An Introduction to Freshwater Taxa

Fishes

Fishes form the most important wetland product on a global scale providing the primary source of protein for nearly 1 billion people worldwide and food security for many more. It is estimated that in Africa inland fisheries land nearly 2.5 million tones per year, which accounts for nearly 25% of the world’s inland waters capture fisheries, providing essential nutrition for the poorest of communities and employment and income for many more. For the purposes of this assessment freshwater fishes are defined as those that spend all or a critical part of their life cycle in fresh waters. Those species entirely confined to brackish waters are also assessed. There are over 14,000 freshwater fish species in the world, and by 2006 only 15% of them had had their risk of extinction assessed using the IUCN Red List Categories and Criteria.

 

  

                                       © Ahmed Gheith

Molluscs

Freshwater molluscs are one of the most threatened groups of freshwater taxa in some regions of the world. They remain fairly unobtrusive, and are not normally considered as being charismatic creatures so rarely attract the attention of the popular media. This is unfortunate as they are essential to the maintenance of wetland ecosystems, primarily through their control of water quality and nutrient balance through filter-feeding and algal-grazing and, to a lesser degree, as a food source for predators including a number of fish species. There are an estimated 6,000 freshwater molluscs for which valid descriptions exist, in addition to a possible 10,000 undescribed species. Of these species, only a small number have had their conservation status assessed (around 13% of freshwater molluscs had been assessed for the IUCN Red List in 2006) and their value to wetland ecosystems is poorly appreciated. The impact of developments such as dams has not been adequately addressed and few are aware of the complex life histories of some groups such as unionid mussels that rely on the maintenance of migratory fish runs to carry their parasitic larvae to the river headwaters. For example, the construction of dams has been documented as playing a major role in the extinction of many of the North American mussels within the last 100 years. Many species are also restricted to microhabitats such as the riffles (areas of fast current velocity, shallow depth, and broken water surface) between pools and runs (areas of rapid non-turbulent flow). The introduction of alien species, wetland drainage and river channelisation, pollution, sedimentation and siltation also impact heavily on unionid mussels.       

      

    

 

                            © Ahmed Gheith

 

Odonates

Larvae of almost all of the known 5,680 species of the insect order Odonata (dragonflies and damselflies) are dependent on freshwater habitats. Although the habitat selection of adult dragonflies strongly depends on the terrestrial vegetation type, their larvae develop in water where they play a critical role in regard to water quality, nutrient cycling, and aquatic habitat structure. A full array of ecological types is represented within the group, which has thus been widely used as an indicator for wetland quality in Europe, Japan, the USA and Australia. Of these 5,680 species, less than 10% had had their risk of extinction assessed using the IUCN Red List Categories and Criteria by 2006. A baseline dataset is needed globally to facilitate development of similar long term monitoring schemes.

  

                                 © Ahmed Gheith

                

Crabs

There are an estimated 1,280 species of freshwater crab. Density estimates are highly variable, but they consistently show that crabs make up a very significant proportion of the invertebrate fauna in terms of overall biomass. The overwhelming importance of detritus in the diet of most species suggests that they are key shredders in many rivers. The detritus shredding guild, apparently almost completely absent from most tropical systems, may be taken up in a large part by crabs in African river systems in particular. This, combined with their general abundance and high biomass, makes them potentially very important to the dynamics of nutrient recycling in African rivers. Of the 100 species native to Africa, around 50% had had their risk of extinction assessed using the IUCN Red List Categories and Criteria by 2006, but many of these assessments were conducted prior to 1998 and were in need of revision.

                                © Ahmed Gheith

Aquatic Plants

Aquatic plants are the building blocks of wetland ecosystems, providing food, oxygen and habitats for many other species. They are also a hugely important natural resource providing direct benefits to human communities. Numerous aquatic plants are highly valued for their nutritious, medicinal, cultural, structural or biological properties. They are also key species in wetland ecosystem services, for example, water filtration and nutrient recycling. An aquatic plant is defined as a macrophyte whose photosynthetically active parts are permanently, or at least for several months each year, submerged in freshwater, or floating on the water’s surface (Cook 1974 & 1990). Using this definition there are nearly 6000 aquatic plant species worldwide (based on 1-2% of all vascular plants being aquatic and a conservative estimate of the total number of vascular plants of 287,655). There were only 36 species of freshwater aquatic plants on the 2006 IUCN Red List.

   

                               © Ahmed Gheith