Longitudinal zonation of lotic insects in the Bandama River system (Ivory Coast)

The longitudinal zonation of three lotic insect groups, viz. Hydropsychidae (Trichoptera), Philopotamidae (Trichoptera), and the Simulium damnosum complex (Diptera) in the Bandama River system (West Africa) is anomalous when compared to the patterns known from other geographical areas: In general from the highest to the lowest riffles downstream there appears to be no clear species replacement. Instead, there is a steady increase of species numbers due to the occurrence of additional species. These distributions are related to the gradient in the length of the period without flow from the north (temporary streams) to the south (permanent running waters), the consequent lack of well-defined sources and the relatively gentle slopes of the channels.