Riverine barrier hypothesis explains the structure of dung beetle communities in Brazilian Coastal Sand-dune forests

Abstract

The Riverine Barrier Hypothesis (RBH) predicts that tropical rivers can be effective barriers to species dispersion, affecting different scales of biological organization, from genes to assemblages. We disentangled the latitudinal from the river presence effect to test the hypothesis that the Doce river mouth acted as a geographical, historical barrier for insects through North and South directions of the Brazilian coast, affecting the current community structure. We sampled dung beetle communities. In 16 Coastal Sand-dune Forest (“Restinga”) patches, across the Doce River Basin in the Brazilian coastline, nine through the South and seven through the North direction of the Doce River's mouth. We analyzed the relationship between the community composition and richness relative to the river (North and South regions) and latitude using linear modeling over a distance matrix (DistLM) approach. The latitude explained 17.74% of the total variation in dung beetle species composition, and the regional position relative to Doce River's mouth exhibited a significant effect (Pseudo F = 2.8479; p = 0.005) only after removing the latitudinal effect, here representing the habitat filtering. Both variables explained together with a total of 32.52% of the variability in dung beetles composition at coastal sand dune forests. The geographical position alone explained 48% of the richness difference between the North and South regions of the Doce River's mouth (Pseudo F = 16.534; p < 0.01). Meanwhile, the latitudinal position of the sites exhibited no significant relationship with species richness. Our findings highlight the role of Doce River as a biogeographical filter in the Doce river basin, while it represents the primary determinant of geographical segregation of the two key-species central to the dung beetle community structure.