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Dudgeon 2014 - law
Philosophy and Current Issue (CTU552)
Multimedia University
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Threats to Freshwater Biodiversity
in a Changing World 30
David Dudgeon
Contents
Definition........................................................................................ 243 Freshwater Ecosystems: Scarcity, Richness, and Vulnerability............................... 244 What Are the Threat Factors?.................................................................. 246 What Are the Global Patterns of Threat?...................................................... 247 What Is Threatened? What Has Been Lost?................................................... 248 What of Climate Change and the Future?...................................................... 250 What Now?...................................................................................... 251 References....................................................................................... 252 Additional Recommended Reading......................................................... 253
Keywords Freshwater ecosystems • Scarcity, Richness and vulnerability • Threat factors • Global patterns of threat
Definition
Fresh waters cover <1 % of the Earth’s surface, yet host around 10 % of known animal species. This biodiversity is threatened by a combination of factors, exac- erbated by the position of rivers and lakes as landscape “receivers.” Climate change will intensify competition for water with humans, causing further declines in freshwater biodiversity.
D. Dudgeon The University of Hong Kong, Hong Kong, China e-mail: ddudgeon@hku
Bill Freedman (ed.), Global Environmental Change, DOI 10/978-94-007-5784-4_108,
Springer Science+Business Media Dordrecht 2014243
Freshwater Ecosystems: Scarcity, Richness, and Vulnerability
Much of the Earth’s surface is covered with water, but only a small fraction – around 2 % – is fresh (i., its salt content is less than 1 part per thousand). However, around two-thirds of the fresh water is frozen solid at the poles (especially Antarctica), and the other third is deep underground. Thus only a tiny portion (0 %) of fresh water remains on the surface in liquid form; it is all that is available for aquatic organisms to live in. To put it another way, freshwater ecosystems (sometimes termed inland waters), constituting no more than 0 % of total global water resources, occupy less than 1 % of the Earth’s surface area. Most of this water resides in lakes, and fully 22 % of it occurs in Lake Baikal, Siberia. Rivers contain a mere 0 % of surface fresh water (or 0 % of all water) equivalent, at any moment in time, to a volume of around 2,120 km 3. The tiny fraction in rivers is the source of most water used by humans and serves as habitat for many organisms found nowhere else. The global scarcity of fresh water has profound implications for nature and for humans. With respect to the former, the habitable area (or volume) – literally, the living space – for freshwater organisms is small. One might therefore expect that freshwater ecosystems would support few types of plants and animals, but, on the contrary, freshwater biodiversity (¼biological variety: used herein to refer mainly to numbers of animal species) is remarkably high. A few statistics make this clear (Dudgeon et al. 2006 ; Balian et al. 2008 ; Strayer and Dudgeon 2010 ). Approxi- mately 125,000 freshwater species have been described and named by scientists; they represent 9 % of known animal species on Earth, including around one-third (>18,000 species) of all vertebrates. Most of these (one quarter of all vertebrates) are fishes, and the rest comprise the entire global complement of crocodilians, virtually all of the amphibians, and most of the turtles. Considering just the bony (actinopterygian) fishes, species richness in the oceans and freshwater is similar (15,000 species each, with none in common) despite the greater productivity of marine environments. This remarkable richness is also disproportionate to the area occupied by freshwaters. The scarcity of fresh water has obvious and important implications for humans. A significant proportion of the global population (0 billion people) lacks ready access to drinking water, and perhaps 40 % (>2 billion) of people do not have adequate sanitation; child deaths resulting from contaminated water may be as high as 1 million annually – as many as 5,000 each day (WHO/UNICEF 2008 ). The demand for water has increased fourfold during the last 50 years, and the global population, which recently topped seven billion, is projected to reach nine billion by 2050 or thereabouts. Water for irrigation will be essential to feed these two billion additional people and improve the nutritional status of many others currently undernourished. Humans already appropriate 54 % of surface runoff and, although estimates of this proportion vary somewhat, increases in the foreseeable future could transgress planetary boundaries for
244 D. Dudgeon
What Are the Threat Factors?
