The need for a better understanding of the role of biodiversity in the functioning of aquatic ecosystems
has been raised recently by several authors. If the question is not entirely new, its formulation is. It is
therefore the opportunity to re-examine the pool of data already available in order to discuss some results
that may be relevant for that purpose, while some major ecological questions have been identified. The
top-down view argues that the effects of fish predation cascade down the trophic chain and are responsible
for controlling the state of the entire ecosystem. The influence of fish predation on prey assemblages (prey
selection, ontogenic shift) has been fairly well documented by various studies of African fish. They are
also challenging subjects for studies of resource partitioning, a well documented example being given by
zooplanktivores in Lake Turkana. The principle of cascading trophic interactions has been documented in
Lake Victoria, after the introduction of a top predator, Lutes niloticus. Another striking example is that
of Lake Nakuru where the introduction of a tilapiine resulted in a substantial increase of bird diversity,
by the extension of the food chain to fish-eating birds. This introduced tilapiine may be considered as a
keystone species, but this concept is questioned. A possible origin of fish biodiversity is adaptive radiation,
that is to say the differentiation of species in morphology and resource use through competition. The
cichlid flocks of the African Great Lakes where hundreds of rather similar species coexist, provide one of
the best known examples of adaptive radiation. The question of niche overlap and functional redundancy
has been raised and various studies have been conducted in the East African Lakes. The role of rare
species has also been questioned. They may provide an insurance of ecosystem stability, which means
that the more stable ecosystems in terms of key functions are those richest in species. In fact, in many
large Nilo-Sudan river basins, there are apparently several groups of species which are able to replace the
current assemblage when it experiences new ecological situations. Finally, the question has been raised as
to what extent pattems of biodiversity are important for ecosystem productivity. The comparison of data
for four African lakes shows no relationship between fish diversity and fishery production, and species
richness does not appear to be a major determinant of basic production trends. At present we have only
anecdotal, sometimes controversial, evidence of the role of fish diversity on the functioning of African
freshwater ecosystems. We need to improve this knowledge on the importance of biodiversity in order to
convince decision-makers that fish biodiversity needs protection