Regional hotspots of Hengduan plant endemism inferring local speciation in response to connectivity

Abstract

The Hengduan Mountains (HDM) is one of the most biologically diverse mountain ranges on the planet, with exceptionally high levels of endemism. We expect that the geological and climate changes of the regions shaped endemism though dispersal and speciation processes by modulating landscape connectivity. Here, we characterise the plant endemism in the HDM, by mapping the distribution of 3,165 endemic species, representing approximately 25% of the total plant species richness. We show that endemic richness is highest along the southern front of the HDM, and especially concentrated along the Shangri-la Plateau and the three-river parallel region at elevations between 2,700 and 4,200 meters a.s.l. We demonstrate a geographically differentiated effect of connectivity on endemic richness and composition. In the endemic hotspot, we find a negative connectivity-diversity relationship, while we find a positive connectivity diversity relationship in the northern and southern HDM. Our result suggests a dominant role of isolation-induced allopatric speciation. Low connectivity may facilitate allopatric speciation in shaping distinct lineage in central HDM; while in the north of HDM, similar cold habitats in high elevation where habitats are more connected than the southern part, have likely facilitated species migration during the Quaternary glaciation. Thus, the degree of connectivity varied within HDM depending on their topographical configuration. Geographic contrasts in diversity further match endemic composition, which suggest the effect of geological history in shaping the diversity and composition of this exceptional flora. Overall, we conclude that landscape connectivity is a key driver of endemic plant speciation in HDM, explaining richness patterns that cannot be explained by temperature and other classic predictors.