Land, Sea and People in the New Millennium.
The present study was undertaken to investigate the observation that dolorite and shale shores in the Transkei tended to have less algal cover and to support more grazers than sandstone shores. The structure of rocky intertidal communities is determined by the interaction of a number of biotic and abiotic factors. The chemical and physical nature of the substratum has been shown to have an influence on colonization, but few studies have attempted to quantify experimentally the differences in succession observed on different substrata. Rock tiles (10 x 10 cm) cut from shale, dolorite and sandstone were placed on three shores, each dominated by one of the rock types, and monitored photographically for 18 months. Within each type two treatments consisting of smooth and rugose surfaces were applied to assess the influence of rugosity on colonization. Experiments were intitiated in summer and in winter to assess the influence of season on the colonization process. The rate of heating when exposed to insolation and degree of water retention of each rock type were determined in the laboratory. Heating rate in shale and dolorite was almost three times that in sandstone, while this rock type retained ten times more water after soaking than the other types. Within a month of placement all rock types supported a thin layer of filamentous algae. After six months only the sandstone supported significant growth of upright algae, while the shale and dolorite were largely devoid of macroalgae. The latter rock types attracted micro-algal grazers such as patellid limpets, which were comparatively rare on sandstone. This situation persisted to the end of the experiment by which time the sandstone was covered in coralline algae. There appeared to be no influence of season on this process. Rugose tiles attracted larger numbers of sessile invertebrates such as barnacles than the smooth tiles, but they were either engulfed by algae or obliterated by grazers depending on rock types The above effects occurred at all sites indicating that site was less important than substratum in influencing colonization. These results suggest that the higher temperatures and degree of dessication on shale and dolorite retards colonization and prolongs early sussessional stages which are attractive to grazers. Grazing then takes over as the factor maintaining the relative bareness of these rocks.
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