The Australian barnacle Elminius modestus

The Japanese brown macroalga, Sargassum muticum

Japanese seaweed, Undaria

Three of the main species affecting rocky shores in the UK are described below. The accidental introduction of non-native species is probably the most difficult anthropogenic impact to control. Fouling species can be transported as adults on the hulls of ships, while larvae and propagules can survive in ballast water and species introduced for aquaculture can bring other species with them. The impact of an introduced species on shore communities varies from case to case.

The Australian barnacle Elminius modestus was probably introduced into UK waters by shipping during the second world war (see Lewis, 1964, for review). It rapidly spread around the UK and mainland Europe and is now abundant in estuaries and bays. It can displace native barnacle species. However, native species perform better than E. modestus on more exposed coasts and these populations provide larvae which settle on available space on sheltered shores. Apart from a slight reduction in the populations of native barnacles, E. modestus seems to have had little effect on the structure of rocky shore communities. Recruitment variation and natural disturbance allow the coexistence of a mixed barnacle fauna.

The Japanese brown macroalga, Sargassum muticum,

The Japanese brown macroalga, Sargassum muticum, was introduced to Europe as a result of oyster transplantation. It spread rapidly thanks to floating fragments capable of reproduction. It is now common on the South coast of England, dominating low shores with a broken stone or boulder substratum. It also grows in deep rock pools. The progress of the species was hindered for a long time by the natural geographic barrier of Lands’ End, but it has recently started to spread up the north Cornish coast. S. muticum grows quickly and can clog coastal waterways. In most of the areas where it grows, S. muticum does not seem to compete directly with native species. However, it might have displaced Chorda filum from unstable habitats and reduced the abundance of Cystoseira spp. and Halidrys spp. in low shore pools. S. muticum provides an ideal microhabitat for many epiphytic species and its presence can sometimes enhance species richness (Withers et al., 1975; Critchley et al., 1990).

Of greater concern is another Japanese seaweed, Undaria, which was deliberately introduced to the French coast and has recently been found on UK shores (Fletcher and Manfredi, 1995). It is a very vigorously growing kelp, has the potential to displace native species, and is spreading quickly along the south coast of England.

Little attention has been given to assessing the types of community that are at risk from introduced species. Gray (1986) identifies the occurrence of ‘bare areas’ as an important feature. Such bare areas occur periodically on all rocky shores, even with low levels of disturbance. Since current understanding of the ecology of invasion is very limited, it is sensible to consider that all communities are at some risk (Holt et al., 1995

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