You are what your host eats: The trophic structure and food chain length of a symbiont community are coupled with the plastic diet of the host ant.

Food chain length provides key information on the flow of nutrients and energy in ecosystems. Variation in food chain length has primarily been explained by environmental drivers such as ecosystem size and productivity. Most insights are obtained from theory or aquatic systems, but the importance of these drivers remains largely untested in terrestrial systems. We exploited red wood ant nests markedly differing in size as natural experiments to quantify the drivers of trophic structure and food chain length of their symbiont arthropod communities. Using stable isotopes, we explored the variation in the trophic positions of four symbiont species with the trophic position of the top predator as a proxy for food chain length of the symbiont community. Nest size did not affect food chain length, nor trophic distance between the symbionts. Instead, food chain length and the trophic positions of the symbionts were strongly affected by the host's foraging decisions. When the host diet shifted from predominantly herbivorous to more predacious, the trophic position of the symbionts and food chain length strongly increased. We show for the first time that a food web can be structured by biotic interactions with an engineering species rather than by abiotic environmental variables.