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

Abstract

Food web ecology has revolutionized our understanding of ecological processes, but the drivers of food web properties like trophic position (TP) and food chain length are notoriously enigmatic. In terrestrial ecosystems, aboveand 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 13 C, δ 15 N) of individual amino acids to trace the flow of essential biomolecules and jointly measure multichannel feeding, food web coupling, and TP 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 TP 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.

Variation in gut microbial contribution of essential amino acids to host protein metabolism in a wild small mammal community

“…microbial) energy channel. The microbial processing of organic matter in terrestrial food webs and the subsequent importance of brown energy channels has gained traction in recent years (Allison, 2006; Hagen et al, 2012; Manlick et al, 2023; Steffan & Dharampal, 2019). This philosophy has been further supported by recent isotopic work (Pollierer et al, 2020; Potapov et al, 2019; Steffan et al, 2017), including a controlled feeding experiment that suggested heterotrophic bacteria are trophic analogues of animals, at least regarding isotopic discrimination associated with nitrogen metabolism (Steffan et al, 2015).…”

Differential seasonal effects of ephemeral legumes in response to moisture-nitrogen coupling in the deserts of northwestern China

“…In addition to precipitation alterations due to climate change, nitrogen deposition is increasingly becoming a serious threat, e.g., the annual mean nitrogen deposition in agroecosystems in the arid zone of Xinjiang can reach up to 19.5 kg/hm 2 (Li et al, 2013), and that in the oases around the desert can reach as high as 30 kg/hm 2 (Xu et al, 2015; Zhou et al, 2014), exceeding many ecosystem nitrogen thresholds(10 kg/hm 2 ). Global climate change and human activities not only affect the natural environment, ecosystem balance, and various ecohydrological processes, but are also key factors for measuring and predicting ecosystem changes, which are relevant to the biogeocirculation processes of almost all types of landscapes and ecosystems on Earth(Daunoras et al, 2024; Higgins et al, 2023; Koltz et al, 2018; Manlick et al, 2023; Zhang et al, 2019).…”

Energy channelling, food chain length and body condition in a northern lake predator

“…Pathways of de novo essential AA synthesis differ among plants, algae, bacteria and fungi, leading to unique essential AA δ 13 C "fingerprints" among these potential energy channels (Besser et al, 2022;Elliott Smith et al, 2022;Larsen et al, 2009Larsen et al, , 2012. Recent work using δ 13 C analysis of essential AAs in consumer tissues has proven robust for tracing the contribution of different energy channels to individuals and communities (Elliott Smith et al, 2021;Manlick et al, 2023;Skinner et al, 2021). Because essential AA δ 13 C fingerprinting relies upon the relative "spacing" between the δ 13 C values different essential AAs, and not measured δ 13 C values, it is hypothesised that primary producers sampled from different geographical locations may indeed exhibit similar "fingerprints".…”