Sets out the aims and objectives of the book, and the needit will fill. The scope is described interms of topics included, geograpic coverage, literature consulted and cited (mainly 2005-2018), and other sources of reference (e.g. the IUCN Red List).

Freshwater biodiversity is threatened by growing human consumption and contamination of fresh water - a globally scarce resource. As human populations increase, the quality and quantity available for freshwater biodiversity declines.The result is a tragedy of the freshwater commons with increasing competition among groups of humans – evident from the hydropolitics of transboundary rivers - and between humans and nature.Humans may even be approaching the planetary boundary for freshwater use.Pollution and contamination are widespread, with emerging threats from microplastics and pharmaceuticals.Dams, drainage-basin disturbance, climate change, alien species, and overexploitation of aquatic animals pose additional threats.Their synergistic effects are evident from a global analysis of rivers: both biodiversity and human water security are at risk in many parts of the world while, in others, investments in infrastructure have enhanced water security although biodiversity remains under threat. Everywhere on Earth where there are substantial human populations, freshwater biodiversity is threatened.In many of these places, human water security is at risk also.

Inland waters and their biodiversity are a valuable resource.They are a source of fresh water, helping to purify it, and provide habitat for organisms (e.g. fishes) that may be eaten or used by humans. To improve the condition of fresh waters globally, it is imperative to link biodiversity conservation to human well-being. The concept of ecosystem services - the benefits humans derive from ecosystems - offers a means to make this link explicit, resolving the conflict between human water use and biodiversity protection. Ecosystem services thus serve as a proxy for biodiversity, assuming that maintaining the former will serve to protect the latter, representing a win-win conservation solution. While relevant for fisheries (a provisioning service), the substitution may be less applicable to supporting services that depend upon maintaining ecological functioning, not maximizing final services for humans. While valuation of biodiversity (and its subsequent monetization) is problematic, payment for ecosystem (or watershed) services can incentivize land-owners to protect sources of clean water for downstream users.

Anthropogenic alteration of river flows, lake levels, and the duration and extent of wetland inundation – collectively, flow regulation – degrades habitats, with manifold direct and indirect effects on populations, community structure and food webs. These can arise from an overall insufficiency of water, or changes in flow that result in there being too much or too little water at particular times.Fluxes of sediments and nutrients are also affected by trapping in dams, while water released downstream may have altered temperature, flow magnitude or periodicity, and erosive power.To mitigate such effects in rivers and associated wetlands, it is essential to allocate sufficient water as an environmental flow (e-flow) in order to sustain native biodiversity and ecosystem functioning.There is general consensus that this allocation should involve sufficient water to mimic the natural flow regime.Obstruction of movement of materials and migrations by animals (mainly fishes) by dams will require different and innovative solutions, and the proliferation of hydropower dams on large rivers (Amazon, Mekong) will have serious consequences for ecology and fishery yields.

Fresh waters are not amenable to protection by fortress conservation, since entire drainage basins can seldom be set aside. Site selection by conservation modelling has shortcomings because of the directional connectivity of river ecosystems. In the many cases where habitat degradation is the primary threat to freshwater biodiversity, restoration measures (that include riparia) may ameliorate matters, allowing species persistence or even recovery. However, rehabilitation is often the best that can be achieved.Mitigation of the barrier effects of dams by fishways have limited effectiveness, but complete removal of dams is an effective means of restoring connectivity.Ex situ (captive breeding, reintroduction, translocation) and in situ conservation measures – sometimes in combination – have been applied to a variety of freshwater animals, using new techniques such as eDNA for monitoring. Various international networks have been established recently to facilitate collaboration on conservation and improve data collection and sharing. There are thus reasons to be hopeful – rather than optimistic – about the prospects for preserving freshwater biodiversity in a warming world.

The biosphere is undergoing an epidemic of human-caused extinctions. They greatly exceed background extinction rates, overstepping planetary boundaries for marine and terrestrial biodiversity loss.This destruction of wildlife abundance and reduction in range occupancy has been termed ‘the great thinning’.Multiple lines of evidence (e.g. numbers of threatened species on the IUCN Red List, the Living Planet Index) reveal that endangerment of freshwater biodiversity is greater than on land or in the sea: charismatic species such as the Yangtze River dolphin have become extinct; large fishes, amphibians and pearly mussels (Unionidae) are also at particular risk.Many Red List species classified as ‘Data Deficient’ may well be endangered.High levels of local endemism and species turn-over among freshwater bodies increases global biodiversity, but means they are not substitutable in terms of their species complements.Some fresh waters (e.g. ancient lakes such as Tanganyika) are hyper-diverse with many endemics.In in the case of Asian peatswamps, further degradation would result in species losses, a significant influence on the global carbon balance and on climatic warming.

