Beyond Isolation by Distance: Riverscape Effects on Genetic Structure of Fall-Run Chinook Salmon
Chanté D. Davis, Clinton W. Epps, Rebecca L. Flitcroft, and Michael Banks
Abstract.—Habitat fragmentation, land use practices, and flow impediments modify the natural course of rivers, disrupting connectivity and subsequently affecting dispersal and gene flow in aquatic organisms. Many of the relationships between the physical river network and the genetic structure of populations are not well understood. Riverscape genetics is a developing field that uses population genetic metrics to assess genetic structure within the context of the environmental variables that drive functional connectivity in a river network. Here, we applied an effective distance network approach to characterize the effects of hydrology in shaping neutral genetic population structure of fall-run Chinook Salmon Oncorhynchus tshawytscha within a small, coastal Oregon catchment. We evaluated whether gene flow was limited by (1) site-specific features occurring within spawning habitat, using a dissimilarity matrix, and (2) the cumulative effect of the environment accrued while traveling en route between reaches. We found that Chinook Salmon that spawned at higher elevations (site specific effects) after traversing steeper gradients (en-route effects) were more genetically distinct from individuals that traversed gradual gradients and spawned at lower elevations. This effect (isolation by resistance) was distinguishable from isolation by distance, which was not detected among spawning groups. Our study enhanced interpretation of habitat heterogeneity in constraining gene flow and spatial genetic structure among reaches within a small, coastal catchment. Given that smaller catchments may hold life history 36 variation that is important to long-term population persistence, there is need to understand these relationships that maintain genetic diversity.