The Ecosystem in Practice: Interest and Problems of an Old Definition for Constructing Ecological Models

Since its inception, the ecosystem concept has been widely used in ecology and is increasingly finding application within other disciplines. In more recent times within ecology, however, it has been suggested the term is now obsolete. We argue that three problems lie at the heart of these criticisms, namely the physics-biology duality problem, the boundary problem and the abstraction problem. The physics-biology duality problem (how to grapple with systems that follow the laws of both physics and biology) is addressed by modern computer science techniques originating from simulation and software engineering. The boundary problem (how to find the limits of an ecosystem in the real world) is solved by a powerful assumption of Tansley, that the ecosystem is an ad hoc construct on the part of an observer for a particular purpose. The abstraction problem (can models of an ecosystem at different levels of detail produce the same outcomes) has no general solution, but can be improved upon by using scaling techniques and standards to facilitate model comparisons. We demonstrate that Tansley's (Ecology 16:284-307, 1935) definition is still relevant to modern ecology almost as is. Tansley's ecosystem is a multi-disciplinary, recursive, scale-independent and observer-dependent object. These properties closely match those of complex systems as defined in mathematics and computer sciences. From Tansley's definition, we propose a formal description of the concepts and relations linked to the ecosystem definition, as an ontology that can serve as a basis for future discussion, modelling and conceptual work.