The purpose of this chapter is to introduce some general concepts to prepare for the development of the DEB theory in the next chapter. I present many tests against experimental data. These tests require careful interpretation of data, making use of the material presented in this chapter.

Individuals: the basic level of organization

From a systems analysis point of view, individuals are special because at this organizational level it is relatively easy to make mass balances. This is important, because the conservation law for mass and energy is one of the few hard laws available in biology. At the cellular and at the population level it is much more difficult to measure and model mass and energy flows. It is argued on {298} that life started as an individual in evolutionary history rather than as a particular compound, such as RNA. The individual is seen as an entity separated from the environment by physical barriers. Discussion should, therefore, start at the level of the individual.

Input/output relationships

Any systems model relates inputs to a system with outputs of that system as a function of its state. Although many formulations suggest that the output is the result of the state of the system and its input, this cause-and-effect relationship is, in fact, a matter of subjective interpretation. The input might as well result from the state and the output; input, state and output change simultaneously, without an objective causality.