The ecology of three sandy beaches on the West Coast of South Africa

• Main Author: Bally, Roderick Peter Alexander
• Other Authors: Brown, Alec C
• Format: Doctoral Thesis
• Language: English
• Published: University of Cape Town 2015
• Subjects: Zoology
• Online Access: http://hdl.handle.net/11427/15452

Bibliography: leaves 367-404. === The ecology of three sandy beaches on the west coast of South Africa is examined in this thesis. The first chapter constitutes an introduction to the subject and presents a brief review of studies carried out on beaches in South Africa in order to place this thesis into its proper context. In the second chapter, methods of sampling sandy-beach macrofauna (in particular sampling strategies) are briefly reviewed and discussed. For this study it was decided to adopt a grid sampling strategy, and the value of this strategy was tested. In the third chapter the sedimentological characteristics of the beach were examined in considerable detail in order to determine whether these differed between beaches, between zones and seasons on individual beaches. Characteristic differences were found for all the above categories, and the reasons for these are discussed in detail. Two zonation classificatory schemes for intertidal sandy-beach fauna have been proposed: that of Dahl (1953) and that of Salvat (1964). The zonation of organisms on west coast beaches is described in chapter 4, and the results obtained are discussed in the light of the above zonation schemes. Using cluster analysis, ordination and information statistic techniques, Salvat's zonation scheme was found to best describe the situation on the west coast, and its advantages over Dahl's scheme are discussed. Chapter 5 deals with the biomass and densities of macrofauna and meiofauna on the three beaches studied and discusses these results in relation to season and results obtained on beaches elsewhere. On a very broad basis, it is found that the results from west coast beaches fall within the range of results obtained elsewhere, although they rank among the higher values. The results also show that mean individual mass of organisms is related to beach exposure - the mean mass increases with increasing exposure. The respiratory metabolisms of the three dominant isopod species are examined in chapter 6. Using multivariate analysis, the effects of mass, temperature and activity on respiratory rates were determined for each species. Analysis of the results showed that the activity potential of each species differed, and this was correlated with the degree of exposure of the zone inhabited by each species. Thus the species living at the top of the shore showed a high activity potential while that living at the bottom of the shore had the lowest activity potential. The results and the reason for these differences are discussed and related to the differences in the conditions pertaining in each species' environment. Traditionally, sandy beaches are investigated by means of one or two transects across the beach, and the results extrapolated to the remainder of the beach. The results obtained in chapter 2 show that there is considerable variation in biomass and density values along the beach, but a special study of this was made in chapter 7. This showed that all species in fact have an irregular distribution both along and across the beach. These distribution patches are described for all species on all three beaches in this chapter. In addition, interspecific competition and predator-prey relationships were investigated, to determine whether these had any influence on the distributions of the species concerned. Animal-sediment relationships are investigated in chapter 8 and two major types of relationship are described. The first or gross type occurs where species are excluded from a beach due to unfavourable grain size, or where the intertidal position of all the species is affected by the sediment coarseness. The second type is where species or size-classes within species select particular grades within a particular intertidal zone. Animal-sediment relationships are discussed for each species, although these were not detected in all cases. · In chapter 9 the biogeography of the sandy beach genera occurring on west coast beaches is investigated. It was found that genera generally made up of intertidal species consisted of fewer species, with generally larger distribution ranges, than genera that tended to consist mainly of sub-tidal species. Of the 15 genera investigated in this chapter, almost all had a greater percentage of species occurring tropically than in temperate and arctic and antarctic climatic zones. When these percentages were corrected for coastline length or oceanic area, however, the concentrations of species were found to be highest in temperate zones, in all cases. The reasons for these findings are discussed in the light of Bretsky and Lorenz's (1970) theory pertaining to genetic-adaptive strategies and mass extinctions. Finally, in chapter 10, the results of this thesis are synthesized and placed into the context of existing knowledge on sandy beach ecology. In addition, trends in sandy beach studies are discussed and future fields of research outlined.