From Cells to the Biosphere: The Expanding Role of Metabolic Scaling Theory in Biology

Metabolic scaling theory (MST) explains how biological rates, times, and structures scale with body size through physical constraints on resource exchange and internal transport. MST is not a theory of one universal exponent. It is a generative framework for explaining why allometric scaling exists, why it is constrained, and why it varies in structured ways. From core constraints, MST predicts coordinated relationships among traits, rates, times, ecological dynamics, ecosystem fluxes, and multivariate covariation. Expanded formulations incorporate architectural variation, finite-size effects, trait heterogeneity, normalization shifts, and distinct constraint regimes to predict scaling functions, in some cases without fitted parameters, rather than isolated exponents. This review synthesizes MST's conceptual evolution, clarifies its assumptions and tests, and shows how structured variation in scaling reveals differences in organismal design, life history, environment, and evolutionary history. MST provides a general quantitative framework linking organismal traits, size, architecture, and energy flux - from populations to the biosphere.