1771Ecology,85(7), 2004, pp. 1771–1789q2004 by the Ecological Society of AmericaTOWARD A METABOLIC THEORY OF ECOLOGYJAMESH. BROWN,1,2,4withJAMESF. GILLOOLY,1ANDREWP. ALLEN,1VANM. SAVAGE,2,3ANDGEOFFREYB. WEST2,31Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131 USA2Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, New Mexico 87501 USA3Theoretical Division, MS B285, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 USAJAMESH. BROWN,MacArthur Award Recipient, 2002Abstract.Metabolism provides a basis for using first principles of physics, chemistry, andbiology to link the biology of individual organisms to the ecology of populations, communities,and ecosystems. Metabolic rate, the rate at which organisms take up, transform, and expend energyand materials, is the most fundamental biological rate. We have developed a quantitative theoryfor how metabolic rate varies with body size and temperature. Metabolic theory predicts howmetabolic rate, by setting the rates of resource uptake from the environment and resource allocationto survival, growth, and reproduction, controls ecological processes at all levels of organizationfrom individuals to the biosphere. Examples include: (1) life history attributes, including devel-opment rate, mortality rate, age at maturity, life span, and population growth rate; (2) populationinteractions, including carrying capacity, rates of competition and predation, and patterns of speciesdiversity; and (3) ecosystem processes, including rates of biomass production and respiration andpatterns of trophic dynamics.Data compiled from the ecological literature strongly support the theoretical predictions. Even-tually, metabolic theory may provide a conceptual foundation for much of ecology, just as genetictheory provides a foundation for much of evolutionary biology.Key words: allometry; biogeochemical cycles; body size; development; ecological interactions;ecological theory; metabolism; population growth; production; stoichiometry; temperature; trophicdynamics.4E-mail: [email protected] H. BROWN ET AL.Ecology, Vol. 85, No. 7PerspectivesINTRODUCTIONThe complex, spatially and temporally varying struc-tures and dynamics of ecological systems are largelyconsequences of biological metabolism. Wherever theyoccur, organisms transform energy to power their ownactivities, convert materials into uniquely organicforms, and thereby create a distinctive biological,chemical, and physical environment.Metabolism is the biological processing of energyand materials. Organisms take up energetic and ma-terial resources from the environment, convert theminto other forms within their bodies, allocate them tothe fitness-enhancing processes of survival, growth,and reproduction, and excrete altered forms back intothe environment. Metabolism therefore determines thedemands that organisms place on their environment forall resources, and simultaneously sets powerful con-straints on allocation of resources to all components offitness. The overall rate of these processes, the meta-bolic rate, sets the pace of life. It determines the ratesof almost all biological activities.Recent progress in understanding how body size,temperature, and stoichiometry affect biological struc-ture and function at the molecular, cellular, and whole-organism levels of organization raises the prospect ofdeveloping a metabolic theory of ecology. Metabolismis a uniquely biological process, but it obeys the phys-ical and chemical principles that govern the transfor-mations of energy and materials; most relevant are thelaws of mass and energy balance, and thermodynamics.Much of the variation among ecosystems, includingtheir biological structures, chemical compositions, en-ergy and material fluxes, population processes,and spe-cies diversities, depends on the metabolic character-istics of the organisms that are present. Much of thevariation among organisms, including their life historycharacteristics and ecological roles, is constrained bytheir body sizes, operating temperatures, and chemicalcompositions. These constraints of allometry, bio-chemical kinetics, and chemical stoichiometry lead tometabolic scaling relations that, on the one hand, canbe explained in terms of well-established principles ofbiology, chemistry, and physics and, on the other hand,can explain many emergent features of biological struc-ture and dynamics at all levels of organization.THEORETICALFOUNDATIONSVirtually all characteristics of organisms vary pre-dictably with their body size, temperature, and chem-ical composition (e.g., Bartholomew 1981, Peters 1983,Calder 1984, Schmidt-Nielsen 1984, Niklas 1994, Gil-looly et al. 2001, 2002, Sterner and Elser 2002). Formore than a century, biologists have been investigatingthe mechanistic processes that underlie these relation-ships. Recent theoretical advances have shown moreexplicitly how these biological characteristics can bequantified, related to each other, and explained in termsof basic principles of biology, chemistry, and physics(e.g., Peters 1983, Sterner 1990, Elser et al. 1996,2000a, West et al. 1997, 1999a,b, 2001, Enquist et al.1999, Gillooly et al. 2001, 2002). Together, the olderconceptual and empirical foundations and the more re-cent theoretical advances provide the basis for a met-abolic theory of ecology. This theory explicitly showshow many ecological structures and dynamics can beexplained in terms of how body size, chemical kinetics,and resource supply affect metabolism. Through var-iation in the rates and biochemical pathways of me-tabolism among different kinds of organisms and en-vironmental settings, metabolic theory links the per-formance of individual organisms to the ecology ofpopulations, communities, and ecosystems.Metabolism and metabolic rateMetabolism is a complex network of biochemicalreactions that are catalyzed by enzymes, allowing theconcentrations of substrates and products and the ratesof reactions to be regulated. A chart of the chemicalreactions of metabolism shows a bewildering numberof substrates, enzymes, and pathways. Nevertheless,the core of metabolism consists of a small number ofreactions that form the basis of the TCA (tricarboxylicacid) cycle (Morowitz et al. 2000). The vast majorityof organisms use the same basic biochemistry, but therates of resource uptake, transformation, and allocationvary.When we speak of energy and energetics, we referto potential energy: the energy contained in photons orchemical