Competition between an invasive urchin and commercially fished abalone: effect on body condition, reproduction and survivorship

Incursion of the urchin Centrostephanus rodgersii into Tasmania, Australia, and its establishment at high densities raises questions about its potential interactions with another large herbivore on subtidal rocky reefs, the commercially fished abalone Haliotis rubra. Surveys on the southeast coast of Australia show a negative relationship between densities of C. rodgersii and H. rubra at several spatial scales, suggesting negative interactions. In intact algal beds, we used enclosures to estimate the effects and relative magnitude of intra- and interspecific competition on the body condition, gonad development and survival of C. rodgersii and H. rubra. An increased density of conspecifics led to declines in the dry gonad weight of C. rodgersii and in the dry foot and stomach content weights of H. rubra. The effects of interspecific competition were asymmetrical. Manipulations of H. rubra densities had no detectable effect on C. rodgersii. In contrast, in enclosures with added C. rodgersii, H. rubra showed reduced total and dry weights of stomach contents and increased mortality relative to controls without urchins. The effects of C. rodgersii on H. rubra could be linked to differences in feeding habits and morphology. C. rodgersii is a generalist herbivore, which, even at low densities, reduced the cover and standing biomass of total, brown and red algae relative to controls without urchins. In contrast, H. rubra is a specialist herbivore, which, even at high densities, had little effect on the cover and standing biomass of algae relative to the effect of C. rodgersii. This study suggests that the invader C. rodgersii is the superior competitor in interactions with H. rubra, and that its presence, even at low densities, affects the abalone fishery