Coupled tectonic and geomorphic forces create a local fish diversity hotspot

Mountain regions are globally recognized as biodiversity hotspots, yet habitat heterogeneity alone cannot fully explain their exceptional diversity. During orogeny, horizontal and vertical tectonic forces, as well as surface erosion, act on the landscape, generating dynamic, transient landscapes with reorganizing drainage networks and changing topography. Because aquatic habitats are tightly coupled with river topography, freshwater species are particularly insightful to study how landscape evolution influences biodiversity.

In this study, we investigated freshwater fish diversity in northern Taiwan, a tectonically active area with high island endemism. Taiwan is dominantly a convergent orogenic belt. However, an exception is the NE corner of the island, where extension associated with the Okinawa back-arc basin has propagated on land. Active crustal stretching and subsidence with normal faulting has resulted in formation of the Ilan Plain and a new drainage system. The largest river in this system is the Lanyang River, which flows parallel to the mountain belt, along an active normal fault system. To investigate if and how basin formation and drainage reorganization impact biodiversity, we collected environmental DNA (eDNA) from 22 riverine sites within and surrounding the Lanyang River basin. We amplified a 420 bp fragment of the mtDNA cytochrome B (cytB) gene and denoised the sequences to retrieve amplicon sequence variants (ASVs). We then examined variations in richness and assemblage composition among basins (interspecific), as well as within one endemic genus (intraspecific).

Our results reveal clear geographic patterns, indicating the existence of geographic barriers, as well as dispersal corridors. The Lanyang River basin emerged as a distinct biodiversity hotspot, with patterns of interspecific and intraspecific diversity suggesting that its high richness has been generated through repeated influx of genetic material from neighboring basins. These patterns are consistent with a history of river capture events driven by drainage divide migration as the Lanyang River basin has increased in size by headwater growth, highlighting how coupled tectonic and geomorphic processes act to shape freshwater fish diversity.