The energy budget: a useful tool?

Studies of energy flow and allocation in biological systems often result in the production of energy budgets. Our aim is to describe the potential mis-representations that energy budgets can produce and draw together the various criticisms levied at energy budgets. While such budgets purport to represent accurately energy allocation we, by discussing the literature, propose that many offer little. This is because in practice they rarely reflect actual energy relationships due to problems with their empirical derivation. These problems include both the omission of some energy budget terms (such as dissolved organic matter, non-lethal predation and metabolic faecal loss) and their underestimation (e.g. mucus production). Recalculation of budgets to account for these terms often results in new conclusions being drawn. Moreover, problems of extrapolation of measurements made in the laboratory to the field, coupled with misconceptions over the expression of temporal and spatial variation in budget terms, produce budgets that are both approximate and specific to an individual or population at the time each budget is constructed. In addition, the set of assumptions that are used in the construction of one budget are rarely the same as those for another and so budgets should be used with extreme care in comparative studies. We suggest that energy budgets have little value in the context of other studies and are of interest and value only as descriptors under a set of what should be well-defined assumptions. We urge caution in their use and propose that more modest studies of energy allocation with precise goals are more appropriate.