2 F
. Bulleri et al. J. Exp. Mar. Biol. Ecol. 255 2000 1 –19 Keywords
: Artificial reefs; Intertidal; Limpets; Patella aspera; Patella rustica; Grazing; Indirect effects; Spatial heterogeneity
1. Introduction
Artificial reefs and breakwaters are common structures in marine coastal habitats. Breakwaters are used to build marinas and harbours and to protect sandy shores from
erosion. Subtidal reefs are thought to attract species previously absent in the area, and are often used to restore over-fished populations by increasing the complexity of the
habitat and the availability of shelter Seaman et al., 1989; Carr and Hixon, 1997. Since these structures act as surrogates of rocky shores, it is important to understand whether
they support assemblages that are comparable to those found on natural substrata and if the ecological processes operating in the two environments are also similar.
Most of the ecological studies on artificial reefs and breakwaters have been confined to subtidal habitats. There have been studies assessing the effectiveness of artificial reefs
in mitigating losses of commercial species due to human disturbance Ambrose, 1994; Carr and Hixon, 1997, some experimental investigations on the effects of dispersal,
recruitment, physical processes and biological interactions in influencing the structure of epibiota on artificial structures Grosberg, 1982; Keough and Butler, 1983; Keough,
1984; Breitburg, 1985; Anderson and Underwood, 1997, and comparisons of assem- blages on these structures with those of rocky shores Connell and Glasby, 1999;
Glasby, 1999. More generally, studies on artificial substrata have contributed to the development and refinement of ecological models to explain patterns in natural habitats
e.g., Sutherland, 1974; Sutherland and Karlson, 1977; Anderson, 1998. In contrast, much less is known about patterns and processes of intertidal assemblages on artificial
substrata.
The present study is part of a research programme on the ecology of epibenthic assemblages on hard substrata, including both rocky shores and artificial reefs, in the
northwest Mediterranean. Here we focus on the effects of limpets in midshore habitats provided by artificial reefs. Understanding grazing on artificial substrata is important for
comparative purposes, since this is one of the most intensively studied and better understood processes on rocky shores. Many studies from different geographical areas
have shown that molluscan herbivores can have profound effects on the structure of assemblages in natural habitats Underwood, 1980; Lubchenco and Gaines, 1981;
Hawkins and Hartnoll, 1983. In addition to the direct effects documented by these studies, intertidal gastropods can also exert a variety of indirect effects. For example,
they can prevent the monopolization of the substratum by ephemeral algae thereby enhancing the establishment of other algae and invertebrates reviewed in Sousa and
Connell, 1992.
Patella aspera Roeding and Patella rustica L. are the most common herbivores in midshore and lowshore habitats of rocky coasts in the Mediterranean. These gastropods
exhibit different patterns of vertical distribution, with P . rustica being more abundant
higher on the shore and P . aspera dominating lower down, but with extensive areas of
F . Bulleri et al. J. Exp. Mar. Biol. Ecol. 255 2000 1 –19
3
overlap at heights on the shore between 0.1 and 0.2 m above the mean-low-water-level Menconi et al., 1999. Previous experiments Benedetti-Cecchi and Cinelli, 1993, 1997;
Benedetti-Cecchi et al., 1996; Benedetti-Cecchi, 2000 revealed the important role of these grazers in regulating patterns of colonization in disturbed patches. The removal of
limpets resulted in the monopolization of the substratum by filamentous algae, whereas in the presence of grazers succession proceeded with the establishment of the fleshy red
alga Rissoella verruculosa Bertolini J. Agardh, barnacles and the Cyanophyta Rivularia spp. More recently, large-scale studies employing scales similar to those of
the present work have shown considerable spatial variability in the effects of limpets among shores tens to hundreds of kilometres apart Benedetti-Cecchi et al., in press.
These experiments were carried out on rocky shores adjacent to the artificial structures studied in the present paper and the two species of limpets, which were found at
densities similar to those here reported see Section 3, were considered as a guild and excluded by means of cages Benedetti-Cecchi et al., in press.
The focus of this paper is on the effects of P . aspera and P. rustica on assemblages of
algae and barnacles developing on artificial reefs in the northwest Mediterranean. The primary aim of this study was to test the hypothesis that removing limpets from these
manufactures led to changes in the structure of assemblages similar to those observed on rocky shores. Furthermore, we examined whether there were inconsistencies in the
effects of limpets at spatial scales comparable to those investigated on rocky shores. These hypotheses were tested with a multifactorial experiment involving the orthogonal
manipulation of the presence absence of the two species of limpets. The experiment was repeated at different locations tens of kilometres apart, on different reefs within each
location a few kilometres apart, and using replicate boulders within reefs tens to hundreds of metres apart as the experimental units. This experiment also allowed us to
test the null hypotheses that P
. rustica and P. aspera had similar effects on assemblages, and that one species had no influence on the distribution of the other on the artificial
reefs these hypotheses have not been tested yet on rocky shores.
2. Materials and methods
