DOI: 10.5061/dryad.2ds6h
title: Data from: Lotic cyprinid communities can be structured as nest webs and predicted by the stress-gradient hypothesis

description: 1. 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.
 2. 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. 3. 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). 4.
 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. 5. 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.
description: All spp. counts (for
 PCOA)Raw count data of fish
 species of all ages captured at 45 sites in the New River basin of NC, VA
 and WV (USA) in summer 2012. These data were used for the principal
 coordinates analysis.Grouped demographic data
 (for SEM and SGH regressions)Stream fish data
 combined into reproductive groups (see Methods section of manuscript) and
 classified as adult or juvenile based on modal breaks in length frequency
 histograms (see Methods section of manuscript). These data were used in
 structural equation modeling, and the juvenile portions of the dataset
 (divided by nest count) were used as response variables in regressions
 testing the stress-gradient hypothesis predictions.Habitat, nest counts and
 sampling effortHabitat variables as
 described in the manuscript. Electrofishing effort (effort) is reported in
 seconds. The variables "agri.rank" and "wsa.rank"
 represent ranked values of each raw measurement of %agriculture and
 watershed area (km2), respectively. These two variables were added to
 calculate the variable "stress.bin", which served as the
 independent variable in regressions testing predictions of the
 stress-gradient hypothesis.Nest web
 observationsCyprinid nesting
 observations, including which species constructed the nest and which
 species used those nests for reproduction. These data were used for
 generating the nest webs.