Physical and biological effects of introduced oysters on biodiversity in an intertidal boulder field

Invasive species have been identified as a serious threat to biodiversity, particularly in protected habitats. The Pacific oyster Crassostrea gigas, which is invasive in many parts of the world, can form very dense populations affecting the abundance and distribution of native organisms. We experimentally separated the effects of the cover and state (physical structure or biological activities) of C. gigas on the development of invertebrate and macroalgal assemblages. Living (biologically active) and dead (physical structure only) oysters were glued in increasing cover to the tops of cleaned boulders and deployed within an intertidal boulder field. After 14 mo, diversity, evenness and assemblage structure of invertebrates and algae were affected by C. gigas, and results varied depending on the cover and state of oysters. Interestingly, boulders with the lowest cover of living C. gigas supported the most diverse assemblages. Differences in assemblage structure were driven by changes in the establishment of several key species including Fucus vesiculosus and Littorina littorea, which were facilitated by oysters and the honeycomb worm Sabellaria alveolata, which was inhibited by oysters. S. alveolata, an ecosystem engineer that can generate reefs protected by the European Union Habitats directive, mainly established on the underside of boulders but was nonetheless greatly reduced by increasing cover of dead or living oysters on the topside. This work highlights the importance of understanding the direct and indirect mechanisms by which invasive species alter biodiversity and how effects vary as invaders increase in abundance.