The Metabolic Theory of Ecology (MTE) and its predictions of the scaling of average population body mass with abundance and energy use are some of the most widely observed and studied biological relationships. However, the scaling exponent value of these relationships have been widely debated and found to vary considerably among various ecological communities and the ecosystems they occupy. Additionally, adherence to MTE predictions is widely contingent on its underlying assumptions and the variables used to describe these relationships. My thesis aims to provide a comprehensive analysis of the size–density and size–energy use scaling relationships of soil invertebrate and tree communities to shed light on the environmental and biological factors underpinning their observed exponents. To do this, I tested whether predictions of size–density scaling derived from MTE are dependent on ecosystem reassembly processes throughout succession and organism life history traits within trees and soil invertebrates. I also analysed size–density and size–energy use scaling in soil invertebrate food webs across four geographical locations to investigate the universality of size–density scaling relationships and their likelihood of accurately indicating energetic equivalence in soil communities of primary and secondary consumers. Additionally, I compared two measures of energy use to investigate size–energy use relationships: population metabolism and trophic energy fluxes in food webs. My findings suggested that size–density scaling, although related to energy use in ecological communities, is likely a poor indicator of energetic equivalence alone, but in combination with size–energy use scaling relationships can provide powerful insights into the energetic structuring of these communities. Additionally, size–density scaling exponents vary considerably across ecosystem succession and organism life form, and expectations should reflect these conditions. Energy flux as an estimate of energy use within communities for analysing size–energy use relationships was a more precise estimate compared to metabolism as it better captures the true energetic demands of organisms, particularly for secondary consumers which face stronger energetic constraints. Finally, analytically estimating energy fluxes with a trophic level–specific focus better captured the differences in patterns of energy use between these trophic levels compared to when they were pooled. Ultimately, this thesis provides new directions for exploring energetic equivalence in terrestrial communities.