Flow regulation and water abstraction from rivers have devastating effects on receiving lakes. The most egregious example is the Aral ‘Sea’: from the 1960s, water for irrigation was diverted from influent rivers; 40 years later, the Aral had dwindled to tiny remnants - fisheries had long since collapsed.Shrinkage of lakes and wetlands across central Asia demonstrate the consequences of expanding irrigation, and highlight conflicts among human water users. Lake Chad in Africa has virtually disappeared as a result of water mismanagement, as has Lake Poopó in Bolivia; Yangtze floodplain lakes have diminished also. All such cases have had serious consequences for migratory waterbirds.In a few instances (Mono Lake in the US, the Mesopotamian marshes), the damage caused can be partially reversed by restoring water supplies, but the causes and drivers of ecosystem degradation are particular to each lake.Even the world’s largest water body, Lake Baikal, is subject to anthropogenic insult.Here, as is typical of lakes globally, a variety of factors – most often eutrophication, land-use change and alien species – contribute to habitat degradation and declines of endemic species.

Non-native, exotic or introduced species fall into the category of aliens, whereas an invasive species is an alien that gives rise to ecological, economic, health or other concerns as a result of its establishment and spread, or has the potential to do so.Their effects include predation, competition and displacement, or hybridization with natives, as well as the transmission of parasites or pathogens.In cases where aliens are ecological engineers, the ramifications of their establishment are such that food-web architecture is disrupted, causing shifts in ecosystem structure and function.Predators (often piscivores) can cause marked changes in lakes (such as Victoria), but filter-feeing bivalves are also nuisance species.Fishes (often deliberately stocked), molluscs, crayfishes and other crustaceans, as well as aquatic macrophytes are frequently invasive, but aliens include a broad array of taxa.Both lakes and rivers in almost all continents are affected, especially those subject to human modification or with compromised water quality.The outcome of such invasions is replacement of natives, and on-going biotic homogenization of a formerly diverse global biota.

Overexploitation is a major threat to freshwater biodiversity.It chiefly affects vertebrates, mainly fishes, as well as reptiles, amphibians and a few mammals and birds, but some invertebrates are also subject to human depredation. Most capture is for food, but some species are taken for medicinal reasons, their skins, or for the aquarium trade. Overfishing became evident in Europe over 1000 years ago, when reductions in mean size and abundance of target species became evident. Size reduction in overexploited stocks is typical of non-fish also: this ‘great shrinking’ is a fingerprint of Anthropocene threats to biodiversity, reflecting both a reduction in the average size of individual species and in the mean body size of species making up assemblages.It results in ‘fishing down’ of food chains as smaller and smaller species are exploited in succession, and is evident in large rivers (Amazon, Yangtze, Ganges and Mekong), where it especially affects migratory species, and lakes (Malawi, Victoria).Statistics on freshwater capture yields are generally underestimates.Subsistence fishing contributes much to human welfare, but inadequate data limits fishery management options.

The hydrological cycle is driven by warmth from the sun, and thus climate change will influence the availability of surface water, the frequency of extreme events, and the extent and duration of aquatic habitat. In addition, most freshwater animals are ectothermic, so climate change will affect habitat quality: will it be too warm? Will species be able to shift their distributions so they can remain within their physiological tolerances?Or, will adaptation be sufficiently rapid to cope with warming?The footprint of climate change is evident after only 1°C of average atmospheric warming: almost 75% of all ecological processes investigated in fresh waters have been affected, with seasonality and distribution of many taxa shifting.Lakes are sentinel ecosystems for climate change because stratification and internal dynamics are affected by temperature, and warming has affected productivity in both Lakes Baikal and Tanganyika. Climate warming will likely favour the spread of alien species, and amplify other anthropogenic threats to biodiversity. The consequences will be most apparent in water bodies already subject to eutrophication, poor water quality, or overexploitation.