The patchiness of some intertidal communities on Manx rocky shores.
This study investigated the factors that maintain the patchiness of various rocky shore
communities, particularly the roles of physical disturbance and biological interactions.
Three communities were studied: a rnidshore red algal turf dominated by Laurencia
pinnatifida, a Cladophora rupestris turf that dominated an area of the lowshore'and a fucoidbarnacle
mosaic in the rnidshore that was interspersed by limpets.
The study of the Laurencia turf showed it to be extremely stable once established and that its
primary mode of recolonisation was by vegetative encroachment. Because of its slow growth
L. pinnatifida did not recolonise and dominate any gaps that occurred in the turf within the
timescale of this study.
The Cladophora rupestris community was not only extremely stable, but was also persistent.
Cladophora rupestris returned as the dominant alga to disturbed areas irrespective of the size,
shape or season of the disturbance and this may have been because of processes involving the
inhibition of other species such as Fucus serratus by early successional species which
Cladophora rupestris could tolerate. Repeated disturbance events may have broken such
inhibitions, enabling other species to colonise the substratum to higher levels of abundance
than would normally have occurred.
In the Laurencia and Cladophora turfs few limpets were required to maintain gaps in the turf,
but these areas were rapidly recolonised by algae if all limpets were removed. The gaps in the
Cladophora turf became dominated by Cladophora rupestris, however, in the Laurencia turf
Laurencia pinnatifida did not recolonise the gaps that had been kept clear by limpet grazing.
In the midshore fucoid, barnacle and limpet community the limpet densities had to be reduced
to half of the natural levels to permit the colonisation of barnacle matrix by algae. The cover
by FUCllS vesiculosus was .inversely related to the density of limpets, with the largest values
being obtained in areas that were devoid of limpets. The effect of modifying the
environmental conditions, by shading and watering the barnacle matrix, on the establishment
of Fucus vesiculosus germlings was also studied. This showed that more shade tended to
result in a greater number of fucoid escapes, possibly because of insolation stress and
photoinhibition of the germlings on the unshaded substratum.
The persistence offucoid patches was found to be affected by both the size of the area and
the duration for which it was protected from limpet grazing.
From the studies I carried out it is apparent that there were several forms of patchiness on the
moderately exposed rocky shores on the south coast of the Isle of Man. These result from the
vertical environmental stress gradient and interactions between plants and animals and in each
case the exact sequence of events that followed a disturbance, depended on the species