When a species is accidentally introduced to an area outside its natural geographical range, it can cause considerable environmental problems. The successful establishment of a non-native species usually depends on a combination of circumstances such as a lack of natural predators, favourable physical factors such as temperature and a good food supply. Species usually become established only if they were introduced from similar latitudes. The above conditions and a fast reproductive rate can lead to rapid increases in populations of introduced species. A classic example on land was the introduction of rabbits to Australia. Species can be introduced both from the areas in which they originate or from secondary sources to which they were previously introduced.
In the UK, marine species have been introduced primarily in association with shipping particularly through the discharge of ballast water and fouling of ships' hulls. Some have been deliberately introduced for commercial reasons whilst others have come in unintentionally along with commercial species, e.g. the slipper limpet Crepidula fornicata.
A recent government study in the UK has identified 53 non-native marine species of which the majority were red algae, polychaete worms, crustaceans and molluscs (JNCC, 1995). Only a few of these species have so far proved to be a nuisance to sea users or a threat to the environment and it appears that marine communities are more able to accommodate alien species without disruption than are terrestrial ones (see also Canals, below). Well-known examples of species that have become a nuisance include Japanese seaweed (Sargassum muticum) which is clogging many inlets in the south of Britain and along North Atlantic coasts, and the slipper limpet (Crepidula fornicata) which is now a pest on European oyster beds. The newest reported arrival to become established in British waters is the large brown seaweed Undaria pinnatifida, first recorded in the Solent in June 1994. Major effects of such introductions identified by the JNCC study were:
• displacement of native species, e.g. the barnacles Semibalanus balanoides and Chthamalus spp. by Elminius modestus (from New Zealand); the brown seaweed Halidrys siliquosa by Sargassum muticum (from Japan);
• introduction of new pests, e.g. the slipper limpet Crepidula fornicata (from America);
• habitat alteration, e.g. by Crepidula fornicata beds;
• degradation of the integrity of the gene pool through hybridization, e.g. of the cord grass Spartina alterniflora (from America) with Spartina maritima to form Spartina anglica;
• associated effects of commercial harvesting, e.g. damage to eel grass (Zostera) beds through dredging for the clam Mercenaria mercenaria (from America);
• trophic alteration, particularly through dietary competition and predation (most introduced species); and
• improved water quality, e.g. from the filtering action of settlements of the serpulid worm Ficopomatus enigmaticus.
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