Chapter 14 - The Estimation of Productivity and the Construction of Energy Budgets

The size of a population and the interactions between populations within an ecosystem may be expressed in terms of biomass (weight of living material) or energy content, as well as in numbers. Biomass and energy are useful to ecologists in that they provide common units for the description of populations of animals and plants of different sizes. By the comparison of energy budgets we may for example compare the strategies of warm blooded mammals and cold-blooded reptiles or even whales and copepods. Such measures are particularly important in food web studies where the focus is on the quantification of the flux of energy or carbon between different trophic levels. A particularly good example of this type of study has been undertaken on the tiny Ythan estuary in Scotland (Baird & Milne,1981; Raffaelli & Hall, 1996). Great effort is required to quantify each pathway within an ecosystem so that a whole community study with the species coverage achieved in the Ythan cannot presently be produced for a large temperate estuary and is almost inconceivable for a tropical floodplain or estuary. Comparing the energy flux of different pathways is one way in which we can assess the relative importance of the various interactions within a web (Paine, 1980, 1992). In studies of general predators, e.g. insectivorous birds, predation activity is often best expressed as biomass or energy. Conversely if the energy requirements are known from metabolic measurements, they may be used to predict food requirements in the field (Stiven, 1961). It must be remembered that the full richness of an organisms requirements and behaviour cannot be expressed in terms of energy. For example, food is more than simply an energy source, its quality in terms of specific amino-acids, vitamins and other constituents will also be important (Boyd & Goodyear, 1971; Iversen, 1974; Schroeder, 1977; Onuf et al., 1977).

14.1 Estimation of standing crop 14.1.1 Measurement of biomass 14.1.2 Determination of energy density 14.2 Estimation of energy flow 14.2.1 The measurement of production 14.2.2 The measurement of feeding and assimilation 14.2.3 The measurement of the energy loss due to respiration and metabolic process 14.3 The energy budget, efficiencies and transfer coefficients 14.3.1 The energy budget of a population (or trophic level) 14.3.2 Energy transfer across trophic links 14.4 Identification of ecological pathways using stable isotopes 14.5 Assessment of energy and time costs of strategies