Predaceous water beetles (Coleoptera, Hydradephaga) of the Lake St Lucia system, South Africa: biodiversity, community ecology and conservation implications

Water beetles are one of the dominant macroinvertebrate groups in inland waters and are excellent ecological indicators, reflecting both the diversity and composition of the wider aquatic community. The predaceous water beetles (Hydradephaga) make up around one-third of known aquatic Coleoptera and, as predators, are a key group in the functioning of many aquatic habitats. Despite being relatively well-known taxonomically, ecological studies of these insects in tropical and subtropical systems remain rare. A dedicated survey of the hydradephagan beetles of the Lake St Lucia wetlands (South Africa) was undertaken between 2013 and 2015, providing the first biodiversity census for this important aquatic group in the iSimangaliso Wetland Park, a UNESCO World Heritage Site within the Maputaland biodiversity hotspot. A total of 32 sites covering the entire spectrum of waterbody types were sampled over the course of three collecting trips. The Lake St Lucia wetlands support at least 68 species of Hydradephaga, a very high level of diversity comparing favourably with other hotspots on the African continent and elsewhere in the world and a number of taxa are reported for South Africa for the first time. This beetle assemblage is dominated by relatively widespread Afrotropical taxa, with few locally endemic species, supporting earlier observations that hotspots of species richness and centres of endemism are not always coincident. Although there was no significant difference in the number of species supported by the various waterbody types sampled, sites with the highest species richness were mostly temporary depression wetlands. This contrasts markedly with the distribution of other taxa in the same system, such as molluscs and dragonflies, which are most diverse in permanent waters. Our study is the first to highlight the importance of temporary depression wetlands and emphasises the need to maintain a variety of wetland habitats for aquatic conservation in this biodiverse region.