Two Processes Regulating Trophic Energy Flow in Pelagic and Terrestrial Ecosystems: Trophic Efficiency and Body Size-Dependent Biomass Production (A Reply to Giacomini)

We proposed two hypotheses to explain why food chains are longer in pelagic than terrestrial ecosystems: greater trophic efficiency of pelagic animal taxa at lower trophic levels and a higher pelagic biomass production rate at lower trophic levels because of smaller pelagic body masses. Giacomini favored the former, invoking in support the energetic equivalence hypothesis. We reply that the energetic equivalence hypothesis does not describe populations at differing trophic levels and so does not refute a significant role for body-mass dependence in explaining faster trophic transfer in pelagic ecosystems. Metabolic scaling as body mass to the exponent 1/4, widely accepted, remains important for trophic dynamic models. We suggest a likelihood method to compare the two hypotheses on the basis of models of whole-ecosystem energetics.