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 So uth Wales, Australia. The specific aim was to test explicitly for the prese nce of a species-area relationship, and examine whether this could be expla ined by the random placement hypothesis (i.e. that the number of species pe r unit area was the same on patches of different sizes). In addition, I tes ted the extent to which differences in numbers of species reflected differe nces in the composition of assemblages. Wooden panels of three different si zes (10 x 10 cm, 20 x 20 cm and 40 x 40 cm) were placed in the field on int ertidal 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 p eriods of time, up to 25 months. I also obtained measures of abundance of i ndividual species and composition of assemblages on panels of different siz es. This allowed specific tests of the hypothesis that the size of the patc h being colonised is important in structuring these assemblages. The streng th of the species-area relationship increased through time on panels submer sed 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 amon g replicates. The random placement hypothesis was supported in certain situ ations, but not in others. When rejected, it was far different reasons on p anels submersed in the two different trials. Panels initiated in October te nded to have proportionally greater numbers of species per unit area on lar ger panels, while the panels initiated in January tended to have more speci es per unit area on smaller panels. There was an identifiable relationship between differences in numbers of species and differences in species compos ition for panels submersed in October. This was not true, however, for pane ls submersed in January, where the species-area relationship did not hold a fter longer periods. The succession of organisms through time was, overall, more important in structuring the assemblages than was the size of the pat ch 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.