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Humans live among ecosystems that provide innumerable benefits to our lives: ranging from the pollination of our crops to providing spiritual values, and everything in between. Collectively, these benefits that people obtain from ecosystems are Ecosystem Services (ES) (Millenium Ecosystem Assessment 2005) . ES can refer to both goods (i.e. timber) and services (i.e. water filtration). They have been discussed in related scientific fields since the 1960’s (De Groot et al. 2002)1) . However, in the last decade the amount of research on the topic, and its applicability to decision-making, has increased dramatically.
The Millennium Ecosystem Assessment (2005)2) divides ES into four distinct categories:
This is the most commonly accepted framework for understanding different types of ES. As our understanding of ES continues to grow, finding ways to explicitly identify and account for each one is an important step to implementing ES in decision-making. To assess these different types of ES, indicators are used for quantifying their flows.
The applied value of the ES concept, and research about specific ES, is that such information can be used to bolster sustainable, place-based decision making (Villamagna et al. 2013)3) . Natural resource planning, infrastructure development, natural area management, tourism development, and more, are interconnected with ES. These sectors both depend, and exert pressures, upon the effective provision of ES. Research in the area of ES is making these interconnections explicit. For example, one can estimate the amount of water that is purified by a wetland area and subsequently used by the local population. The value of this service is an important counterbalance to the economic factors that would push for its development into, say, housing. Oftentimes, ES help present natural values in a way that they can be effectively quantified, and thus compared, with traditional economic values (Chan et al. 2012)4) . By studying ES, the trade-offs, ES values, and relationships between actions and outcomes become more clear, enabling decision-makers to better address the issues they face.
Ecosystem Services include such a diverse range of goods and services that consistently defining ways to identify, characterize, and value them is incredibly important for effective application. Unfortunately, the research on ES is anything but consistent, using different terminology, definitions, and frameworks (Villamagna et al. 2013)5) . This creates an ongoing challenge for practitioners. Some methods for categorizing, assessing, and valuing ES are reviewed below.
There are several large scale attempts to frame and evaluate ES across the Alpine Space, Europe, and the globe. Some important examples of such initiatives are:
For the purposes of the ES evaluation in the AlpES Project, each ES is split into three aspects: supply, flow, and demand. The indicators used to assess ES are often different depending upon the category one is hoping to assess. Some notes on specific indicators are thus also included below.
Supply is the amount of an ES that can be delivered by an ecosystem. The supply of ecosystem services is strongly linked to natural conditions, e. g. land cover (vegetation), hydrology, soil conditions, fauna, elevation, slope and climate (Burkhard et al. 2010)6) . In order to better quantify the distinct pieces of supply, it is further broken down into 1.) potential and 2.) stock. This differentiation can aid in decision making.
Ecosystem Service Flow is the de facto, or actual, amount of an ES (or bundles of ES) that is utilized from an ecosystem in a given time period (Burkhard et al. 2014, Albert et al. 2016)10) . Flow is easiest to conceptualize in provisioning ES; for example, the amount of fire wood taken out of a forest would be equal to the flow of the fuel wood provision ES. In other words, it is the amount or rate of an ES that is supplied to some beneficiary (Potschin et al. 2016)11) . Indicators for flow are thus often simply measurements of the amount of an ES used from a particular area in a given period.
Demand for an Ecosystem Service is the amount of the good or service that is currently consumed or used in a particular area over a given period, regardless of where the ES are derived from (Burkhard et al. 2012)12) . For example, the amount of firewood burned in a village over a year, even if the majority of this wood is imported. Demand is a characteristic that can be measured across a multitude of scales, ranging from local demand for recreation opportunities, to global demand for carbon sequestration.
As part of the AlpES project, eight ES services have been selected for evaluation and mapping across the Alpine Space:
Now that the ES have been selected, indicators for each will be developed. They will then be evaluated and mapped for the Alpine Space as part of the AlpES project.For each ES indicator, metadata are available here.