Establishment and persistence of species-rich patches in a species-poor landscape: role of a structure-forming subtidal barnacle

Some sessile invertebrates are capable of maintaining space in barren habitats produced by sea urchins, thereby creating species-rich patches in a species-poor landscape. We sought to determine the role of a large and common barnacle, Austrobalanus imperator, in the establishment and persistence of these species-rich patches. Barnacle density was modified in 2 experiments at sites in southeastern Australia. The first experiment concerned community establishment and involved the addition of barnacles in 4 densities (zero [control], low, medium and high) to plots on vertical rock surfaces. The addition of barnacles at ecologically realistic densities and spatial arrangements rapidly resulted in statistically significant increases in invertebrate cover and diversity. After 56 mo, the diversity of invertebrates was significantly higher on plots that received high densities of barnacles relative to controls. However, invertebrate cover no longer differed between treatment plots, despite evidence that barnacles modify the grazing intensity of sea urchins. The second experiment assessed the persistence of sessile invertebrates following the removal of barnacles from well-established assemblages dominated by sponges. The removal of barnacles did not hinder the rapid recovery of sponges to pre-manipulation levels, indicating that barnacles played an inconsequential role in well-established assemblages. We conclude that barnacles play an important functional role in this system, as they promote the recruitment of sessile invertebrates. It was also clear that the development of invertebrate assemblages on natural vertical surfaces was very slow, which raises the question of whether we are examining these important occupiers of space at appropriate temporal and spatial scales.