WHAT DO SPECIES DO IN ECOSYSTEMS

Three aspects of the role of species in ecosystems are reviewed. II) Theoretically, what are the possible relationships between ecosystem processes (Likens' (1992) ''transformation and flux of energy and matter'') and the species richness of communities? (2) Summaries of two experiments with artificially constructed terrestrial ecosystems in the controlled environment facility known as the Ecotron an described. The first draws attention to the role of earthworms as ''ecosystem engineers''; the second explores changes in ecosystem processes in mesocosms assembled with three different levels of biological diversity. (3) Finally, a brief summary of the concept of organisms as ecosystem engineers is provided. The review is one of a growing number of publications that attempt to break down the artificial and potentially damaging barriers that exist between ecosystem ecology and population biology. Loss of plant species may change and impair ecosystem processes under average environmental conditions, and reduce the ability of ecosystems to withstand, and recover from, extreme events. The very existence of some ecosystems depends on particular species - the major autogenic and allogenic engineers that create the system - and all ecosystems are probably modulated and modified to a significant extent by at least one species of engineer.