This chapter illustrates the importance of ancient lakes as natural laboratories for the
study of community structure. The major problem in this field, assessing the relative
importance of competition and predation, can be success-fully attacked by the use of human
perturbation as a natural experiment. Using East African Lake Victoria as an example, the
effects are reviewed of eutrophication; overfishing and invasive aliens on several communities
within this complex ecosystem. The long-term study by the authors' research team on the
zooplanktivorous fish community of Lake Victoria provided the opportunity to use the explosive
population increase of an introduced predator, the Nile perch Lates niloticus, as a
natural experiment. Changes in population densities, habitat choice and life histories of the
zooplanktivores suggest a shift from a community structured by competition to one structured by
predation. Although the complexity of their foodwebs may complicate the separation of the
structuring factors, it may be concluded that ancient lakes, which are almost inevitably
exposed to human perturbation, offer unique opportunities to study community structure.