MEPS 508:117-128 (2014)  -  DOI: https://doi.org/10.3354/meps10855

ABSTRACT: Herbivores can have highly variable effects across their ranges, sometimes with unanticipated effects on trophic dynamics that in turn affect management and conservation programs. In seagrass beds, small invertebrate grazers (mesograzers) are expected to benefit the habitat-forming plants by removing competing algae; however, harm by mesograzers has been documented increasingly in a number of regions. In San Francisco Bay, California, USA, a gammaridean amphipod (Ampithoe valida), native to the US East Coast and putatively introduced to Pacific coastlines, reaches outbreak densities and consumes large quantities of eelgrass Zostera marina, while it is not known to do so elsewhere in its range. Using a series of mesocosm experiments, we manipulated predator (fish) identity and density, herbivore assemblage, and habitat complexity to test how variation at multiple trophic levels influences the abundance and role of A. valida. Reductions in A. valida abundance by native San Francisco Bay shiner surfperch and bay pipefish were less pronounced than by a pinfish brought in from North Carolina, USA; however, the former 2 predators enhanced eelgrass biomass, while consumption of both amphipods and eelgrass by pinfish netted limited benefits to eelgrass. Increasing density of the surfperch in a separate experiment did not further reduce A. valida abundance but nonetheless strengthened positive effects on eelgrass, presumably through behavioral response to increased threat. The presence of a second (introduced) grazer reduced predation pressure on A. valida, weakening the trophic cascade to the detriment of eelgrass. Increasing habitat complexity by substituting flowering shoots reduced predation success, leading to higher A. valida abundances. Our results point to the importance of predator control of A. valida to reduce eelgrass damage; however, the strength of the trophic cascade across this grazer’s range is likely to be modulated by composition of predator and grazer assemblages as well as flowering rates and phenology.

KEY WORDS: Amphipod · Trophic interaction · Eelgrass · Predator · Trophic cascade