Scientists' warning to humanity on the freshwater biodiversity crisis

Abstract

Freshwater ecosystems provide irreplaceable services for both nature and society. The quality and quantity of freshwater affect biogeochemical processes and ecological dynamics that determine biodiversity, ecosystem productivity, and human health and welfare at local, regional and global scales. Freshwater ecosystems and their associated riparian habitats are amongst the most biologically diverse on Earth, and have inestimable economic, health, cultural, scientific and educational values. Yet human impacts to lakes, rivers, streams, wetlands and groundwater are dramatically reducing biodiversity and robbing critical natural resources and services from current and future generations. Freshwater biodiversity is declining rapidly on every continent and in every major river basin on Earth, and this degradation is occurring more rapidly than in terrestrial ecosystems. Currently, about one third of all global freshwater discharges pass through human agricultural, industrial or urban infrastructure. About one fifth of the Earth's arable land is now already equipped for irrigation, including all the most productive lands, and this proportion is projected to surpass one third by midcentury to feed the rapidly expanding populations of humans and commensal species, especially poultry and ruminant livestock. Less than one fifth of the world's preindustrial freshwater wetlands remain, and this proportion is projected to decline to under one tenth by midcentury, with imminent threats from water transfer megaprojects in Brazil and India, and coastal wetland drainage megaprojects in China. The Living Planet Index for freshwater vertebrate populations has declined to just one third that of 1970, and is projected to sink below one fifth by midcentury. A linear model of global economic expansion yields the chilling prediction that human utilization of critical freshwater resources will approach one half of the Earth's total capacity by midcentury. Although the magnitude and growth of the human freshwater footprint are greater than is generally understood by policy makers, the news media, or the general public, slowing and reversing dramatic losses of freshwater species and ecosystems is still possible. We recommend a set of urgent policy actions that promote clean water, conserve watershed services, and restore freshwater ecosystems and their vital services. Effective management of freshwater resources and ecosystems must be ranked amongst humanity's highest priorities.

Keywords: Aquatic biodiversity; Conservation; Ecosystem services; Freshwater; Groundwater; Wetlands.

Fig. 1

The expanding human freshwater footprint. a Trends in global freshwater resources, assessed as Global Freshwater Withdrawals (blue) and Global Area Equipped for Irrigation (gray). Trend in terrestrial resources assessed as Human Appropriation of Terrestrial Net Primary Productivity (green). Note freshwater utilization is now a stronger limiting factor on human populations than terrestrial utilization. b Trends in global biodiversity from a 1970 baseline. Living Planet Index for freshwater vertebrate populations (blue), for terrestrial vertebrate populations (green), and Global Wetland Loss (gray). Global Wetland Loss model fitted to data from 1700 to 2000 and plotted from 1970 to 2050 for compatibility with Living Planet Index datasets. Note rate of biodiversity decline is faster in freshwater than terrestrial systems, and occurring alongside reductions in Global Wetland Loss. The linear model projects global wetlands to disappear entirely by midcentury. Historical estimates as solid curves with 95% prediction intervals. Data as cumulative proportions of global totals. See footnote for methods. Methods for model specifications used to generate future projections (dashed curves) and literature references. Global Freshwater Withdrawals for human use as percentage of total annual river discharge to the sea (c. 36 000 km³) (Milliman and Farnsworth ; van Vliet et al. 2013). Historical estimates from (Shiklomanov and Rodda ; Scanlon et al. 2007) adjusted to match the revised human-withdrawal estimate for 2010 of 10 688 km³ year¹ (Jaramillo and Destouni 2015). Irrigation as global area equipped for irrigation or land ready for cultivation as a percentage of total global arable land (c. 14 million km²). Historical estimates from (Scanlon et al. ; Ellis et al. ; Siebert et al. 2015). Historical estimates of human share of terrestrial net primary productivity from (Krausmann et al. 2013). Population trends as estimated from the Living Planet Index (LP1 from Loh et al. ; WWF 2016) evaluated against a 1970 baseline. LPI measures trends in the geometric mean of population abundances. LPI-FW estimates from 3324 monitored freshwater populations representing 881 species (darker curve with circles), and LPI-T for 4658 terrestrial populations of 1678 species (lighter curve with triangles). Note freshwater populations declined an estimated 81% from 1970 to 2012. Wetlands as percentage of pre-industrial total global wetland area at 1700. Historical estimates from (Davidson 2014)

Fig. 2

Examples of critically endangered or recently extinct (†) freshwater megafauna (≥ 30 kg). Freshwater animals with large adult body sizes have higher extinction risk due to complex habitat requirements and slow life‐history strategies (e.g., long life span and late maturity). The main conservation threats to these species come from human activities, including overharvesting, pollution, introducing invasive species, and habitat modifications (e.g., water diversions, land-use changes, climate change). The species illustrated roughly represent the proportions of endangered freshwater vertebrates, with greater threats to fishes (a–c) than to tetrapods (c–f), and to Eurasian (a–c, f) than non-Eurasian (d–e) species Image sources: Acipenser naccarii, †Psephurus gladius, Andrias davidianus from Wikimedia Commons; Crocodylus intermedius and Podocnemis lewyana from Threatened Reptiles of Colombia (http://reporte.humboldt.org.co/biodiversidad/); Pangasianodon gigas from PGNGuru (http://www.pngguru.com/), †Lipotes vexillifer from PNGGuru (http://www.pngguru.com/)