Effects of patch size on colonisation in estuaries: revisiting the species-area relationship.

The effects of patch size on the colonisation and succession of intertidal invertebrates and algae were investigated in an estuary near Sydney, New South Wales, Australia. The specific aim was to test explicitly for the presence of a species-area relationship, and examine whether this could be explained by the random placement hypothesis (i.e. that the number of species per unit area was the same on patches of different sizes). In addition, I tested the extent to which differences in numbers of species reflected differences in the composition of assemblages. Wooden panels of three different sizes (10 x 10 cm, 20 x 20 cm and 40 x 40 cm) were placed in the field on intertidal oyster leases in each of two different experimental trials: spring (October 1994) and summer (January 1995). Independent replicate measures of the number of colonising species on panels were obtained after different periods of time, up to 25 months. I also obtained measures of abundance of individual species and composition of assemblages on panels of different sizes. This allowed specific tests of the hypothesis that the size of the patch being colonised is important in structuring these assemblages. The strength of the species-area relationship increased through time on panels submersed in October, but the trend was reversed for panels submersed in January. There was a significant interaction between factors of patch size and time of submersion for multivariate measures of differences in composition among replicates. The random placement hypothesis was supported in certain situations, but not in others. When rejected, it was for different reasons on panels submersed in the two different trials. Panels initiated in October tended to have proportionally greater numbers of species per unit area on larger panels, while the panels initiated in January tended to have more species per unit area on smaller panels. There was an identifiable relationship between differences in numbers of species and differences in species composition for panels submersed in October. This was not true, however, for panels submersed in January, where the species-area relationship did not hold after longer periods. The succession of organisms through time was, overall, more important in structuring the assemblages than was the size of the patch being colonised. The species-area relationship should not necessarily be regarded as a truism - it did not always hold in this system. The initial timing of experiments with respect to recruitment and succession influenced the results.