasive Aquatic Species Styela clava. In Ireland these include a copepod parasite of oysters Mytilicola orientalis, now established in Dungarvan Bay, the nematode Angullicola crassa that infests the swimbladder of eels and the alien ascidian Styela clava that fouls oyster beds. |
Many volunteer recording projects are in operation throughout Britain and Ireland, which are invaluable to scientists studying changes in marine ecosystems. Biologists are still trying to characterize the capability of non-native species to invade, with hope of predicting and stopping such invasions (ISSG, 2002).