The causes of insect endemicity with the example of Madagascar

Abstract

Abstract Biodiversity is disappearing at an exceptional rate, engendering a considerable effort of the scientific community to conserve the rarest and most threatened species. Biodiversity Hotspots were designed for this purpose and Madagascar, showing the highest levels of endemism and imbalance across taxa, rapidly became their flagship. The processes that lead to such exceptional biodiversity patterns on the island remain poorly understood and the lack of efficacy of traditional taxonomy to catalogue tropical endemic biodiversity hinders the scientific advances in this domain. This study tackles insect endemism in Madagascar, aiming at identifying the evolutionary and ecological factors responsible for their present diversity patterns. Aquatic beetles, living in standing or running water, submit to differences in habitat stability, potentially affecting dispersal patterns and hence endemism patterns. We chose them as a target group for these reasons, and first achieved a DNA level inventory of the fauna. We identified 169 species, showing high congruence with the morphological taxonomy and corresponding to 74% of the total estimated species richness. The mitochondrial dataset revealed high spatial turnover at the species and haplotype levels. A phylogenetic tree from three gene markers was used for ancestral state character reconstruction and suggested the dependent evolution of endemism and habitat use. Secondly, our results highlight a combination of low dispersal activity and important climatic constraints for the lotic fauna, explaining the local scale endemism in this group, whereas the lentic species turnover correlated the least to both climate and geographic distance. Last, a distribution modelling approach revealed the uniqueness of Malagasy climate niches and the absence of conservation of environmental envelopes at higher phylogenetic levels. We conclude that endemism in Madagascar is best explained locally by the dependence to environmental variability of the lotic species, and that dispersal capacities are the primary hindrance to colonising external suitable habitats.