Function and "functioning" in ecology: What does it mean?

Many investigations in current ecological research focus on ecological functions, the functions of biodiversity, or the functioning of ecological systems. However, within ecology "function" is used in several different meanings. The most important ones are those of function as denoting either processes, roles, services or the "functioning" of whole systems. Some of these meanings pose considerable problems in terms of their ability to apply them in empirical research. These problems together with the ambiguity of the term "function" constitute considerable impediments for generalizations of research results. In order to improve this situation, the current paper describes the different meanings of "function" and discusses the requirements necessary to unambiguously apply the concept(s) in practice. This is illustrated by the example of current discussions on the "functions of biodiversity".