Intraspecific diversity in prey body size influences survivorship by conferring resistance to predation

Intraspecific diversity can have as strong an effect on communities and ecosystems as more well-studied effects of species diversity. Intraspecific variation in prey body size may have particularly strong effects on populations by influencing the outcome of predator-prey interactions, but this has received little attention. We tested the hypotheses that (1) body size of the Sydney Rock Oyster, Sacosstrea glomerata, influences predation risk; (2) increasing intraspecific variation in body size of the oyster will enhance total survivorship; (3) oyster size phenotypes will differ in the magnitude of survivorship benefits they incur from association with other phenotypes; and (4) size-specific survivorship benefits of associating with other body sizes will vary with predator access. In a fully factorial field experiment, we manipulated the diversity of S. glomerata oyster body sizes (1-3 size classes per treatment) and predator access to prey (small vs. large mesh cages). Oyster total and individual phenotype survivorship was documented over an 8-week period. Overall, the relationship between diversity in S. glomerata body size and survivorship was positive and was related to changes in density of individual body sizes when grown in mixtures. In fact, no phenotype had lower percent survivorship in mixtures than in monoculture. Nevertheless, individual phenotypes displayed differing responses to predator access treatments and phenotype mixing that reflected differences in the types of predators (e.g., shell drilling or removing) to which they were most susceptible. In general, the greatest survivorship benefits of phenotype mixing were seen in treatments where the most disparate (i.e., small, large) size classes were mixed. Our study adds to growing evidence of the importance of intraspecific diversity in mediating key population processes such as predation. Importantly, different oyster phenotypes benefit from increasing phenotypic diversity depending on the predators that can access the oysters, suggesting that flexible complementarity among oyster phenotypes may be a useful tool for increasing the resistance and resilience of oysters in a range of environments which vary in their predator composition. These results will help to inform strategies for enhancing survivorship of species transplants in restoration projects.