Returners and New Arrivals After the Crash: Intermediate Hosts and Global Invaders Dominate Gastropod Fauna of Lake Naivasha, Kenya

Aquatic alien species (AAS) have had a major impact on freshwater ecosystems, including Lake Naivasha in Kenya. Here, the ecosystem has undergone tremendous changes and multiple species introductions over the past 100 years, and molluscs have experienced a major decline in species diversity. The East African Rift Lakes have experienced a steady rise in lake levels since 2010. We investigated the impact of recent ecosystem changes on the lakes’ molluscs by determining the current mollusc diversity and its composition. We also reconstruct the history of faunal change and turnover over the last 100 years and discuss the future of molluscs in the lake and the implications in a One Health context. The combined effects of rising water levels and the decline of the crayfish Procambarus clarkii are likely to be responsible for the resurgence of Lake Naivasha’s mollusc fauna. The current fauna consists of three global invaders of American origin, one of which is new to East Africa (Pseudosuccinea columella) and another of which has only recently begun to spread (Pomacea canaliculata). A further three species are native to Africa, two of which are known from historical records, while one is new to Lake Naivasha (Bulinus forskalii). All native species are of public health (Biomphalaria sudanica) and veterinary health (Bulinus tropicus, Bulinus forskalii) concern. The current biodiversity of molluscs has reached the same level as in historical times, but the species composition is remarkably different.