(A) Schematic illustrating different scenarios of nutrient preference. There are two families (FS and FA) and two resource classes (RS and RA). Without resource specialization, FS and FA have equal access to RS and RA. With symmetric specialization, each family prefers its own resource class with the same strength. With asymmetric specialization, one family (FS) has better access to its own resource class (RS) relative to that of the other family (FA) on its own resource class (RA). (B) A mechanistically explicit consumer-resource model that incorporates resource competition, resource specialization and nonspecific cross-feeding (Materials and methods) recovers the predicted additivity pattern at both the species (left) and family (right) level of taxonomic organization. The observed relative abundance of each species or family in 300 communities grown on a different pair of nutrients (100 AA, 100 SS, and 100 SA) is plotted against the abundance predicted from the same communities grown on each of the relevant single nutrients (S, A). Each family specializes equally on its preferred nutrient (qS = qA = 0.9) as in previous work (Marsland et al., 2020a). In Figure 4—figure supplement 4, we illustrate representative consumption matrices for different choices of qA and qS. (C) 22 strains were isolated from the assembled communities and their growth rates on minimal M9 media supplemented with one the 10 carbon sources were measured. qS represents the growth rate advantage of Enterobacteriaceae on sugars relative to the other dominant family (colored), while qA represents the growth rate advantage of the other family on the acids relative to Enterobacteriaceae (Materials and methods). When qS is positive, Enterobacteriaceae grow faster on the sugar than the other family, while when qS is negative, Enterobacteriaceae grow more slowly on the sugar than the other family. When qA is positive, the other family grows faster on the acid than Enterobacteriaceae, while when qA is negative, the other family grows more slowly on the acid than Enterobacteriaceae. Each dot corresponds to a sugar-acid pair for a Enterobacteriaceae-other family pair (n = 24). The growth rate advantage of Enterobacteriaceae on sugars is significantly greater than the growth rate advantage of the other families on acids (i.e. qS > qA, mean of differences = 0.069, paired t-test, n = 24, p-value<0.0001). (D) Here we repeat the same simulation as shown in (B), this time using different combinations of qA and qS (0.05, 0.15, 0.25, 0.35, 0.45, 0.55, 0.65, 0.75, 0.85, 0.95). Heatmap shows the mean dominance level (δ) for different combinations of qA and qS. When δ < 0, the sugar dominates (purple); when δ > 0, the acid dominates (orange).