The “perfect storm” is constituted by a number of separate but interacting elements:
- Pollution of all types caused by inorganic or organic substances from point (e., end of pipe) or diffuse (overland runoff or seepage) sources, often comprising complex mixtures, with direct consequences ranging from lethal through acute to chronic (crossref. needed). Impacts can be indirect if pollutants reduce habitat suitability; for instance, soil runoff clogs riverbed sediments used by spawning fish.
- Flow regulation used generally to encompass water abstraction for irrigation and other purposes; construction of large and small dams for water storage, flood control, or hydropower generation; long-distance transfers of water between drainages; and river channelization or canalization with associated dykes or levees that separate rivers from their flood plains. In extreme cases, a complex river corridor can be transformed into a massive, concreted drainage ditch. Dams and weirs are barriers to the movement of organisms and material within river networks, presenting a critical constraint for migratory fishes. Dams also degrade rivers by transforming the section upstream into an impoundment of standing water, while the flow downstream depends upon dam operations and may not resemble the original regime; sediment loads, oxygen content, and temperature of released water are likewise altered. Natural flow or inundation patterns – to which animals are adapted and upon which they depend – are modified, seasonal patterns of flow variability or water level fluctuations are reduced, and river dewatering or lake bed drying may even occur.
- Overexploitation impacts animals used for food, mainly fishes but also frogs, some reptiles, and a few crustaceans and molluscs. Overfishing typically results from high catch effort, with larger, more long-lived species (often predators) tending to decline first, whereupon the fishery shifts to smaller, fecund species with short life cycles. Migratory species are particularly vulnerable, since they are often caught during movements that take place prior to breeding, so diminishing the capacity for stock replenishment. Declines also result from use of damaging fishing gear (fine-mesh nets, electrical devices, explosives, or poisons) often adopted as methods of last resort after larger fishes have been depleted. Crocodiles and turtles have been hunted, close to extinction in some cases, for their hides or other body parts, and unionid or “pearly” mussels in the United States were exploited for their pearls and nacreous shells (used to manufacture buttons) during the late nineteenth and early twentieth century; some species have yet to recover.
- Drainage-basin alteration such as vegetation clearance affects the water balance within drainage basins and usually increases erosion. Changes in runoff quantity are accompanied by reduced quality (contaminants from farmland, towns, and cities) so degrading aquatic receivers. Clearance of riparian zones (along lake shores or river banks) and floodplains impacts semiaquatic animals (otters, herons, etc.) that live along water margins and amphibiotic species (frogs, dragonflies, etc.) that spend their adult phase on land.
246 D. Dudgeon
- Invasive species are nonnative to a particular region but have been introduced (accidentally or deliberately) by humans and become established. They are also known as introduced, exotic, or alien species, but the use of the term “invasive” denotes nonnative species that established themselves and spread at the expense of native species. Typically, invasives are effective competitors or efficient predators or possess an attribute lacking among members of the receiving community, but others cause damage by introducing parasites or diseases (including fungal chytridiomycosis that affects frogs). There is scant taxonomic constraint upon what makes a successful invasive. The category encompasses aquatic plants, snails, mussels, crayfish, mosquitoes, turtles, frogs, a few water- fowl, and many fishes (see the Global Invasive Species Database). The estab- lishment of large, predatory Nile perch (Lates niloticus) – categorized among the 100 “World’s Worst” invaders – in Lake Victoria, East Africa, and the conse- quential disappearance of >200 species of endemic cichlid fishes, is but one example of the potential for damage.
- Interactive effects among these five threat factors, and their multifarious com- ponents (including many not listed above), are pervasive since they can act simultaneously upon the same habitat. Indeed, the extent of drainage-basin alteration and pollution are often correlated. Habitat alteration may make ani- mals more vulnerable to pollutants or, conversely, sublethal effects of contam- inants may compromise their ability to adjust to changed conditions. New conditions may, in turn, facilitate establishment of invasives, and the greater the extent of habitat alteration, the less likely are native species to persist. In addition, pollution and habitat alteration limit the ability of fishes to withstand or recover from exploitation, and dams may prevent them for accessing breeding sites up- or downstream. In short, the five threat factors can combine to produce synergistic outcomes that can be difficult to predict and exceed the sum of their individual impacts.
