‘Ecosystem functioning’ means different things to different people. Also – and not so surprising – there are different ways and variables by which the function(ing) of ecosystems is assessed. In this chapter I will broaden the view of the different meanings of ecosystem functioning beyond the specific context of the biodiversity–ecosystem functioning (BEF) discourse and try to distinguish and clarify the most important meanings of the concept. For this purpose I will take a close look at the two parts of ecosystem functioning, namely the ecosystem concept and the concept of function(ing). Both concepts are highly ambiguous. Understanding their semantic and philosophical intricacies is an important prerequisite for clearly conceptualising and assessing ecosystem functioning.

The most general definition of an ecosystem – what I will call its ‘generic definition’ in the following – defines an ecosystem simply as an assemblage of organisms together with their abiotic environment. While this definition is widely accepted, it is not very helpful when it comes to assessing ecosystem functioning in practice, for example, in BEF research, ecosystem management, or restoration ecology. More specific and concrete definitions are required. There are of course many specific definitions of what an ecosystem is, but there are also considerable differences between the definitions. These differences have important consequences for research and application. I will exemplify this by means of a case study on ecosystem management in the Greater Yellowstone Ecosystem (Section 4.3.1), and also introduce a methodology to clarify and compare the different meanings of ‘ecosystem’ (Section 4.3.2).