Abundance and diversity of the aquatic beetles in a Mediterranean stream system (Northern Tunisia)

Summary. The understanding of community structuring factors is a fundamental issue in community ecology. We investigate the biodiversity of the water beetle in the Ichkeul Lake stream system located in Northern Tunisia, with emphasis on the effect of several environmental variables on their assemblage structure. This area is of a great importance since it was included in the list of preserved waterbodies by RAMSAR convention on wetlands, MAB (Man And Biosphere), UNESCO (United Nations Educational, Scientific and Cultural Organization). Consequently, the study of its biodiversity constitutes a required way to better conduct this protection aim. Special attention must be paid to species endemic to North Africa, as all of them require conservation measures. Sampling was carried out over the course of a year. Dissolved oxygen, conductivity, pH, temperature and turbidity were measured for each study site. Water permanence, velocity, aquatic vegetation and substrate type were considered. The investigated streams showed a spatial heterogeneity in water depth, water velocity, turbidity, mineralization and substrate type, but homogeneity in temperature and dissolved oxygen. A total of 68 coleopteran species were identified from the investigated area, belonging to different chorotypes and phenological categories. The coleopteran assemblage was assessed in terms of four community parameters: abundance, richness (S), diversity (Shannon–Wiener index: H′), and evenness (Pielou index: J′). Species composition was analyzed by multivariate analysis. Sites were arranged into two groups based on their species affinity by non-parametric multidimensional scaling (MDS) ordination on the basis of Bray–Curtis similarity at a similarity of 50%. Principal component analysis (PCA) highlighted the species response to the environmental variables. It revealed the distribution profile of sampled species; on the first axis, F1, a maximum number of variables are projected. Its positive pole is defined by the majority of species, with tolerance of high salinity. On its negative pole only three species are present. Its positive pole is defined by lentic phytophagous species, showing a preference for well vegetated waters. Its negative pole is marked by the species L. orientalis, associated with the majority of water parameters.