Summary1. Ecosystem services are defined as the benefits that humans obtain from ecosystems. Employing the ecosystem service concept is intended to support the development of policies and instruments that integrate social, economic and ecological perspectives. In recent years, this concept has become the paradigm of ecosystem management. 2. The prolific use of the term 'ecosystem services' in scientific studies has given rise to concerns about its arbitrary application. A quantitative review of recent literature shows the diversity of approaches and uncovers a lack of consistent methodology. 3. From this analysis, we have derived four facets that characterise the holistic ideal of ecosystem services research: (i) biophysical realism of ecosystem data and models; (ii) consideration of local trade-offs; (iii) recognition of off-site effects; and (iv) comprehensive but critical involvement of stakeholders within assessment studies. 4. These four facets should be taken as a methodological blueprint for further development and discussion. They should critically reveal and elucidate what may often appear to be ad-hoc approaches to ecosystem service assessments. 5. Synthesis and applications: Based on this quantitative review, we provide guidelines for further development and discussions supporting consistency in applications of the ecosystem service concept as well as the credibility of results, which in turn can make it easier to generalise from the numerous individual studies.

Ecologists usually argue for a proactive approach to species conservation-it should start before a species is endangered and under substantial risk of extinction. In reality, however, conservation often only starts when species populations are already in a critical state. This may be the result of a policy process in which those actors who see only little or no benefits from conserving species try to delay conservation as long as possible to avoid its cost. A frequent consequence is that populations decline to critical levels so that once conservation policies set in due to legal obligations, political pressure or any other reason, additional conservation measures are required to re-establish the populations. We show that the costs associated with this policy process may be higher than those of a proactive policy. This is somewhat surprising because the costs of maintaining populations at a level at which they are not endangered may occur over a longer period. However, the costs of bringing species populations back to those levels may be so high that they outweigh the costs of the proactive approach. We develop simple cost functions that capture the main economic and ecological parameters relevant to our argument and apply them for an assessment of the costs of common hamster (Cricetus cricetus) conservation in the region of Mannheim, Germany. We find that a proactive approach would have saved between €17.2 and €36.4 mn compared to the existing policy where conservation was delayed until legal requirements forced local policy makers to implement a comprehensive hamster protection programme.

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