What Are the Global Patterns of Threat?
A recent global study of river health (Vo ̈ro ̈smarty et al. 2010 ) addressed the relative intensity of anthropogenic threats to both human water security and biodiversity. The two analyses each combined 23 weighted threat factors or stressors (termed “drivers”) within four categories: drainage-basin alteration, pollutants, water-resource development (i., dams and flow regulation), and biotic threats such as overfishing. However, the weighting applied to each driver varied between the two analyses, since their impacts differ greatly depending on whether they are felt by humans or (say) river fishes. For example, building a dam could be beneficial for human water security, whereas the effects on river fishes are negative. Conversely, mercury tends to accumulate along food chains posing a danger to apex consumers (crossref.); it is thus a greater threat to humans than to most fresh water organisms. Despite these separate driver weightings, low levels of water human security and high endangerment of biodiversity were generally
30 Threats to Freshwater Biodiversity in a Changing World 247
Fig. 30.
A global geography of river threat, showing the patterns of spatial concordance of aggregate threat from 23 drivers (see text) to human water
security and freshwater biodiversity. Areas shaded
gray
have no appreciable river flow. Image from
riverthreat
30 Threats to Freshwater Biodiversity in a Changing World 249
groups (e., freshwater bivalves: 38 % classified as threatened by the IUCN) there are grounds for concern about species loss. Declines in freshwater species, including the first human-induced extinction of any species of cetacean (the Yangtze river dolphin, Lipotes vexillifer), are a reliable indicator of unsustainable use of freshwater by humans with consequences that have outpaced attempts at management (Dudgeon et al. 2006 ; Strayer and Dudgeon 2010 ). To this can be added a substantial extinction debt due to human actions that have reduced populations to levels from which they can no longer recover, as well as losses that occurred in the past that have been overlooked or forgotten. One example is the gradual disappearance, since medieval times, of beaver (Castor canadensis) from much of its former range in Europe. Another is American shad which supported a major commercial fishery along the western coast of the United States during the nineteenth century that has long since collapsed. Imperfect knowledge of past conditions in freshwater gives rise to “shifting baseline syn- drome” whereby we are deceived by the false impression that conditions in the immediate past reflect conditions in the intermediate and distant past, so that we underestimate the extent of human impacts (Humphries and Winemiller 2009 ). The shifting baseline reduces expectations of what species should be present in fresh- water, even in the case of economically important species, such as shad, soon after dams or other insults have eliminated them from particular rivers (Limburg and Waldman 2009 ).
What of Climate Change and the Future?
Climate change was not included in the list of threat factors given above nor is it taken into account in the global river threat analysis shown in Fig. 30. Impacts on freshwater will arise from rising temperatures and alterations in rainfall and increased frequency of extreme climatic events, as well as medium-term effects such as glacial melt. There is already evidence of warmer water temperatures, shorter periods of ice cover, and shifts in the geographic ranges and seasonality of freshwater animals in northern latitudes (reviewed by Heino et al. 2009 ). Climate change does not augur well for freshwater biodiversity in regions where the human footprint is pervasive, since this is where conflicts over water will be most intense and the outlook for biodiversity correspondingly bleak. Warmer temperatures will mean greater water use by plants (crops, pasture, and natural vegetation), correspondingly less runoff or percolation to sustain rivers and lakes, and more water abstraction for irrigation. Changes in temperature and/or flow and inundation patterns could cause shifts in the timing of breeding or migration by fishes, and even the disappearance of seasonal cues for such life-cycle events. Consequences for reptiles, such as turtles and crocodiles in which the sex ratio is determined by temperature, could be extremely serious. Ultimately, conditions in rivers and lakes may no longer be suitable for species that evolved there, and there will be limited opportunities for overland dispersal by aquatic animals to more suitable habitat.
