An ecosystem is normally defined as a complex of all living (plants, animals, microorganisms) and non-living (soil, climate) components interacting *s as a functional unit in a certain area. Each contributes to maintaining the overall ecosystem's health and productivity. Ecosystems such as forests, wetlands and grasslands play an important role in the global water cycle. Recognizing this role and the interactions between the two is critical to managing water resources sustainably.
It is often conceptualized that ecosystems provide a range of "services" that can be categorized as: i) Provisioning that refers to consumer goods, such as food and water; ii) Regulating that includes, among others, water purification and preventing erosion; iii) Habitat that provides the environment for life cycles of species or maintains genetic diversity, through quality and quantity of natural vegetation or substrate for fish and iv) Cultural that refers, for example, to the aesthetic, tourism and spiritual services (TEEB, 2010).
The estimated economic value of ecosystem services globally, made in 2011, was $124.8 trillion, which was close to double the global gross domestic product in the same year (Costanza and others, 2014). It is widely acknowledged now that various ecosystems, both aquatic and terrestrial, are in decline, primarily due to economic development. There is no shortage of statistics. Since 1900, the world has lost around 50 per cent of its wetlands (WWDR 3, 2009). Between $4.3 and $20.2 trillion per year worth of ecosystem services were lost between 1997 and 2011 due to land use change (Costanza and others, 2014). An estimated 20 per cent of the world's aquifers are being over-exploited leading, among others, to land subsidence and saltwater intrusion (Gleeson and others, 2012). Over half of the world's large river systems are adversely affected by dams (Nilsson and others, 2005). Inefficient use of water for crop production has caused salinization of 20 per cent of the global irrigated land area (FAO, 2011). The decline of ecosystems results in a range of adverse impacts on humans, since billions of people live in water scarce regions, and/or areas with high water quality risks (Guppy and Anderson, 2017; Veolia and IFPRI, 2015)
It is common today to hear in scientific discourse about "payment for ecosystem services", "ecosystem approach", "green and grey infrastructure", "nature-based solutions", and many other terms that directly or indirectly involve the notion of ecosystems (Lautze, 2014). This discourse is a reflection of the growing concern about the status of global ecosystems and increasing understanding of the critical role that ecosystems play in development at large, and in water resources development, in particular.
As a natural [e.g. aquatic] ecosystem is being modified, some of the original services and associated benefits extracted from it are...