Aggregation in model ecosystems. I. Perfect aggregation

Determining appropriate levels of aggregation or complexity constitutes a major problem in describing ecological systems. We give necessary and sufficient conditions for perfect aggregation in non-linear dynamical systems. This condition has a form explicitly related to that for aggregation of linear dynamics in automatic control theory and to that for aggregation of linear functions in economics. Application of the theory to several ecological examples supports the general principle of aggregating variables that functionally are either similar or complementary.