Lotic cyprinid communities can be structured as nest webs and predicted by the stress-gradient hypothesis

Little is known about how positive biotic interactions structure animal communities. Nest association is a common reproductive facilitation in which associate species spawn in nests constructed by host species. Nest-associative behaviour is nearly obligate for some species, but facultative for others; this can complicate interaction network topology. Nest web diagrams can be used to depict interactions in nesting-structured communities and generate predictions about those interactions, but have thus far only been applied to cavity-nesting vertebrate communities. Likewise, the stress-gradient hypothesis (SGH) predicts that prevalent biotic interactions shift from competition to facilitation as abiotic and biotic stress increase; this model has been hardly applied to animal communities. Here, both of these models were applied to nest-associative fish communities and extended in novel ways to broaden their applicability. A nest web was constructed using spawning observations over 3 years in several streams in south-western Virginia, USA. Structural equation modelling (SEM) was then implemented through an information-theoretic framework to identify the most plausible nest web topology in stream fish communities at 45 sites in the New River basin of the central Appalachian Mountains, USA. To test the SGH, the per-nest reproductive success of 'strong' (nearly obligate) nest associates was used to represent interaction importance. Eigenvectors were extracted from a principal coordinate analysis (PCoA) of proportional species abundances to represent community structure. Both of these metrics were regressed on physical stress, a combination of catchment-scale agricultural land use and stream size (representing spatiotemporal habitat variability). Seventy-one per cent of SEM model evidence supported a parsimonious interaction topology in which strong associates rely on a single host (Nocomis), but not other species. PCoA identified a gradient of community structure dominated by Nocomis and associates, to communities dominated by other reproductive groups. Both metrics of interaction importance responded positively to physical stress. This study demonstrates that nest webs can be useful in a variety of systems and that SEM can be a quantitative extension of this framework. Likewise, the SGH can be used to understand positive interactions in animal communities and can be extended to predict proportional representation of facilitating and beneficiary species in communities.