Habitat structure and the nature of communities on intertidal boulders

• Published in: Journal of experimental marine biology and ecology 1986-12, Vol.104 (1), p.97-123
• Main Authors: McGuinness, K.A., Underwood, A.J.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Community organization on intertidal boulders; Fundamental and applied biological sciences. Psychology; Habitat structure and composition; Marine; Micro-habitat diversity; Sea water ecosystems; Species-area curves; Synecology
• ISSN: 0022-0981; 1879-1697
• DOI: 10.1016/0022-0981(86)90099-7

Habitat structure may have important effects on the structure of natural communities. At least two aspects of habitat structure should be considered-the composition of the habitat (i.e. the material from which it is formed) and the number of different micro-habitats it contains. A number of assumptions has been made about how the latter aspect of habitat structure affects the number and abundances of species in natural assemblages. These assumptions have rarely been tested; nor, indeed, have there been many tests of the direct effects of habitat composition. The communities of organisms on intertidal boulders on two rock-platforms were studied. These platforms differed in the major type of rock present; boulders on one shore were predominantly sandstone, those on the other were mostly shale. The effects of this difference in composition of habitat were examined by exchanging bare boulders between the two shores. The green alga Ulva lactuca L. became much more abundant on sandstone boulders than on shale on both shores. This probably reflected the fact that sandstone retained more water during low tide and was a better substratum for growth of the alga. Spirorbid worms settled on the undersides of rocks but became more abundant on shale. This may have been because the shale rocks were more pitted and therefore provided greater protection from abrasion. Sampling showed that small natural boulders had less micro-habitat diversity than large boulders. Furthermore, the tops of these larger boulders were emersed for a significantly longer period during low tide. The effects of these factors were examined using solid concrete blocks as experimental boulders. The surfaces of the blocks were modified to create different densities and types of micro-habitats (i.e. great and small densities of pits and grooves). In addition, blocks of two different thicknesses were used to examine the effects of this characteristic of size of block. Both experimental factors affected the number of species and the abundances of some common species that grew on the blocks. The effects were complex and varied throughout the experiments. In general, however, the undersides of blocks with more micro-habitats supported more sessile animals. The tops of these blocks at high levels on the shore typically had more grazing gastropods. These effects were probably the result of species in pits and grooves being protected from disturbance and physical stress. In contrast, the number of species of