greatly simplified? The concept of biodiversity opens
up a vast array of questions about the functional
role of diversity in the ecological functioning of
ecosystems, such as responses to perturbations or
biological productivity. The massive loss of species,
communities and ecosystems, as a consequence of
landscape transformation and water management,
makes it urgent to investigate the role of aquatic
species in ecosystems. At present, there are few data
with which to examine such questions, and even
fewer for African freshwater systems. However, the
re-examination of the pool of data already available is
a unique opportunity to discuss some results that may
be relevant for that purpose.
Of course, the function of biodiversity in ecosystems
cannot be clarified by examination of only one
group of animals, such as fish. The whole biota
should theoretically be considered, and fish should
be treated as part of the ecosystem, even while
focusing on their particular ecological role. In the
past, fishery biologists, particularly those working in
tropical countries, traditionally have tended to consider
fish in isolation, as a natural renewable resource, rather
than as integral components of the aquatic ecosystem,
interacting with other biotic and abiotic components of
the whole system. This attitude led to major ecological
disasters, such as he introduction of Lates nilotieus
in the Lake Victoria, and its dramatic consequences
on the native rich endemic cichlid fauna. A better
understanding of the role of fish diversity in the
functioning of aquatic ecosystems should therefore be
a prerequisite before further manipulation of African
inland waters is undertaken (Lévêque, in press).
This paper does not provide new field data, but is
an attempt to use the already published information in
order to discuss some of the questions arising from the
biodiversity concept. Given the assumed importance
of biological diversity in ecosystem functioning, some
major ecological questions have been identified:
- what is the linkage between species diversity and
ecosystem function: is species composition responsible
for particular ecological characteristics and processes
of the system under study, when compared with other
systems? Another related question could be: is the
redundancy in function among species useful and
necessary for ecosystems or is there an overriding
influence of certain species in controlling both the
structure and function of ecosystems (dominant species
and keystone species) (di Castri and Younès, 1990a
and b; Lawton and Brown, 1993; Paine, 1969);
- how is system stability and resistance affected by
species diversity, and to what extent could the integrity
and sustainability of ecosystems be maintained in
spite of species deletions resulting from degradation of
environmental conditions (Solbrig, 199 l)? The ability
to resist external episodic extreme perturbations could
depend on system heterogeneity and species diversity