The necessity and utility of a hierarchical approach to model building in ecology is discussed. Some of the basic properties of hierarchies are described and criteria to discern subsystems are shown. A new approach of measuring the resilience in terms of natural probability of occurrence of a certain perturbation has been suggested, together with a stochastic definition of stability.

The application of the modelling process in ecology is outlined. Aspects of modelling as structure-function, systemic attributes and simulation language are examined. The different levels of model evaluation, its ramifications and implications are traced. The model depends on the process concerned and the sources of the most common errors are mentioned.

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