Data on the condition and trends of freshwater species are for the most part poor at the global level, although some countries (e.g., Australia, Canada, New Zealand, South Africa, and the United States) have better inventories and indicators of change of freshwater species. Much of the problem originates from the fact that large numbers of species have never been catalogued and baselines on population status rarely exist, with the exception of a few highly threatened species (e.g., river dolphins) or species of commercial value (e.g., Pacific salmon in the United States).
The leading global effort to monitor the conservation status of species, the World Conservation Union (IUCN) Red List, has limited coverage of freshwater species, although a large effort is ongoing to fill this gap. Because of its harmonized category and criteria classification (i.e., all contributing experts follow the same methodology and guidelines), the IUCN Red List is the best source of information, at the global level, on the conservation status of plants and animals. This system is designed to determine the relative risk of extinction, with the main purpose of cataloguing and highlighting those taxa that are facing a higher risk of extinction globally (i.e., those listed as Critically Endangered, Endangered, and Vulnerable).
The 2006 Red List highlighted that freshwater species have suffered some of the most marked declines. For instance, of the 252 endemic freshwater Mediterranean fish species, 56% are threatened with extinction, and seven species are now extinct. This represents the highest proportion of imperiled species of any regional freshwater fish assessment that IUCN has conducted so far. Similarly, in East Africa, one in four freshwater fish is threatened with extinction. Odonates, another taxonomic group assessed by IUCN, also show high levels of imperilment, with almost one-third of the 564 species assessed being listed as threatened.
In 2004, IUCN completed the first global assessment of more than 5500 amphibian species, which was updated in 2006 to include 5918 species. This assessment considerably improved our overall knowledge of the condition of freshwater species, though its scope and representativeness are limited by lack of information, with 107 species still listed as Data Deficient and therefore unassigned a threat category. The Global Amphibian Assessment serves to reinforce the reality of the imperiled status of freshwater species, with close to a quarter of all assessed species listed as threatened, 34 as extinct, and as many as 165 species described as probably extinct. Overall, 43% of all amphibian species are declining in population, indicating that the number of threatened species can be expected to rise in the future.
Even large freshwater mammals are at increasing risk. For instance, the common hippopotamus, which until recently was not thought to be endangered, was listed in 2006 as threatened because of drastic and rapid declines in its population figures, with recorded reductions of up to 95% in the populations of the Democratic Republic of Congo, because of illegal hunting for meat and ivory. Overall, 41 species of freshwater mammals, including many otter species, freshwater dolphins, two freshwater feline species, as well as freshwater ungulates and rodents are threatened with extinction.
Data on freshwater reptiles, namely freshwater turtles and crocodilians (i.e., crocodiles, caimans, and gharials) also show declining trends. According to the IUCN/SSC (Species Survival Commission) Tortoise and Freshwater Turtle Specialist Group and the Asian Turtle Trade Working Group, of the 90 species of Asian freshwater turtles and tortoises, 74% are considered threatened, including 18 critically endangered species, and 1 that is already extinct: the Yunnan box turtle. The number of critically endangered freshwater turtles has more than doubled since the late 1990s. Much of the threat has come from overexploitation and illegal trade in Asia. The status of crocodilians presents a similar pattern, particularly in Asia. Of the 17 freshwater-restricted crocodilian species, as of 2007, 4 are listed by IUCN as critically endangered (3 of which are in Asia), 3 as endangered, and 3 as vulnerable. The most critically endangered is the Chinese alligator. The major threats to crocodilians worldwide are habitat loss and degradation caused by pollution, drainage and conversion of wetlands, deforestation, and overexploitation.
While information on freshwater plants and invertebrates are not readily available to portray population trends, available data give insight into the condition of freshwater ecosystems and species. In terms of freshwater plants, while many macrophytic species are probably not threatened at a global or continental scale, many bryophytes with restricted distributions are rare and threatened. In the United States, one of the few countries to assess more comprehensively the conservation status of freshwater molluscs and crustaceans, The Nature Conservancy has assessed that one-half of the known crayfish species and two-thirds of freshwater molluscs are at risk of extinction, with severe declines in their populations in recent years. Furthermore, of the freshwater molluscs, at least 1 in 10 is likely to have already become extinct (Master et al., 1998).
While the Red List focuses only on threatened species and therefore does not look at population trends of nonthreatened species, it does provide a good measure of progress in attenuating species loss. Other measures of the change in vertebrate species populations, such as WWF's Living Planet Index (LPI), show a similar downward trend.
As these indices and examples show, freshwater species are in serious decline all over the world. However, available data and information are predominantly from temperate and developed regions. Some progress is being made to collect and compile information elsewhere, but progress is slow and resources needed are high, particularly in developing countries where capacity is limited.
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