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

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.