The coupling of green and brown food webs regulates trophic position in a montane mammal guild.

Food web ecology has revolutionized our understanding of ecological processes, but the drivers of food web properties like trophic position and food chain length are notoriously enigmatic. In terrestrial ecosystems, above- and belowground systems were historically compartmentalized into "green" and "brown" food webs, but the coupling of these systems by animal consumers is increasingly recognized, with potential consequences for trophic structure. We used stable isotope analysis (δ C, δ N) of individual amino acids to trace the flow of essential biomolecules and jointly measure multichannel feeding, food web coupling, and trophic position in a guild of small mammals. We then tested the hypothesis that brown energy fluxes to aboveground consumers increase terrestrial food chain length via cryptic trophic transfers during microbial decomposition. We found that the average small mammal consumer acquired nearly 70% of their essential amino acids (69.0% ± 7.6) from brown food webs, leading to significant increases in trophic position across species and functional groups. Fungi were the primary conduit of brown energy to aboveground consumers, providing nearly half the amino acid budget for small mammals on average (44.3% ± 12.0). These findings illustrate the tightly coupled nature of green and brown food webs and show that microbially mediated energy flow ultimately regulates food web structure in aboveground consumers. Consequently, we propose that the integration of green and brown energy channels is a cryptic driver of food chain length in terrestrial ecosystems.