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
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).
Japanese seaweed, Undaria
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.,