Habitat structure and the nature of communities on intertidal boulders.

Keith Mcguinness, Tony Underwood

Research output: Contribution to journalArticlepeer-review


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 algae was often greatest on blocks with the fewest micro-habitats, possibly because grazers were least abundant on these blocks. All of these findings were demonstrated to be a response to the experimental addition of the new micro-habitats themselves and not a function of increased surface-area. There were often more species of algae on the thinner concrete blocks, and this was probably a response to the shorter period of emersion of the thinner blocks during low tide.
Overall, the experiments detected a variety of influences of several aspects of habitat structure. Some of these were pronounced, leading to major changes; others were less important. Notably, the effects of micro-habitat diversity were not always in the direction predicted by theory. These results indicate that further investigation is required of the effects of habitat structure on natural communities.
Original languageEnglish
Pages (from-to)97-123
JournalJournal of Experimental Marine Biology and Ecology
Issue number1-3
Publication statusPublished - 1986
Externally publishedYes


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