• New   marine   life   is   arriving   into   our   waters   both   by   migration,   range   extension, and human introductionThe action bringing a non-native species to a new environment, as opposed to an extension of range..
• The number of species of non-indigenous flora, fauna and algae is increasing in marine habitats and some are causing major ecological changes.
• Distributions of some non-native species are currently limited by water temperature.
• Warmer UK waters over the last three decades are facilitating the establishment of some of these species.
• Future temperature increases could enable a wider range of species to invade and become established, replacing current native species.

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What is happening now

We would suggest a medium level of confidence regarding "what is happening now". This applies to non-native species overall as the information available in the UK is moderate, and consensus is also only moderate.

What could happen in the future

However, we would allocate a high level of confidence as to future effects; there is a growing body of evidence from the rest of the world that climate change can facilitate marine invasions, and the potential risks from new introductionsThe action bringing a non-native species to a new environment, as opposed to an extension of range. in the future are both high and potentially disastrous.

One of the major problems of assessing the potential impact of climate change on non-native species is the lack of knowledge regarding where many of the species are established. There has been no full scale base-line survey of the presence of non-native species in the marine environment so the current distribution of many species is not known.

There is an urgent requirement for monitoring of the range of and effects of climate change on, established invaders. Only then can detailed risk assessments and contingency plans be prepared for future invaders. Further, the question of how climate change will interact with other ecological pressures (such as invasive species or habitat fragmentation) to create synergistic effects also needs to be considered (Sutherland et al. , 2006).

Commercially, some economically important species have been introduced, but some associated pests and parasites adversely affecting native species have also been unintentionally introduced. Control methods, where applied to nuisance species, are fairly ineffective and no non-native marine species have yet been successfully eradicated from British waters. Of the species deliberately introduced for aquaculture, only a few bivalve molluscs have become established in the natural environment beyond the confines of their cultivation. For example Crepidula fornicata has become a dominant mollusc in estuaries on the south coast and especially in the Solent, outcompeting oysters.

The Pacific oyster, Crassostrea gigas , is an important invasive species in its own right because it is extensively cultivated in Scotland. Cultivation of this species is controlled in that it occurs in containment (i.e. on trays or in bags) and is only allowed to go ahead after a consultation process. It is assumed that the low temperature of Scottish waters would mean that this species would be unable to establish itself. There has been no successful spat fall recorded in Scottish waters although maturation of the gonad and the occasional release has been noted but not settlement or establishment of populations. As this species has become established in other countries such as the Netherlands and Germany as well as areas in the south of the UK there is potential for an increased risk of the species becoming established in Scotland as water temperatures increase. This could lead to the out-competition of native filter feeders (Eno et al. , 1997) and the extensive modification of estuarine habitats.

The introductionThe action bringing a non-native species to a new environment, as opposed to an extension of range. of non-indigenous marine plankton via ballast water can also have a considerable ecological and economic effect on regional systems (Edwards et al. 2001). Some of these species can form Harmful Algal Blooms (HABs) and as a consequence of regional climate warming it is thought that many more non-indigenous species may become established in the future (e.g. Gymnodinium catenatum)

Paul Elliott
Aquatic Ecology Group, Department of Zoology , Cambridge University, Downing Street, Cambridge, CB2 1TL

Philip C. Reid and Martin Edwards
Sir Alister Hardy Foundation for Ocean Science, The Laboratory, Citadel Hill, Plymouth, PL1 2PB.

Tracy McCollin
Fisheries Research Services Marine Laboratory, PO Box 101, 375 Victoria Road, Torry, Aberdeen, UK, AB11 9DB