Spartina alterniflora genotypic identity affects plant and consumer responses in an experimental marsh community

Summary Competition and herbivory are ubiquitous processes known to interactively shape plant performance, distribution and community assembly. Likewise, plant genetic variation and associated trait differences can impact both plant–plant and plant–herbivore interactions individually, yet few studies have explored these interactions simultaneously. Saltmarsh communities are an ideal system to study these questions, as they are dominated along the Atlantic and Gulf coasts of the United States by a foundation plant species, Spartina alterniflora, with high levels of genetically based trait variation. Furthermore, consumer pressure and plant competitors are known to influence both the distribution and production of Spartina. We manipulated Spartina genetic identity, neighbour identity (needlerush, Juncus roemerianus) and consumer presence (snail, Littoraria irrorata) in a 1-year field experiment in St. Joseph Bay, FL, to test how the strength and direction of neighbour and consumer interactions vary by plant genotype. Consumer effects on Spartina were generally stronger and more variable than those of the plant neighbourhood, and these effects were generally consistent across Spartina genotypes. However, genotype-specific variation in morphology, phenology and palatability significantly influenced both consumer and neighbour responses to Spartina: the consumer Littoraria was more likely to climb palatable Spartina genotypes, and neighbouring Juncus had shorter stems in the presence of highly productive Spartina genotypes. Synthesis. Our results add to the growing list of examples high-lighting the role of intraspecific variation in species interactions and suggest that variation in these interactions may promote the maintenance of plant genetic variation and community composition.