Summary Ecosystem services are defined as the contributions that ecosystems make to human well-being and they are increasingly being used as an approach to analyse the relationship between humans and ecosystems. While ecosystem services are mainstreamed, operationalization of the ecosystem service concept for different policy purposes has to be further advanced. Among others, interest increases in integrating ecosystems and the services they provide into accounting schemes and into conservation planning. In this thesis, I address three challenges to operationalize the ecosystem service concept for accounting and conservation. These are first shortly described and addressed later on in the different chapters of this thesis. The first challenge relates to controversies around the conceptual basis of the ecosystem service concept. Ecosystem services is a normative concept and such concepts lead to controversies. These need to be clarified and addressed because contesting a concept likely reduces its acceptance and applicability. The second challenge relates to capturing the heterogeneous spatial distribution across an area of both the potential of ecosystems to provide ecosystem services (i.e. capacity) and of the actual use of ecosystem services (i.e. flow). The operationalization of ecosystem services thus requires geographic analysis. This spatial information on ecosystem services can feed into different policy purposes. Applying the ecosystem service concept in conservation planning is an evolving new practice. The third challenge is to develop appropriate methods to incorporate spatial information on ecosystem services into conservation planning. This thesis aims to explore and further develop the conceptual basis of ecosystem services, and to create and apply spatial models of multiple ecosystem services for accounting, management and conservation. These interdisciplinary objectives are addressed by critically reflecting on ecosystem services, conceptual reasoning, further methodological development of spatial modelling, as well as applying the resulting spatial models in plausible conservation scenarios. In Chapter 2, I explore the conceptual basis of ecosystem services and describe and reflect on seven recurring critiques of the concept and respective counter-arguments. Critical arguments and counter-arguments are summarised from a literature review and they are contrasted. The seven chosen critiques are as follows. First, the concept is criticized for being too anthropocentric, whereas others argue that the concept goes beyond instrumental values and includes elements of intrinsic values that relate to ecosystem being valued for their own sake. Second, some argue that the concept promotes an exploitative human-nature relationship, whereas others state that it re-connects society to ecosystems and emphasizes humanity’s dependence on ecosystems. Third, concerns exist that the concept conflicts with biodiversity conservation objectives, whereas others emphasize complementarity between the concepts of biodiversity and ecosystem services and the practical application of both concepts in planning. Fourth, the concept is questioned because of its supposed focus on economic valuation, whereas others argue that ecosystem-services science includes various types of value systems. Fifth, the concept is criticized for promoting commodification of nature, whereas others point out that most ecosystem services are not (directly) connected to market-based instruments. Sixth, vagueness of definitions and classifications of ecosystem services are stated as a weakness, whereas others argue that vagueness enhances creativity and transdisciplinary collaboration. Seventh, some criticize the normative nature of the concept, implying that all outcomes of ecosystems and their processes are desirable. The normative nature is indeed typical for the concept, but should not be problematic when adequately acknowledged. Disentangling and contrasting different arguments contributes to a more structured debate between opponents and proponents of the ecosystem services concept and helps to further conceptualize the ecosystem service concept. In Chapter 3, I develop and test a framework to analyse ecosystem service capacity and flow in a spatially explicit way. This study was conducted in the overall context of ecosystem accounting. Ecosystem accounting aims to monitor extent, condition and properties of ecosystems that deliver ecosystem services over time. Guidelines and standards for ecosystem accounting are currently being developed under the auspices of the United Nations and this chapter is closely aligned to the recent System of Environmental-Economic Accounting Experimental Ecosystem Accounting (SEEA) guidelines. Understanding the capacity of ecosystems to generate these services and the resulting flow of ecosystem services is an essential element for understanding the sustainability of ecosystem use as well as developing ecosystem accounts. I conduct spatially explicit assessments of nine ecosystem services in Telemark county, Southern Norway. The modelled ecosystem services are moose hunting, sheep grazing, timber harvest, forest carbon sequestration and storage, snow slide prevention, recreational residential amenity, recreational hiking and existence of areas without technical interference. I conceptually distinguish capacity to provide ecosystem services from the actual flow of services and empirically assess both. This is done by means of different spatial models, developed with various available datasets and methods, including (multiple layer) look-up tables, causal relations between datasets (including satellite images), environmental regression and indicators derived from direct measurements. Capacity and flow differ both in spatial extent and in quantities. The distinction of capacity and flow of ecosystem services provides a parsimonious estimation of over- or underuse of the respective service. Assessment of capacity and flow in a spatially explicit way can thus support monitoring sustainability of ecosystem use, and this is an essential element of ecosystem accounting. In Chapters 4 and 5, I explore methods to operationalize the ecosystem service concept for conservation planning. These chapters are based on the models for regulating and cultural services that were