Variable prey density modifies the effects of seagrass habitat structure on predator-prey interactions

In seagrass habitats, high structural complexity often enhances prey survival by reducing predator-prey encounter rates and predator foraging success. However, faunal density and biomass typically increase with seagrass structural complexity, such that variation in prey or predator density may contribute to or alter relationships between structural complexity and prey survival. We determined whether variability in prey density alters relationships between seagrass structural complexity and (1) prey survival, (2) predator-prey encounter rates, and (3) behavioral components of predator-prey interactions in eelgrass Zostera marina habitat. Using transplanted eelgrass in laboratory mesocosms, we compared trends in meso predator (juvenile giant kelpfish Heterostichus rostratus) feeding success and behavior, and predator avoidance behavior by prey (grass shrimp Hippolyte californiensis) between experiments in which we (1) held prey density constant while increasing eelgrass shoot density, or (2) increased prey density while increasing eelgrass shoot density. We found that increasing prey density (but not eelgrass density) enhanced proportional prey survival and decreased the probability that mesopredators would attack prey, whereas increasing eelgrass density (but not prey density) decreased the likelihood that prey would attempt to escape from mesopredator attack. Predator-prey encounter rates were influenced both by prey density and eelgrass density, whereas mesopredator attack success was not influenced by either eelgrass density or prey density. Our results suggest that variable prey density modifies relationships between habitat structure and prey survival through a variety of behavioral mechanisms, and that factors that covary with habitat structure, such as prey density, may alter the effects of habitat structure on predation.