Occupation of patches in the epifaunal communities on pier pilings and the bivalve Pinna bicolor at Edithburgh, South Australia

Abstract

The reoccupation of artificially cleared patches in a subtidal epifaunal community was investigated in two field experiments on the pilings of Edithburgh pier, South Austrlia. In most cases, the greatest proportion of the patch was reoccupied by the vegetative extension of established sponge and tunicate colonies adjacent to it. Larval recruitment by sponges, bryozoans, tunicates and serpulids contributed to the reoccupation but resulted in only a small proportion of the mean percentage cover. The relative abundances of individual species established in any patch were shown to be a function of the (1) position in space, (2) age, (3) time of creation, (4) initial size of the patch.There was a large amount of between-patch variation in all cases. Overgrowth interactions occurred frequently within patches, and for many pairs of species, neither species consistently overgrew the other. Overgrowth interactions were tested statistically, and a large number of pairs of species were found to be competitively equivalent. This represents a possible situation additional to the alternatives recognized in the literature, namely competitive hierarchies or networks. Interactions between species should be regarded as stochastic, with a wide range of possible outcomes. The situation at Edithburgh is likely to produce greater between-patch variability than either a network or a hierarchy.Despite this large variation, super-specific taxa differ fairly consistently in capacity for overgrowth. Tunicates overgrow sponges, which overgrow bryozoans, which overgrow serpulids. The occupation of most patches was directional in the sense that bryozoans and serpulids invaded first, but tunicates and sponges excluded them and came to dominate the patch. These realtionships are used to predict patterns of abundance for substrata which are small and isolated, and these predictions are compared with the epifauna of the bivalve Pinna bicolor, which provides such substrata adjacent to the pier.