developed in Chapter 3. The variation in spatial distribution between ecosystem services can be high. Hence, spatial identification of important sites for conservation planning is required. The term ‘ecosystem service hotspot’ has often been used for this purpose, but this term is defined ambiguously. An ecosystem service hotspot can refer to either areas containing high values of one service or areas with multiple services. In Chapter 4, I review and classify methods to spatially delineate hotspots. I test how spatial configuration of hotspots for a set of ecosystem services differs depending on the applied method. The outcomes are compared to a heuristic site prioritisation approach (Marxan). The four tested hotspot methods are the threshold value approach, Gi* statistic, intensity, and richness. In a conservation scenario, I set a target of conserving 10% of the quantity of five regulating and cultural services for the forest area of Telemark. Spatial configuration of selected areas as retrieved by the four hotspots and Marxan differed considerably. Pairwise comparisons were at the lower end of the scale of the Kappa statistic (-0.003 – 0.24). The outcomes also differed considerably in mean target achievement ranging from 7.7% (richness approach) to 24.9% (threshold value approach), cost-effectiveness in terms of land-area needed per unit target achievement and compactness in terms of edge-to-area ratio. Differences in spatial configuration among different hotspot methods probably lead to uncertainties for decision-making. These differences also have consequences for analysing the spatial co-occurrence of hotspots of multiple services and of services and biodiversity. While determining hotspots according to one approach might lead to high degrees of spatial overlap with another ecosystem service or biodiversity, other delineation methods might lead to considerably lower degrees of overlap. In Chapter 5, I analyse how the incorporation of ecosystem services as conservation features affect conservation of forest biodiversity and how different opportunity cost constraints change spatial priorities for conservation. In this study, spatially explicit cost-effective conservation scenarios for 59 forest biodiversity features and five ecosystem services in Telemark County were created with the help of the heuristic optimisation planning software Marxan with Zones. A mix of conservation instruments where forestry is either completely (non-use zone) or partially restricted (partial use zone) were combined. Opportunity costs were measured in terms of foregone timber harvest, an important provisioning service in Telemark. Including a number of ecosystem services shifted priority conservation sites compared to a case where only biodiversity was considered, and increased the area of both the partial (+36.2%) and the non-use zone (+3.2%). Furthermore, opportunity costs increased (+6.6%), which suggests that ecosystem services are not a side-benefit of biodiversity conservation in this area. Opportunity-cost levels were systematically changed to analyse their effect on spatial conservation priorities. Conservation of biodiversity and ecosystem services trades off against timber harvest. Currently designated nature reserves and landscape protection areas achieve a very low proportion (9.1%) of the conservation targets scenario, which illustrates the high importance given to timber production at present. A trade-off curve indicated that large marginal increases in conservation target achievement are possible when the budget for conservation is increased. Forty percent of the maximum hypothetical opportunity costs would yield an average conservation target achievement of 79%. This study shows how a heuristic optimisation approach can aid conservation planning for a number of ecosystem services and biodiversity. Based on the critiques on the ecosystem service concept, their respective counter-arguments and the conclusions from the chapters of this thesis, I suggest to further clarify, describe and develop a thick, rich and vague concept of ecosystem services. The ecosystem service concept is a thick concept as it is both descriptive, referring to flows of energy, matter and information from ecosystems to society, and normative, referring to ecosystem services being valuable and preferable to conserve. A rich ecosystem service concept envisions the concept as a platform for plural values emphasising multi-functionality of ecosystems. A rich conceptualization of ESs also includes sustainability principles, such as for instance renewability and aspects of intra- and intergenerational justice. For the purpose of interdisciplinary collaboration, the concept needs to be both vague enough to be flexibly adopted by different disciplines, and at the same time rich enough to ensure that researchers from different disciplines know they are working with the same phenomenon. My thesis shows that conceptual and empirically testable ways to address the critique on the ecosystem service concept exist. I conclude that a thick, rich and vague conceptualization of ecosystem services is a way forward and an adequate foundation for science that builds on the ecosystem service concept. I have proposed and empirically tested methods to spatially model ecosystem service capacity and flow and that capacity and flow differ in distribution and abundance. The distinction and empirical assessment of capacity and flow, if measured with aligned indicators, improves the understanding of over- or underuse of ecosystem services. Furthermore, I have shown possible consequences of operationalizing the ecosystem service concept for conservation. To include ecosystem services in conservation or management decisions and to bear the consequences of this inclusion is mainly a societal choice. This choice requires a societal discourse on which ecosystem services and how much of these services should be conserved. This thesis provides knowledge that can feed into such a deliberative discourse about ecosystem services. A thick, rich and vague ecosystem service concept contributes to the philosophical basis of this discourse, ecosystem accounting contributes to the cognitive basis, and the proposed methods for ecosystem service conservation can help guiding action to effectively sustain the provision of ecosystem services.