Fig. 1

An illustration of the taxa-function relationship and response curves. a The functional profile of a community is a linear combination of the functional profile of each taxon in a community (the copy number of each gene in each taxon’s genome) weighted by the abundance of each taxon in that community. Note that here, a taxon need not be a species, but can instead represent any subpopulation of the community with shared genomic content (such as a single strain of a particular species). b The taxa-function relationship can be modeled as a high-dimensional landscape, linking each community composition to the corresponding functional profile. Here, we show an extremely simplified two-dimensional abstraction of this model to illustrate the impact of this landscape on taxa-function robustness. In this illustration, each coordinate on the plane corresponds to a specific taxonomic composition, with points close to one another corresponding to communities with similar taxonomic compositions. The height represents the functional profile of the community (and in this simplified illustration can denote, for example, the abundance of some function). Notably, the local topology of this landscape around a specific taxonomic composition (e.g., point 1) determines whether minor changes in that composition (represented as small movements on this plane; black arrows) will induce small (point 1) or large (point 2) functional shifts. c Depending on the exact distribution of genes across the genomes of species in a community, changes to the taxonomic composition of a community can produce functional shifts of varying magnitudes. For example, if the distribution of genes differs markedly between two communities (e.g., high vs. low redundancy), similar taxonomic composition perturbations to both communities may produce drastically different functional shifts. d To model the relationship between taxonomic perturbations and functional shifts in a given community, a taxa-function response curve is obtained by fitting a power function to a large array of measurements of functional shifts associated with many different taxonomic perturbations of varying magnitudes. e The taxa-function response curve can be decomposed into two factors: attenuation, which describes how quickly functional shifts increase in magnitude as taxonomic perturbations increase, and buffering, which indicates how well functional shifts are suppressed at smaller taxonomic perturbations