ABSTRACT
Organisms in ecosystems are not living isolated but are continuously in interaction with other organisms either of the same species or with organisms of other species. When we build ecological models, it is crucial to describe these interactions, because the model results are strongly dependent on a close to correct description of at least the most important exchange processes among the state variables. The resources in ecosystems are limited according to the mass and energy conservation principles, which imply that the organisms are competing for the resources. Therefore, competition is an important biological interaction. Many population dynamic models have been constructed to describe this competition (e.g., Lotka-Volterra model) (Jørgensen and Fath, 2011 and Section 19.3). An equally important interaction process is, however, cooperation, because without cooperation no ecosystem will endure. Cooperation is necessary to cycle the resources-for instance the biologically important elements-(see Chapter 12). Without cycling, the limiting elements would be used very fast and further development of the ecosystem would stop, and due to the Second Law of Thermodynamics, the ecosystem would inevitably decompose completely and reach thermodynamic equilibrium. This means that the system is without gradients, free energy, and exergy and the entropy is at maximum. Cycling processes are described in Chapters 13 through 17, and how these processes ensure the cycling is presented in Chapter 12. Chapter 18 considers the processes of particularly toxic substances in ecosystems and their influence on other biological processes.
