Terrestrial-focused protected areas are effective for conservation of freshwater fish diversity in Lake Tanganyika

Freshwater protected areas are rarely designed specifically for this purpose and consequently their conservation benefit cannot be guaranteed. Using Lake Tanganyika as a test case we investigated the benefits of terrestrial-focussed protected areas on the alpha and beta taxonomic and functional diversity of the diverse endemic rocky shore cichlid fishes. Lake Tanganyika has limited protected shorelines and continued human population growth in its catchment, which has potential for negative impacts on habitat quality and key biological processes. We conducted 554 underwater surveys across a gradient of human disturbance including two protected areas, along 180 km of Tanzanian coastline, sampling 70 cichlid species representing a diverse range of life-histories and trophic groups. Alpha diversity was up to 50% lower outside of protected areas, and herbivores appeared most affected. Turnover dominated within-locality variation in beta diversity, but the nestedness component was positively related to human disturbance indicating an increase in generalist species outside of protected areas. Within protected areas the decline in zeta diversity (the expected number of shared species across multiple surveys) was best described by power law functions, which occur when local abundance is predicted by regional abundance; but declined exponentially in unprotected waters indicating a dominance of stochastic assembly. Despite not being designed for the purpose, the protected areas are clearly benefitting cichlid taxonomic and functional diversity within Lake Tanganyika, probably through local reduction in sediment deposition and/or pollution, but as cichlids can be poor dispersers protected area coverage should be expanded to benefit isolated communities.