2.7 | Testing for a phylogenetic signal
Inferences made from phylogeny-informed null modeling approaches are
predicated on the existence of a positive phylogenetic signal in species
niche-space (Webb, Ackerly, McPeek, & Donoghue, 2002); the notion that
closely related species possess similar suites of traits and occupy
similar niches (Tucker, Davies, Cadotte, & Pearse, 2018). We tested for
a phylogenetic signal with respect to abundance in the presence of
sandwort using the R package phylosignal (Keck, Rimet, Bouchez, &
Franc, 2016). To approximate association with a sandwort-based diet, we
estimated the abundance averaged niche space of each ASV. Briefly,
abundances were relativized to the total sequence count for each ASV
across all samples. Relative abundances among horses with access to
sandwort were multiplied by 1 and relative abundances among horses
without sandwort access were multiplied by −1. The sum of these values
within each ASV were assigned as a ‘niche-score’ for each ASV which
varied continuously between −1 (ASV only present in horses with access
to sandwort) and 1 (ASV only present in horses without access to
sandwort). Sandwort was selected since: 1) different dietary components
are expected to vary in their polysaccharide composition, thereby
selecting for different microbial metabolic functions (Julliand &
Grimm, 2017) and 2) sandwort presence was observed in preceding analyses
to be an important correlate of phylogeny-informed and
phylogeny-independent β-diversity.