Direct and indirect impacts of a non-native predator: foraging by Carcinus maenas on native bivalves of south-east Australian estuaries

Predation is a key process shaping ecological communities. In addition to the consumptive effects of predators, where predators modify the structure of ecosystem engineers, they may have large cascading impacts on native biodiversity. In Australia, Carcinus maenas, the European shore crab, is a voracious benthic predator of bivalves that may play a role in structuring benthic communities. Along the southeast coast, the crab inhabits mangrove forests and preys upon with Xenostrobus securis, an ecologically important Australian bed-forming mussel that facilitates diverse associated invertebrate communitics. My thesis examines: (1) how traits of habitat-forming bivalves and environmental context interact to influence associated invertebrate communities: (2) foraging patterns by C maenas on X. securis and the mechanisms that drive these; and (3) direct and indirect effects of C. maenas predation on X securis mussel beds and their associated invertebrate communities. Using a meta-analysis of existing literature, I show that although habitat-forming bivalves have universally positive effects on associated communities, the magnitude of their effect varies according to variation in traits of the bivalves and habitat context. Hence, predation that modifies the density and/or size structure of habitat-forming bivalves has the potential to have large cascading impacts to associated communities. Foraging experiments reveal that C. maenas displays size-specific patterns of bivalve predation that, contrary to the suggestion of earlier studies, are opportunistic as opposed to energy-maximising. Crab gender and satiation as well as the spatial configuration of bivalve prey also influence predation. The impact of C. maenas predation on X. securis mussel beds is greater than native crabs, Paragrapsus laevis but neither crab has a significant consumptive effect on associated invertebrates. Instead, crab predation indirectly modifies the associated invertebrate assemblage structure by changing the structure of the bivalves. Contrary to the assumption that crab predation would produce negative indirect effects on mussel bed communities, where predation resulted in increasednumbers of dead bivalve shells, associated invertebrate abundance increased. Overall, these results demonstrate that non-native predators may have significant impacts on native biodiversity, not only as a result of direct, consumptive effects, but also indirect effects resulting from changes in the population structure of habitat-forming species. Hence, indirect effects need to be factored into models of impact and the development of management strategies.