Population energy use scales positively with body size in natural aquatic microcosms

Aim We test the ‘energetic equivalence rule’ (EER) – the idea that the amount of energy used by a population per unit area per unit time is independent of body mass – in meio‐invertebrate communities from a series of natural, multitrophic aquatic ‘rock pool’ microcosms. Our study represents the first rigorous test of the EER at local scales of observation in a community of naturally coexisting species. Location Discovery Bay, Jamaica. Method We estimated population energy use (PEU) for every occurrence of every species of meio‐invertebrate fauna found in each of 29 microcosms (233 observations of 31 species) using estimates of population density obtained in January 2005 in combination with published metabolism–mass relations for closely related taxa. Results In the rock pool system as a whole, population density decreased (ancova: b = –0.38 (–0.55 to –0.19), r2 = 0.19, P < 0.001) and PEU increased with body mass (ancova: b = 0.55 (0.36–0.73), r2 = 0.28, P < 0.001). Main conclusions The positive PEU–body mass relation found here suggests that larger organisms are energetically dominant and points to the importance of size‐structured competition in these systems. Our results contrast those obtained in the few other previously published tests of the EER and challenge the idea that all species use similar amounts of energy regardless of their size.