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If “stationarity is dead” (because climate change is altering means and extremes of temperature, rainfall, and river flow), then we must accept that conditions in lakes and rivers will alter more quickly than their inhabitants will be able to adapt to them. Given that dispersal opportunities to new habitats are constrained for most freshwater animals, can and should we consider their assisted translocation to potentially suitable sites (Olden et al. 2011 ) where their long-term persistence would be more likely? There is an urgent need to address all of these issues, if we are to avoid becoming overseers of more dramatic declines and extinctions of freshwater species than witnessed thus far.
References
Balian EV, Le ́veˆque C, Segers H, Martens K (2008) The freshwater animal diversity assessment: an overview of the results. Hydrobiologia 595:627– Dudgeon D, Arthington AH, Gessner MO, Kawabata Z-I, Knowler DJ, Le ́veˆque C, Naiman RJ, Prieur-Richard A-H, Soto D, Stiassny MLJ, Sullivan CA (2006) Freshwater biodiversity: importance, threats, status and conservation challenges. Biol Rev 81:163– Heino J, Virkkala R, Toivonen H (2009) Climate change and freshwater biodiversity: detected patterns, future trends and adaptations in northern regions. Biol Rev 84:39– Humphries P, Winemiller KO (2009) Historical impacts on river fauna, shifting baselines and challenges for restoration. Bioscience 59:673– Limburg KE, Waldman JB (2009) Dramatic declines in North Atlantic diadromous fishes. Bioscience 59:955– Olden JD, Kennard M, Lawler JJ, Poff NL (2011) Challenges and opportunities in implementing managed relocation for conservation of freshwater species. Conserv Biol 25:40– Poff NL, Richter BD, Arthington AH, Bunn SE, Naiman RJ, Kendy E, Acreman M, Apse C, Bledsoe BP, Freeman M, Henriksen J, Jacobson RB, Kennen JG, Merritt DM, O’Keeffe JH, Olden JD, Rogers K, Tharme RE, Warner A (2010) The ecological limits of hydrologic alteration (ELOHA): a new framework for developing regional environmental flow standards. Freshw Biol 55:147– Rockstro ̈m J, Steffen W, Noone K, Persson A ̊ , Chapin FS III, Lambin E, Lenton TM, Scheffer M, Folke C, Schellnhuber H, Nykvist B, De Wit CA, Hughes T, van der Leeuw S, Rodhe H, So ̈rlin S, Snyder PK, Costanza R, Svedin U, Falkenmark M, Karlberg L, Corell RW, Fabry VJ, Hansen J, Walker BH, Liverman D, Richardson K, Crutzen P, Foley J (2009) Planetary boundaries: exploring the safe operating space for humanity. Ecol Soc 14:32, http://www. ecologyandsociety/vol14/iss2/art Strayer DL, Dudgeon D (2010) Freshwater biodiversity conservation: recent progress and future challenges. J North Am Benthol Soc 29:344–358, bioone/doi/abs/ 10/08-171. Vo ̈ro ̈smarty C, McIntyre PB, Gessner MO, Dudgeon D, Prusevich A, Green P, Glidden S, Bunn SE, Sullivan CA, Reidy Liermann C, Davies PM (2010) Global threats to human water security and river biodiversity. Nature 467:555–
252 D. Dudgeon
Additional Recommended Reading
Global Invasive Species Database. issg/database/welcome/ IUCN Red List. iucnredlist Rivers in Crisis. Mapping dual threats to water security for biodiversity and humans. http://www. riverthreat WHO/UNICEF (2008) Progress on drinking water and sanitation: special focus on sanitation. World Health Organization and United Nations Children’s Fund Joint Monitoring Programme for Water Supply and Sanitation. UNICEF/WHO, New York/Geneva. who/ water_sanitation_health/monitoring/jmp2008/en/index WWF (2010) Living Planet Index 2010. World Wide Fund for Nature, Gland. assets. org/downloads/lpr2010
30 Threats to Freshwater Biodiversity in a Changing World 253
Dudgeon 2014 - law
Course: Philosophy and Current Issue (CTU552)
University: Multimedia University
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