We dedicate this book to professor C. T. de Wit (1924 - 1993) who
initiated Production Ecology as a school of thought at the Wageningen
Agricultural Univer- sity (see Rabbinge et at., 1990). To acknowledge
the leading role of C. T. de Wit, a recently formed graduate school at
this university in Production Ecology was named after him. Production
Ecology is the study of ecological processes, with special attention to
flows of energy and matter as factors that determine the productivity of
ecological systems. Agro-ecosystems are a special case of ecosystems
which are much better suited for the productivity approach than natural
ecosystems are. This is the reason for the strong role of agricultural
research in production ecology. On the other hand, it must be recognized
that the spatial heterogeneity of natural ecosys- tems and their species
richness may alter some ecophysiological relationships. However, the
basic physical, chemical and physiological processes will be the same.
De Wit introduced the state variable approach as the basis for
simulation mod- elling. In this approach the floating character of
nature is schematized into a series of snapshots over time in which the
states are frozen at each separate moment. The current state determines
how the rates of change will lead to the next snapshot. This way of
thinking enables a clear and workable representation of interacting
simul- taneous processes, without compromising on the mathematics.