Positive and negative effects of a dominant competitor on the settlement, growth, and survival of competing species in an epibenthic community

a r t i c l e i n f o Because dominant competitors can monopolize resources, any positive effects they have on other species can have large community impacts. The solitary tunicate Ascidia ceratodes is a dominant competitor in the fouling community in Bodega Harbor, CA. This tunicate preempts primary substratum from competitors, but its thick tunic also allows other species to grow on its surface. The net effect of Ascidia on the community as a whole therefore depends on the balance between competitive and facilitative effects. In this study we evaluate the facilitative effects of Ascidia on different life stages of common competing species. We quantified larval settlement onto Ascidia compared to unoccupied space; we quantified the growth rate on Ascidia of small colonies of two common species; and we measured whether established colonies could escape overgrowth by Ascidia by growing onto its tunic. We found evidence for high rates of settlement on Ascidia, with some species showing higher and others lower settlement relative to that observed on free space. The growth rate of settlers was generally lower on Ascidia compared to primary substratum. We also found that colonial species established on primary space commonly escape overgrowth from Ascidia by growing onto Ascidia, as this occurs in about half of all encounters. This study indicates that the total effect of Ascidia on the community will depend on species-specific and life stage-specific effects, with likely long-term consequences for the diversity and composition of the community. A fundamental question in ecology is how different species coexist while requiring the same resources. A majority of the research on this topic has focused on resource competition and the different kinds of resource partitioning that can maintain diversity (e. However, competition has not provided a complete understanding of species diversity patterns in nature, and more recent work has emphasized the effects of positive interactions, such as facilitation, on Facilitative interactions can maintain diversity via several mechanisms. Dominant species can create better abiotic conditions for interacting species by modifying the physical environment (Hacker and Gaines, 1997); dominant species can provide additional space for epiphytic and epizootic organisms to settle and grow (Kimbro and Grosholz, 2006); and species that are resistant to predation can provide an associational refuge for other species that are susceptible to predators (Hay, 1986). In this study, we examine how the facilitative provisioning of additional substrate may help maintain species diversity …