a, Original food-chain length result from Fig. 5d, showing mean trophic position (from stable-isotope data) of the apical consumer in each treatment. Bars, least-squares means (±s.e.m.) from the generalized least-squares model in Extended Data Table 2a (n = 15 islands). b, c, Corresponding plots of mean per-sample dietary Shannon diversity (b) and species richness (c), from DNA metabarcoding, of the apical consumer on each island (n = 15 islands). Dietary diversity and richness are used here as proxies for trophic omnivory; these metrics were analysed with the same model structure as in a and show the inverse pattern, which is consistent with the possibility that changes in food-chain length were driven by changes in trophic omnivory by apical consumers (higher omnivory and shorter food chains on control and +GA+CT islands, and vice versa on +GA and +CT islands). The green anole × curly-tailed lizard interaction terms from the models are shown in the top right. Letters denote significant differences (P ≤ 0.05) between treatments in pairwise two-sided t-tests. d, Food-chain length as a function of island area, as in Fig. 5c, but here including only islands with curly-tailed lizards (n = 8) and analysed using ANCOVA to highlight the area × treatment interaction. Food-chain length was uncorrelated with ecosystem size on +GA+CT islands (see also Fig. 5c, Extended Data Table 2). e, f, Corresponding ANCOVA analyses of mean per-sample dietary diversity (e) and richness (f) of curly-tailed lizards, consistent with the possibility that changes in top-predator omnivory influenced food-chain length (n = 7 islands). Mean per-sample diversity (e) was greater on small islands (which is consistent with higher trophic omnivory in small ecosystems10) and on +GA+CT islands, and decreased with island area on +CT but not +GA+CT islands (as expected if higher levels of trophic omnivory on +GA+CT islands of all sizes resulted in shorter food chains and contributed to the pattern seen in a). Per-sample dietary richness (f) showed a similar response to the island area × treatment interaction. For d–f, ANCOVA statistics are shown in the top right. Points, island-level means; error bars, ±1 s.e.m. g, h, The trophic position of curly-tailed lizard was negatively correlated with dietary diversity (g) and richness (h) in linear regressions, consistent with our conjecture that arthropod prey breadth is a proxy for trophic omnivory by the top predator. Points, means; error bars, ±1 s.e.m.; n = 7 islands. Regression statistics are shown in the top right. Island 204 (open circles) was represented by 3 curly-tailed lizard isotope samples (n ≥ 5 samples for all other islands) but only one faecal sample (n ≥ 3 samples for all other islands). Thus, in b, c we used brown anoles as the apical consumer on island 204 (trophic position 2.37 versus 2.43 for curly-tailed lizards). In e–h, we omitted island 204 from statistical analyses but show it for reference. In all panels, including the curly-tailed lizard data from island 204 (or omitting island 204 entirely from b, c) gives similar statistical results. Island names corresponding to each point are shown in d–h.