Introduction

All biological entities require matter for their construction and energy for their activities. This is true not only for individual organisms, but also for the populations and communities that they form in nature. The intrinsic importance of fluxes of energy (this chapter) and of matter (see Chapter 18) means that community processes are particularly strongly linked with the abiotic environment. The term ecosystem is used to denote the biological community together with the abiotic environment in which it is set. Thus, ecosystems normally include primary producers, decomposers and detritivores, a pool of dead organic matter, herbivores, carnivores and parasites plus the physicochemical environment that provides the living conditions and acts both as a source and a sink for energy and matter. Thus, as is the case with all chapters in Part 3 of this book, our treatment calls upon knowledge of individual organisms in relation to conditions and resources (Part 1) together with the diverse interactions that populations have with one another (Part 2).

A classic paper by Lindemann (1942) laid the foundations of a science of ecological energetics. He attempted to quantify the concept of food chains and food webs by considering the efficiency of transfer between trophic levels - from incident radiation received by a community through its capture by green plants in photosynthesis to its subsequent use by herbivores, carnivores and decomposers. Lindemann's paper was a major catalyst for the International Biological Programme (IBP), which, with a view to human welfare, aimed to understand the biological basis of productivity of areas of land, fresh waters and the seas (Worthington, 1975). The IBP provided the first occasion on which biologists throughout the world were challenged to work together towards a common end. More recently, a further pressing issue has again galvanized the community of ecologists into action. Deforestation, the burning of fossil fuels and other pervasive human influences are causing dramatic changes to global climate and atmospheric composition, and can be expected in turn to influence patterns of productivity on a global scale. Much of the current work on productivity has a prime objective of providing the basis for predicting the effects of changes in climate, atmospheric composition and land use on terrestrial and aquatic ecosystems (aspects that will be dealt with in Chapter 22).

The decades since Lindemann's classic work have seen a progressive improvement in technology to assess productivity. Early calculations in terrestrial ecosystems involved sequential measurements of biomass of plants (usually just the above-ground parts) and estimates of energy transfer efficiency between trophic levels. In aquatic ecosystems, production estimates relied on changes in the concentrations of oxygen or carbon dioxide measured in experimental enclosures. Increasing sophistication in the measurement, in situ, of chlorophyll concentrations and of the gases involved in photosynthesis, coupled with the development of satellite remote-sensing techniques, now permit the extrapolation of local results to the global scale (Field et al., 1998). Thus, satellite sensors can measure vegetation cover on land and chlorophyll concentrations in the sea, from which rates of light absorption are calculated and, based on our understanding of photosynthesis, these are converted to estimates of productivity (Geider et al., 2001).

Before proceeding further it is necessary to define some new terms. The bodies of the living organisms within a unit area constitute a standing crop of biomass. By biomass we mean the mass of organisms per unit area of ground (or per unit area or unit volume of water) and this is usually expressed in units of energy (e.g. J m-2) or dry organic matter (e.g. t ha-1) or carbon (e.g. g C m-2). The great bulk of the biomass in communities is almost always formed by plants, which are the primary producers of biomass because of

Lindemann laid the foundations of ecological energetics progressive improvements in technology to assess productivity some definitions: standing crop and biomass,...

their almost unique ability to fix carbon in photosynthesis. (We have to say 'almost unique' because bacterial photosynthesis and chemosynthesis may also contribute to forming new biomass.) Biomass includes the whole bodies of the organisms even though parts of them may be dead. This needs to be borne in mind, particularly when considering woodland and forest communities in which the bulk of the biomass is dead heartwood and bark. The living fraction of biomass represents active capital capable of generating interest in the form of new growth, whereas the dead fraction is incapable of new growth. In practice we include in biomass all those parts, living or dead, which are attached to the living organism. They cease to be biomass when they fall off and become litter, humus or peat.

The primary productivity of a community is the rate at which biomass is produced per unit area by plants, the primary producers. It can be expressed either in units of energy (e.g. J m-2 day-1) or dry organic matter (e.g. kg ha-1 year-1) or carbon (e.g. g C m-2 year-1). The total fixation of energy by photosynthesis is referred to as gross primary productivity (GPP). A proportion of this is respired away by the plants (autotrophs) and is lost from the community as respiratory heat (RA - autotrophic respiration). The difference between GPP and RA is known as net primary productivity (NPP) and represents the actual rate of production of new biomass that is available for consumption by heterotrophic organisms (bacteria, fungi and animals). The rate of production of biomass by heterotrophs is called secondary productivity.

Another way to view energy flux in ecosystems involves the concept of net ecosystem productivity (NEP, using the same units as GPP or NPP). This acknowledges that the carbon fixed in GPP can leave the system as inorganic carbon (usually carbon dioxide) via either autotrophic respiration (RA) or, after consumption by heterotrophs, via heterotrophic respiration (RH)—the latter consisting of respiration by bacteria, fungi and animals. Total ecosystem respiration (RE) is the sum of RA and RH. NEP then is equal to GPP - RE. When GPP exceeds RE, the ecosystem is fixing carbon faster than it is being released and thus acts as a carbon sink. When RE exceeds GPP, carbon is being released faster than it is fixed and the ecosystem is a net carbon source. That the rate of ecosystem respiration can exceed GPP may seem paradoxical. However, it is important to note that an ecosystem can receive organic matter from sources other than its own photosynthesis - via the import of dead organic matter that has been produced elsewhere. Organic matter produced by photosynthesis within an ecosystem's boundaries is known as autochthonous, whereas that imported from elsewhere is called allochthonous.

In what follows we deal first with large-scale patterns in primary productivity (Section 17.2) before considering the factors that limit productivity in terrestrial (Section 17.3) and aquatic (Section 17.4) settings. We then turn to the fate of primary productivity and consider the flux of energy through food webs (Section 17.5), placing particular emphasis on the relative importance of grazer and decomposer systems (we return to food webs and their detailed population interactions in Chapter 20). We finally turn to seasonal and longer term variations in energy flux through ecosystems.

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