Functional traits of herbivores and food chain efficiency in a simple aquatic community model

Large-bodied zooplankton play important roles in the flow of nutrients and energy along the food chain in freshwater ecosystems. However, the importance of functional traits of zooplankton in trophic flow is not well understood. We used the minimal food-chain model, which includes producers (algae), first-order consumers (grazer zooplankton), and secondary consumers (fish), to reveal which functional traits of phyto- or zooplankton affect one of the most important ecosystem processes for functioning, the trophic transfer efficiency (TTE) across three trophic levels. Numerical simulations indicated that, regardless of the level of nutrient loading and the fish density, the conversion coefficient of grazer zooplankton was one of the most important and general factors for determining TTE. The antipredator defense and maximum grazing rate of zooplankton greatly affected TTE, but the effect depended on the level of nutrient loading and the fish density. For an ecosystem that had high nutrient loading and high fish density and in which the zooplankton density was regulated by the top-down effect, antipredator defense and maximum grazing enhanced TTE. Opposite relationships were observed for other states of the ecosystem. In general, those functional traits of the first-order consumers that affected vertical ecological interaction were important for trophic flow in the ecosystem, whereas the sensitivity of trophic flow to these traits depended largely on whether the zooplankton were regulated by the bottom-up or the top-down effect.