Quantifying Isolation-by-Resistance and Connectivity in Dendritic Ecological Networks

ABSTRACT Statistical methods that quantify individual movements have limited applicability within dendritic ecological networks (DENs), to include riverscapes. We promote an approach herein by deriving an analytical framework (=DistNet and ResistNet) that more appropriately quantifies differentiation by meshing individual DEN segments with fitted distances and annotated environmental features. We first explored the spatial arrangement of diversity in a river network by fitting pairwise distances against graph edges. We then derived effective resistance models (e.g., as a composite of an array of possible environmental covariates) by using a genetic algorithm for heuristic model selection, invoking circuit theory (CircuitScape) to form a graph-based complement to ResistanceGA (a landscape ecology package). Although numerous pairwise distance metrics can be employed, we utilized genetic distances encompassing 13,218 ddRAD loci from N=762 Speckled Dace (Cyprinidae: Rhinichthys osculus) sampled at 78 Colorado River localities (µ=9.77/site). When distances for each locus were fitted into the network, we found intraspecific divergences due to stream reaches were greater than expected when distances alone. In fitting model-averaged ‘effective resistance’ models to the same network, we showed the manner by which a host of anthropogenic and environmental variables contribute to patterns of connectivity underlying riverscape genetic differentiation. Our framework represents a contemporary approach to deriving metapopulation/ metacommunity structure within DENs, in that it allows: (a) The extension of projections into unsampled temporal/spatial components; (b) Comparisons to be made among species and/or drainages, both of which benefit multi-species management; and (c) the quantification of locus-specific patterns, from which adaptive hypotheses could then be derived. “The intimate relation of hydrological conditions to fish life is proverbial; to be hopelessly or dangerously maladjusted to one’s environment is to be a ‘fish out of water’” (Hubbs 1941).