Detection of energetic equivalence depends on food web architecture and estimators of energy use

Ecologists have long debated the universality of the energetic equivalence rule, which posits that population energy use should be invariant with average body size due to negative size-density scaling. We explore size-density and size-energy use scaling across 183 geographically-distributed soil invertebrate food webs (comprising 55,054 individual soil invertebrates) to investigate the universality of these fundamental energetic equivalence rule assumptions across trophic levels and varying food web structure. Additionally, we compare two measures of energy use to investigate size-energy use relationships: population metabolism and energy fluxes. We find that size-density scaling does not support energetic equivalence in soil communities. Furthermore, evidence of energetic equivalence is dependent on the estimate of energy use applied, the trophic level of consumers, and food web properties. Our study demonstrates a need to integrate food web energetics and trophic structure to better understand how energetic constraints shape the body size structure of terrestrial ecosystems.