Fish Community Structure and Food Web Ecology in the Lower Okavango Delta, Botswana

Alteration of the flow regime may affect species distributions, mechanisms of community assembly, and patterns of energy flow within and between fluvial ecosystems. Changes in the periodicity and predictability of flows may reduce a river’s capacity to produce ecosystem services affecting human welfare. This dissertation asseses how periodic wetting and drying influence fish assemblage structure and food web dynamics in rivers of the lower Okavango Delta, Botswana. This information should aid in the development of appropriate management interventions that balance biodiversity conservation with sustainable use of ecosystem services. During low-flow periods, ephemeral aquatic habitats had higher α diversity and higher species turnover compared to permanent habitats, indicative of fish aggregation and random changes in species abundance at a local scale as habitat patches became smaller and more isolated. Assemblage nestedness was higher in permanent habitats, suggesting fish had dispersed non-randomly among patches in that habitat type as water levels fell. During high-water periods, fish assemblages were homogenized both at local and regional scales, suggesting a greater influence of mass effects. Controlling for the influence of season and habitat type on species abundance showed most negative species co-occurrences involving predators and potential prey, suggesting that predation mortality or predator avoidance could be a primary driver of species segregation. Most aggregations involved intra-guild interactions among non-predatory species, suggesting species affinity to similar habitats. The biomass of most fish populations was supported by food chains originating from either seston or C4 grasses. The degree of dependence on a single source was positively related to trophic position during the dry season in the Boro River (a highly ephemeral river), suggesting that species higher in the food web assimilated material disproportionately from one source during this period. During the same period, the degree of dependence on a single source was inversely related to trophic position in Boteti River (a more permanent river), suggesting that fish foraging habits shift in response to seasonal patterns of connectivity and habitat availability. I infer from these findings that maintenance of a relatively natural flow regime will be necessary for conserving aquatic ecosystem structure and function in this system