Energy Flow

by

William S. Currie

• LAST REVIEWED: 17 March 2021
• LAST MODIFIED: 23 May 2012
• DOI: 10.1093/obo/9780199830060-0047

Introduction

Energy flow in ecology encompasses a broad range of empirical and theoretical topics. It is difficult to draw a distinct boundary around the topic because it overlaps with many other areas of ecology, biology, physical sciences, and global change. The scales addressed by the study of energy flows and transformations include the molecular scale, the metabolism and physiology of organisms and microorganisms, the organization of trophic webs and ecosystems, and the surface energy balance of landscapes, regions, and the globe. The flow of energy was a key organizing principle in the growth of ecosystem science in the mid- to late 20th century. Food webs deal with the flow of energy from primary producers to consumers, detritivores, and predators. Ecosystem approaches consider aggregated primary production, respiration by autotrophs and heterotrophs, net ecosystem production, and changes in these quantities over time. Environmental biophysics includes the study of radiation fluxes, the surface energy balance, and the coupling of vegetation processes to the atmosphere. The concept of energy flows suggests energy transfers among elements of a system, while the broader concept of energetics also includes metabolism, controls on production, relationships between energy flows and carbon cycling, and a diverse set of ecological and biophysical phenomena related to energy in organisms and ecosystems. Within these topics there are several distinct areas of research and scholarship, with some areas overlapping a great deal and others overlapping only a little. Many efforts over the last half century have sought to unify wide-ranging aspects of ecological energetics, some with highly theoretical approaches that relate to ecosystem development or degree of organization. Other efforts have sought to link ecological energy flows with industrial energy flows or the provision of human needs in a linked social-ecological framework. In research on ecosystems and trophic webs historically, energy units, such as calories, are used to describe both biological and abiotic energy flows. In current research carbon is typically used as a proxy for energy in biological energy flows, while joules are typically used to describe abiotic flows and transformations of energy, such as those in the surface energy balance.

General Overviews

Energetics is such a broad topic that general overviews covering the entire range of the topic are virtually nonexistent. Each of the general overviews listed here covers some aspects of energetics while omitting others. Odum 1997 is an excellent starting point for students and researchers new to the study of energetics, because it covers a broad base of topics with a highly readable style. Hall 2004 provides a historical perspective in reviewing key energetics concepts in ecology and ecosystem science and is also an excellent starting point for students or researchers new to the topic. Odum 1983 is somewhat out of date in its framework, but it is listed here for historical reasons, because it was highly influential for a generation of ecologists. Ricklefs 2008 is an introductory textbook suitable for advanced undergraduate or beginning graduate students in ecology, as is Barbour, et al. 1999. Jørgensen 1997, in contrast, is a highly theoretical treatment that seeks a broad synthesis across energetics and other theoretical frameworks used in the study of ecosystems. Smil 1991 is widely recognized and cited. It is written for the scientifically literate general reader and has perhaps the broadest view of energy flows, but for ecologists it is less scientifically detailed compared to other citations listed here. Smil 2008 is an update, again for the lay reader, and it also provides a very broad view of energetics in the biosphere, the earth system, and in human societies.

• Barbour, Michael G., Jack H. Burk, Wanna D. Pitts, Frank S. Gilliam, and Mark W. Schwartz. 1999. Light and temperature. In Terrestrial plant ecology. 3d ed. By Michael G. Barbour, Jack H. Burk, Wanna D. Pitts, Frank S. Gilliam, and Mark W. Schwartz, 377–410. Menlo Park, CA, Benjamin/Cummings.

  Broad and thorough overview of basic principles that govern the light and radiation environment, including atmospheric effects, cloud cover, season, aspect, and latitude. Brief introduction of methods for measurement of radiation.

• Hall, Charles A. S. 2004. Ecosystems and energy: History and overview. In Encyclopedia of energy. Vol. 2. Edited by Cutler J. Cleveland, 143–155. New York: Elsevier.

  Overview and historical account reviewing general principles of energy flows in ecosystems, trophic webs (including grazing and detrital webs), and organism metabolism, production, and respiration. Focus is primarily on tracing the history of understanding and concepts. Suitable for general readers and an excellent starting point to learn about energy flows in ecology.

• Jørgensen, Sven Erik. 1997. Integration of ecosystem theories: A pattern. 2d rev. ed. Dordrecht, The Netherlands: Kluwer Academic.

  DOI: 10.1007/978-94-011-5748-3

  This text focuses on ecosystems from a highly theoretical perspective, and it is wide-ranging in its treatment of energetics. Includes chapters on energy, entropy, and exergy. Jørgensen searches for linkages among these topics with other theoretical frameworks, including network and hierarchy theory, catastrophe theory, and ecosystem development and dynamics, among others.

• Odum, Eugene P. 1997. Energetics. In Ecology: A bridge between science and society. By Eugene P. Odum, 79–120. Sunderland, MA: Sinauer.

  Eugene P. Odum has been a leading and highly influential ecologist for more than half a century. This insightful chapter serves as an excellent starting point for study of energy flows in ecology and human-environment systems. Very wide-ranging, touching on ecosystems, agriculture and food production, biofuels, the energy needs of human societies, energy flow through natural systems and human systems, and linkages to ecosystem services and economics.

• Odum, Howard T. 1983. Systems ecology: An introduction. New York: Wiley.

  A widely cited book that was highly influential in the study of ecosystems and ecological energetics. It introduced a systems perspective for studying energy flows and transformations in ecosystems and human societies.

• Ricklefs, Robert E. 2008. The economy of nature. 6th ed. New York: Freeman.

  Ecology textbook for undergraduate or beginning graduate students. Chapter 3 on the physical environment covers photosynthesis, heat in the environment, and energy in organisms. Chapter 22 on energy in ecosystems covers the trophic pyramid, gross and net primary production, respiration, and basic linkages of energetics to biogeochemistry.

• Smil, Vaclav. 1991. General energetics: Energy in the biosphere and civilization. New York: Wiley.

  As the title suggests, this is a wide-ranging work that covers biological energy flows and transformations as well as energy uses and transformations in human societies. A widely cited work for the general synthesis and fundamental descriptions that it provides. Many of the topics are updated in Smil 2008.

• Smil, Vaclav. 2008. Energy in nature and society: General energetics of complex systems. Cambridge, MA: MIT Press.

  Provides a general introduction and synthesis of energy concepts and energy flows in the biosphere, the lithosphere, and human societies. Ecologically relevant topics include the earth’s energy balance, energy flows in autotrophs and heterotrophs, human energetics, and systems of agricultural production. Written for the scientifically literate general reader.