Intertidal assemblages on coastal defence structure in Singapore. II. Contrasts between islands and the mainland.

Abstract

In Singapore, breakwaters have replaced natural rocky shores as the predominant hard- substratum intertidal coastal habitat on both the main island as well as the smaller islands to the south. These form isolated and non-contiguous areas of relatively homogenous rock (granite), separated by sandy shore or water. Insular assemblages of species at island locations are expected to be more stochastic in organisation than those on the mainland, due in part to variable patterns of colonisation and extinction. As such, there should be differences between the mainland and islands in terms of assemblage composition, structure and in the variability of abundances of taxa at a range of spatial scales. This study examines the structure of assemblages on seawall habitats located on the Singapore mainland and its southern islands. Analyses revealed differences between island and the mainland in the structure, but not the composition of intertidal assemblages, and also in the magnitude of variation in abundances of individual taxa at the spatial scales examined. Differences were more pronounced in assemblages lower on the shore than in high shore and supralittoral assemblages. In addition, assemblages tended to be more similar within locations, but there was no tendency for variability in the mean abundances of any taxa to be consistently higher at any habitat or spatial scale. On the whole, the results do not support the model of higher stochasticity of assemblages on artificial intertidal rocky habitats on islands. Instead, it revealed that high spatial variability at mainland locations could be linked to highly localised environmental conditions. Community organisation on these seemingly homogenous structures is shown to be complex and idiosyncratic. Factors and mechanisms influencing assemblage structure in natural rocky intertidal habitats are likely to operate differently on these artificial habitats, and should be examined critically prior to management decisions assuming or relying on the surrogacy of these structures as alternative marine habitats.