Habitat complexity mediates predation of juvenile abalone by starfish

Predation strongly influences the abundance and distribution of prey populations due to its disproportionately large effects on survival during the early life-history stages. However, the intensity of predation can vary dramatically among habitats. The habitat can directly affect the interaction between predator and prey; but also, by determining the distributions of predators and prey, the habitat can mediate the likelihood of a predatory encounter. Here, we used laboratory experiments to identify the likely predators of juvenile abalone Haliotis iris on temperate reefs in central New Zealand. Predator performance in the laboratory was assessed in conditions without prey refuge, in simulated juvenile habitat as well as in the presence of alternate prey. We then used surveys to compare the abundance of predators and juvenile abalone to explore if negative associations between predator and prey in the laboratory manifest in the field. Last, we manipulated algal habitat complexity at 2 depths and quantified the effect of predator exclusion on juvenile abalone survival in the field. We found that starfish were the likely predators of juvenile H. iris in our study system. Furthermore, predation of juvenile abalone by starfish was lowest in habitats with the greatest structural complexity, and there is evidence that predation by starfish in cobble habitats was size-dependent. Overall, we found that habitat variability mediates predation on juvenile abalone by determining the likelihood of an encounter between predator and prey.