Growth, reproduction and survival are activities requiring energy, for which living organisms need substrates (nutrients, light, food). Activity includes all the physiological work an animal does, not just for movement but also for growth and physiological regulation of its internal environment [3]. Energy generated in this way is essential not only for individual organisms, and for the populations and communities they form in nature, but also for whole ecosystems because trophic fluxes rely on energy.

Empirically based population and evolutionary studies of energy budgets use ‘net production’ or ‘scope for growth’ models [4, 5]. Scope for growth (SFG) is defined as ‘the difference between the energy of the food an animal consumes and all energy utilisations and losses’. In SFG models, the assimilated energy is immediately available for maintenance, the remainder is used for growth and or is deposited as reserves. Many mechanistic models for growth and reproduction in marine animals are based on the SFG concept [6, 7, 8, 9].

Another category of models that have been developed are the dynamic energy budget (DEB) or “assimilation models” [10, 11]. A DEB model describes the rates at which the organism assimilates and utilises energy for maintenance, growth and reproduction, as a function of the state of the organism (e.g. age, size, sex, nutritional status) and its environment (temperature & food density). One of the most detailed theories of dynamic energy budgets is the k–rule DEB theory developed by Bas Kooijman (DEB, Kooijman 2000 [12]). This theory assumes that the various processes of gain and loss of energy are dependent on surface area (assimilation) or on body volume (maintenance). The model assumes that the assimilated products enter a reserve pool (Fig. 1), from which a fixed proportion k of the available energy is allocated to somatic maintenance and growth combined, and the remaining 1-kappa to either maturation (for embryos and juveniles) or to reproduction and maturity maintenance (for adults). DEB models have been successfully used for bacteria, invertebrates and vertebrates. Contrary to SFG, the DEB model assumes that part of the respiration relates to overhead costs of production processes (growth and reproduction).

 

Figure 1. Schematic representation of the k-rule DEB theory (after Kooijman, 2000).


Another theory, the Metabolic Theory in Ecology (MTE), has been proposed to describe how individual organisms take up and use energy (e.g. [13]). This theory is founded on a mechanistic description of how the metabolic rate of organisms varies with body size and temperature. The MTE assumes that the energy supply rate to the cells follows a ¾ scaling relationship with body mass. Both MTE and DEB theory have been critically compared [14].