The simulation methods with difference equation and Markov chain were offered to analysis of compartmental models in this study. It was shown that the two simulation methods with linear difference equation and Markov chain could equally applied to a class of compartmental models in which outflows (rate variables) are directly proportional to the state (level) variables. Some advantages and disadvantages of the simulation with each method were discussed and illustrated. In general, it was concluded that deeper understanding of the compartmental models could be gained from mutually complemental applications of the two simulation methods.

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Agriculture can be defined as an industry where products are generated using the activities of organisms. Modern agriculture has chosen to rely on fossil energy as the primary means to stimulate the activities of these organisms. Is it impossible to replace current technologies based on fossil energy with the use of proper interactions between organisms to enhance agricultural production? If not, then modern agriculture, which utilizes only the simplest biotic responses, can be transformed into another system of agriculture, in which the use of complex bio-interacions becomes the key technology instead of heavy use of fossil energy. In the first part of this paper, the concept of use of complex bio-interactions was clarified, and in the following sections, 5 actual examples of the bio-interactions operating in agriculture and the bio-environment interactions, especially in grassland, were selected. The bio-interactions used in the examples are still not complex, but in future, much more complicated interactions certainly will be revealed. It is essential to maintain a balance between 'production' and 'environment' problems in agriculture. The concept of system and use of system analysis will be instrumental in the studies on bio-interactions in agriculture.

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