Trophic cascade strength is influenced by size frequency distribution of primary consumers and size-selective predation: examined with mesocosms and modeling.
Understanding variation in trophic cascades is critical for developing management strategies for invasive and introduced species as well as in designing biomanipulation experiments. We evaluated, in fish driven trophic cascades, the responses of invertebrate biomass, body size, and production both experimentally and through a model. Experimental mesocosm treatments of small (Moina), large (Daphnia pulex), and mixed size zooplankton communities were characterized with and without size-selective fish (bluegill, Lepomis macrochirus). Experimental responses were typical of trophic cascades showing reduced biomass, body size and production of zooplankton and generally enhanced chlorophyll a. Although zooplankton biomass was similarly reduced in all fish treatments, production for the fish D. pulex treatment was lower than the other fish treatments because larger taxa have a lower ratio of production to biomass (P/B). As would be expected based on zooplankton production among fish treatments, response of chlorophyll a in treatments with D. pulex was faster and final concentration higher than in Moina or mixed size treatments. A model of changes in production under a range of reductions in body size and/or biomass, from no reduction to 99% reduction, supported experimental results indicating invertebrate production can be resilient to reduction of biomass when body size is reduced. Strength of trophic cascades may therefore, in part, be related to interactions among reduction in biomass, size-selective predation, and size frequency distribution and plasticity of prey community. Because size-selective predation is a common trait in trophic cascades, some level of compensation is likely in most trophic cascades.