Systems in ecology are irrreducible, which means that we are not able to go from observations to fundamental principles which govern the system reactions. This implies that we have to work realistic models and attemptt o make conclusions on system principles on the basis of the model reactions. This is called experimental mathematics. Ecosystem may react to changes in forcing functions (external variables) by changes in species composition and ecological structure. The species which are best fitted to the new conditions will take over. ‘Fitted’ may be expressed by use of the thermodynamic concept exergy, which is the free energy relative to its environment. It has been possible to show by use of models that the changes in species composition and structure as a consequence of changes in forcing functions always are accompanied by an increase in exergy. The changes in the forcing functions may cause an increase or decrease in exergy, but the changes in the species composition always give an increase in the exergy. The examinations were carried out by the use of models with biological properties and with increasing complexity. Many different changes were used for the examinations and always the same pattern was observed. The results are not only of interest as thermodynamic descriptions of ecological observations by pertubations of ecosystems, but can also be used in practical modelling, as the exergy can be used to describe the trends in the changes of species composition.