Fresh water is vital for the function of all terrestrial ecosystems – the flora and the fauna that make up those ecosystems, and, of course, for humans. Humanity relies on water not just for drinking, but also for food production, dealing with waste, providing energy and transport, to name but a few. To meet its needs humanity harnesses water through dams, irrigation networks, and pumps and pipes that supply drinking water and remove wastes. It is estimated that humanity consumes 1000–1700 m3 of the globe's surface and groundwater resources per year; that is between 22% and 150% of the annual global supply of fresh water (Hoekstra & Wiedmann 2014). This proportion is likely to increase as the global human population increases in the next 30 years and the demands for water in developing countries catches up with that of developed countries. According to the Intergovernmental Panel on Climate Change, changes in climate will amplify existing stress on water availability and will exacerbate different forms of water pollution, with impacts on ecosystems, human health, and water system reliability in large parts of the world (Stocker et al. 2013).

For a number of years, academics have tried to understand the linkages between the water system and human needs and the impacts that anthropogenic activities have on the water system itself. In the early days, the scientific approach sat within individual domains (e.g. hydrology for the water cycle (Thompson 1999); ecology for ecological impacts of water pollution (Abel 1996)). Given the complexity of the interactions and the centrality of humans in the water environment, more recently interdisciplinary approaches have come to the fore (e.g. Ferrier & Jenkins 2010; Renaud & Kuenzer 2014). The latest of such approaches is what we define in this book as ecosystem services-based approaches. These encompass a range of ways of understanding, assessing, and managing ecosystems at which core is the notion of ecosystem services, understood as the benefits that humans obtain from ecosystems.

Understanding water ecosystem services requires both an elucidation of the interrelationships between hydrology, landscapes and ecology, and a contextualization of how water influences human livelihoods and wellbeing and how ecosystems themselves are affected by human activities. Ecosystem services-based approaches, as defined in this book, aim to understand these complex relationships to support more efficient and sustainable decision-making. Society needs to recognize the requirement to balance and manage the benefits derived from water resources, rather than simply managing the resource itself (United Nations Environmental Programme 2009). These benefits come from the realization of a whole range of provisioning, regulating, and cultural services provided by ecosystems. This way of interpreting water systems and water resources represents a change from traditional sectoral control policies and approaches to delivering an integrated view of natural resource management.

In this book the editors and authors took up the challenge, ten years after the publication of the Millennium Ecosystem Assessment (2005), of reflecting on what has been achieved, what lessons have been learnt, and how to improve the application of ecosystem services-based approaches for managing water ecosystems in the future. By proposing a structured definition of ecosystem services-based approaches (in Martin-Ortega et al.) and exploring the forefront of their application at the conceptual level and through a series of national and regional case studies from across the world, the authors have completed a comprehensive vision of the current knowledge and challenges of applying ecosystem services-based approaches to address water challenges.

In this concluding chapter we reflect upon the key messages that have emerged from the discussions contained in this book and on the way forward. We organize this discussion around the four parts in which the book is structured.

20.1 HOW CAN ECOSYSTEM SERVICESBASED APPROACHES HELP ADDRESS MAJOR GLOBAL CHALLENGES?

Capon et al., Febria et al., and Salman and Martinez explore how ecosystem services-based approaches have been, and can be, applied to address three of the critical global challenges that humanity currently face, namely climate change, biodiversity loss, and meeting the growing population's food and energy demands (notably in the developing world).

Just as the proof of the pudding is in the eating, so it is in science that the testing of hypotheses is at the core of the development of theory. As set out in this book’s Introduction (Chapter 1), most ecological theory has been developed outside Australasia; this biogeographical region thus provides an excellent case study for investigating whether the generally accepted hypotheses from ecological theory really are applicable in a different context. Furthermore, many excellent observational studies have been carried out in Australasia; the history of invasions in that region can be viewed as a series of experimental tests of ecological theory. Indeed, the Australian native flora and fauna was so different from that of the rest of the world, that Darwin (before he became an atheist) asked himself in his diaries whether there had been two Creators instead of one (see Chapter 1).

In Chapter 1, we explained the reasoning behind the selection of the hypotheses that are reviewed in the main chapters in this book. At the beginning of this project all the authors were asked to formulate hypotheses, to give their feedback on the hypotheses, the way they were formulated and whether the hypotheses could be assessed by them; as a result of this collective effort authors could agree on assessing this present set of hypotheses. The hypotheses are derived from general ecological theory (see Chapter 1) but the material at hand cannot be viewed simply as ‘data’ such that the hypotheses can be tested as a part of statistical inference. This type of inference is familiar and easy for most scientists to address, and that is where most authors of this book clearly felt more secure than when they had to arrive at judgements from observations or from historical reconstructions. The overall conclusions, based on a meta-analysis of the assessment/testing of the 11 hypotheses by the authors of the chapters on species that moved into or out of Australasia as presented in the following tables, are clear: after weighing up the evidence many of the authors of chapters in this book concluded that there was no support for the individual hypothesis, or that they had to reject the hypothesis.