Season and prey type influence size dependency of predator−prey body mass ratios in a marine fish assemblage

Abstract

Marine and freshwater food webs are strongly structured by size-dependent preda- torprey interactions. Predatorprey body mass ratios (PPMR) are important parameters in size- based food-web models, but studies evaluating the temporal stability of PPMR or its relationship to predator feeding modes are scant. Using a large data set of predatorprey pairs from a diverse fish community sampled in summer, fall, and winter, we showed that community-level PPMR var- ied with predator mass in a nonlinear (dome-shaped) manner. PPMR was higher in the summer relative to the fall and winter for all predator body size classes regardless of whether prey were fish or invertebrate. Further, the size dependency of PPMR was dome-shaped for invertebrate prey but positive and linear for fish prey. We empirically show that community-level PPMR is dynamic rather than fixed, which is in agreement with general expectations set by simulation studies of biomass spectra. However, we are presently unable to identify the specific processes underlying these patterns. Size-based models of marine ecosystems offer considerable promise over traditional taxa-based approaches, and our analyses provide insight into major patterns of variation in PPMR in a temperate marine system.