ENERGY FLOW AND ECOSYSTEMS Alan P. Covich Colorado State University I. Ecosystem Boundaries: Inputs, Outputs, and photoautotrophs These, such as green plants and some Transformations of Energy bacteria, use solar energy and inorganic compounds II. Multiple Energy Pathways to synthesize organic matter as primary producers. III. Internal and External Nutrient Cycling primary productivity The rate of synthesis of organic IV. Biodiversity Effects on Energy Flow matter by plants (biomass per unit area of habitat V. Why Species Matter per unit time or, in some cases, biomass per unit VI. Future Studies volume per unit time). secondary productivity The rate of assimilation and growth by animals (biomass per unit area of habitat per unit time or, in some cases, biomass per unit GLOSSARY volume per unit time). trophic cascades These occur when changes in the chemoautotrophs Microbes that use inorganic com- presence or absence (or shifts in abundance) of a pounds as a source of carbon and energy and func- top predator alter the production at several lower tion as primary producers. trophic levels; primary and secondary production at decomposition The biotic breakdown of dead organic lower levels are alternately constrained or uncon- matter (detritus) by bacteria and fungi that releases strained by the feeding activities of consumers at carbon dioxide and nutrients for recycling. upper levels. ecosystems Composed of species assemblages (pro- trophic levels Groups of individuals classified as pri- ducers and consumers) that interact with each other mary producers or primary or secondary consumers and their associated abiotic environment within within food webs; individuals feeding both as pri- well-defined natural or conceptual boundaries. mary and secondary consumers are omnivores. A food chains Composed of species that are connected single species may be represented on more than one by the flow of energy and material from producers trophic level. to consumers. food webs ‘‘Flow maps’’ that depict connections among multiple food chains. functional groups Aggregations of species that perform similar ecosystem processes, such as grazers, suspen- ECOSYSTEMS ARE THERMODYNAMICALLY OPEN, sion or filter feeders, leaf shredders, predators, and hierarchically organized communities of producers, decomposers. consumers, and decomposers together with the abiotic Encyclopedia of Biodiversity, Volume 2 Copyright 2001 by Academic Press. All rights of reproduction in any form reserved. 509 510 ENERGY FLOW AND ECOSYSTEMS factors that influence species growth, reproduction, and functions. Field testing of many concepts related to dispersal. These abiotic factors include the flow of en- understanding the importance of key species in ecosys- ergy and the circulation of materials together with the tem functioning is just beginning. Results of these long- geological, hydrological, and atmospheric forces that term ecosystem studies can provide guidelines for the influence habitat quality, species distributions, and spe- stewardship of biodiversity. cies abundances. Energy flows through many species, and the way in which this flow affects the persistence of ecosystems is influenced by land-use changes, precip- itation, soil erosion, and other physical constraints such I. ECOSYSTEM BOUNDARIES: INPUTS, as geomorphology. Energy flow through ecosystems is essential for OUTPUTS, AND TRANSFORMATIONS nutrients to cycle through food webs. These food OF ENERGY webs are often subwebs of more complex species assemblages and may be only partial descriptions of An ecosystem approach can be used to address many more complex hierarchies of energy flows. The hierar- different questions spanning scales from the global bio- chy of species’ interactions in natural food webs sphere to small ponds or patches of habitat. As the typically results in some important feedback loops questions change, so do the boundaries and the com- and recycling of nutrients and materials within the plexities of species interactions within and among dif- conceptually defined boundaries of an ecosystem. ferent compartments or across trophic levels. Ecosys- Many species of producers and consumers are usually tem boundaries are often defined to include natural interconnected and some may be interdependent. Food species assemblages and to analyze inputs and outputs webs are diagrams that can function as ‘‘flow maps’’ of energy and materials for cross-site comparisons of to document which species interact with other species, efficiencies in energy transfers and studies of changing either directly or indirectly, as energy flows through conditions. These analyses often take the form of mathe- the community and determines the movement of matical models such as computer simulations or indi- nutrients and other materials. vidual-based models of species and their specific func- Different species have important functional values, tions within the biotic assemblage and environmental such as for organic matter production (plant and animal conditions under study. growth) or organic matter breakdown (decomposition), One ecosystem may export nutrients and organic oxygen production, nitrogen fixation, and nutrient cy- matter (stored energy) to other ecosystems so that cling. These species have intrinsic values as the unique cross-site linkages often become important. For exam- end products of evolution, and native species are likely ple, in studies of nutrient cycling in terrestrial ecosys- to have adapted specific ways to respond to local or tems, the definition of boundaries would likely have regional environmental disturbances. Conceptually, the some compartments of organic matter production by loss of even a single native species, or the introduction living plants and their relationships with herbivores of a nonnative species, could alter how the other re- and carnivores. Macro- and micronutrient inputs would maining native species continue to perform different likely be derived from the atmosphere through dry de- ecosystem functions. Disruptions of ecosystem pro- position of particulates—nitrogen gas being taken up cesses are known to have occurred after certain well- by some nitrogen-fixing species of microbes. Other adapted, native species were lost through local extinc- sources of nutrients, especially phosphorus, would tion following intense (pulse) or prolonged (press) dis- come from weathering and erosion of soil and bedrock turbances. However, predicting which species are essen- deposits, with movement among other compartments tial to ecosystem functions has generally remained derived from actions of wind and water. This combina- difficult because information is lacking on many species tion of interactions illustrates the importance of defin- interactions as well as on life history, adaptations to ing clear boundaries for subsystems within the complete different disturbances, and dispersal abilities among key ecosystem so that measurements of movements (fluxes) species. Ecosystem studies can provide a broad perspec- among compartments can be measured accurately. Gen- tive regarding species relationships and recycling of erally, ecosystems and their boundaries are abstractions essential nutrients. Species’ shifts in patterns of abun- that can only be useful and insightful when combined dance (or local extinctions) following natural and an- with sufficient knowledge regarding the natural history thropogenic disturbances illustrate how some key spe- and general ecology of communities and their physical cies regulate nutrient cycling and other ecosystem environment. There is wide recognition that a combina- ENERGY FLOW AND ECOSYSTEMS 511 tion of direct field observation, experimentation, and ideas (Schulze and Mooney, 1994; Orians et al., 1996; modeling is essential when conducting ecosystem Lawton, 1997; Palmer et al., 1997; Naeem, 1998; studies. Wall, 1999). Currently, fundamental questions dealing with rela- tionships between energy flow and the species-specific A. A Historical Perspective: roles of organisms are attracting increased attention. For example, can results of controlled small-scaled The Ecosystem Concept (fine-grained) experimental studies of productivity be How energy moves from one group of species to another used to predict responses of other natural assemblages has been an active area of study at least since Charles at larger scales (coarse grained) of ecosystem dynamics? Darwin and Alfred Russel Wallace first wrote in the Does energy flow through an ecosystem increase, de- 1850s about the interconnections among species. They crease, or remain the same if one species goes locally were intrigued with the general proportions of popula- extinct but the abundances of other ‘‘similar species’’ tion abundances that were thought to exist among dif- change rapidly to compensate for the lost species? Un- ferent groups of large predators and their prey, and they der what environmental conditions do species substi- emphasized competition and predation as important tute for one another and compensate functionally for factors for regulating species interactions. These early the lost species? These and many other questions are observers lacked a conceptual approach to what was beginning to be answered, but studies related to biodi- later viewed as ecosystem-level dynamics. Darwin’s versity and the persistence of species assemblages re- studies of earthworms and their roles in soil develop- main incomplete. A series of symposia during the 1990s ment and his views on the roles of multiple species dealt with the relationships between