I was asked to introduce this volume by examining "why a knowledge of ecosys tem functioning can contribute to understanding species activities, dynamics, and assemblages." I have found it surprisingly difficult to address this topic. On the one hand, the answer is very simple and general: because all species live in ecosystems, they are part of and dependent on ecosystem processes. It is impossible to understand the abundance and distribution of populations and the species diversity and composition of communities without a knowledge of their abiotic and biotic environments and of the fluxes...