The IUCN Red list includes all known freshwater species known to the region from the selected taxonomic groups. However, there are limitations to the data, due mainly to incomplete knowledge of freshwater species. The following details should be noted:
The rate of species discoveries remains very high, and taxonomic uncertainties abound. Whilst some areas, particularly lakes, are very well studied and scientists have a firm grasp of the ecosystem and its inhabitants, other areas are poorly known and the true assemblages are unknown to date. Molluscs in particular are constantly undergoing taxanomic clarification, and these assessments can therefore only be based on the best knowledge availble at the time of the assessment. In addition, many species names, especially in the tropics, actually represent complexes of several species that have not yet been disaggregated. In the IUCN Red List, these are treated as single species, pending resolution of their taxonomic status. With every update the intention is to include all newly described, however this is not always possible due to time and funding constraints, access to literature, and not being able to keep up to date with the very latest descriptions.
Aquatic species are mapped to the river basin catchment area, as this is a more useful management unit. This represents some taxa better than others, for instance the terrestrial nature of Odonata means that they may not be restricted to the catchment areas of the rivers they are known to inhabit. For many restricted range species, even though they are only known from a very small area, their range will still be reflected as the minimum Hydro1K level 6 basin area, which may be an overestimation of its actual range. Additionally, many lacustrine species are restricted to certain depth levels, and particularly in the case of species found in the Great Lakes, the representation of the entire lake basin exaggerates the actual range of these species.
The information on the relative importance of different threatening processes to freshwater species is incomplete. In these assessments, we coded all threats that seemed to be having an important impact, but not the relative importance of such threats. For example, many species are known to have declined catastrophically in suitable habitats, but these are in most cases also subject to some sort of habitat loss. However, for these species, habitat loss appears at the moment to be a secondary threat, and the factors causing very rapid disappearance of populations, even in suitable habitats, appear to constitute the driving threats. Likewise, many species that have declined seriously because of over-harvesting are also subject to habitat loss, though over-harvesting is probably the dominant current threat.
A total of almost 20% of freshwater species within Africa were considered Data Deficient (DD). Because many DD species are likely to have small distributions or populations, or both, they are intrinsically likely to be threatened. Although the percentage of globally threatened or extinct freshwater species is already very high (approximately 22%), it is almost certainly an under-estimate of the real number. The data in the IUCN Red List, and the analyses resulting from it, therefore tend to under-estimate threat levels, probably very significantly. The results presented here are therefore the best estimates and predictions that can be made, based on incomplete information. Future updates of the data will almost certainly reveal higher levels of threat, and more serious declines.