Uncovering the hidden biodiversity of streams at the upper distribution limit of fish

Abstract

AbstractAimBiodiversity changes can occur gradually or as distinct jumps as species distributions end and others begin. However, comprehensive assessments of freshwater biodiversity and how assemblages change along a continuum have been hindered by the connectivity and networked nature of streams and their large numbers of rare and cryptic species found there. We hypothesize that the boundary where fish distributions end marks an ecological shift in freshwater assemblages and could unify stream concepts.LocationTrask River watershed, Oregon, USA, temperate North America.TaxonFishes, amphibians, crayfishes, bivalves, mammals, pathogens.MethodsWe targeted freshwater species across taxa using multigene eDNA metabarcoding of 48 different DNA barcodes to identify assemblages to understand species patterns distributed along a longitudinal gradient in watersheds around the upper fish distribution boundary. Detections from multiple genetic sources improve the detection probability for rare species.ResultsWe detected a shift in the assemblage from lower reaches towards headwaters where fish distributions end, leading to fewer species across taxa at reduced eDNA signal. We uncovered a hidden biodiversity in watersheds by detecting a sparsely distributed amphibian pathogen Batrachochytrium dendrobatidis, the elusive mammal Mountain Beaver, fish species that went undetected using traditional approaches, and newly discovered cryptic lineages of sculpins. Salmonids were detected further upstream with eDNA than expected.Main conclusionsOur findings unify stream concepts owing to the marked species shift at the upper extent of fish elevating the importance of this boundary. The detection of salmonids further upstream suggests a distribution extension. The sculpin detections reveal a cryptic species complex with ramifications for fish conservation, potential pockets of endemism and possible identification of new species. We show extensive diversity in watersheds, some of which was previously hidden, transforming our understanding of species presence and distributions affecting our ability to track, protect and manage them across watersheds.