Does the Ecosystem Service Concept Reach its Limits in Urban Environments?

There is a rapidly growing body of literature on the theory about the ecosystem service concept and the practical assessment of
ecosystem services in different contexts ranging from natural to urban environments. Yet, where does the concept reach its limits?
This paper critically reflects the application of the ecosystem service concept in urban environments illustrating the handling of
urban structures (incl. built-up areas) and the risk that the normative principle of the concept could be missed. It is shown that in
theory urban structures refer to a variety of ecosystem concepts. As a starting point for ecosystem service assessments, these could
be classified into natural, managed, constructed and overbuilt systems. Since ecosystem service concepts do not directly refer to
a specific ecosystem definition, but to biophysical structures and processes, all of these classes could be included. However, the
dependency on context and scale makes a differentiation in practical ecosystem services assessment challenging. We conclude
that the ecosystem service concept does not reach its limits in urban environments, but urban environments represent an extreme
case characterized by multifunctionality and a high degree of modification that enables to uncover research challenges applying
in any environment. There is a need for a more transparent reporting of theoretical and methodological assumptions to facilitate
the comparability between ecosystem service assessments. Comprehensive approaches that consider multiple ecosystem services
and include human input, human modification, the ecosystem status as well as their interactions are required to understand the
spatial relations between ecosystem services delivered by different ecosystems.