There is evidence of significantly reduced abundance of populations of flora and fauna attributed to wetland loss and pollution impacts, particularly eutrophication, chemical and metal toxicity, and the hazards of plastic and other wastes (WWF 2016). Although wetlands have the capacity to filter and improve water quality, continuous breaking down of organic matter and other nutrients can lead to a tipping point in water quality, beyond which a wetland can no longer regenerate itself, with species assemblages potentially changing markedly.
Fragmentation of rivers through dam building and water diversion, with resultant wetland habitat losses and degradation, has a significant impact on fish populations, especially migratory and endemic fish species. Fish populations are also being overexploited for food. Amphibian species are experiencing dramatic declines through habitat loss, invasive species, disease and pollution, followed by climate change (WWF 2016) (see Figure 9.15). Reptiles and many bird species are deeply affected by loss of wetlands, while aquatic mammals such as otters also suffer local extinctions from habitat loss and overexploitation.
The Living Planet Index (LPI) measures population abundance trends of 881 freshwater species monitored worldwide across 3,324 different populations (see chapter 6). Recent analyses indicate an 81 per cent decline in LPI in freshwater ecosystems between 1970 and 2012, the highest of any habitat type monitored using this index (WWF 2016). The LPI declined by 41 per cent over this period for migratory fish, based on measurement of over 162 fish species (Figure 9.16). Some improvement is evident from 2008 onwards, in response to removing weirs, installing fish ladders, and improving the up- and downstream passage of migratory fish in many places. Migratory species of birds and mammal populations in certain managed wetlands are also starting to recover, in response to habitat conservation and restocking. In contrast, the decline in amphibian and invertebrate wetland species, including insects, is much higher (WWF 2016).