High‐resolution food webs based on nitrogen isotopic composition of amino acids

Abstract

Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well-understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher-order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free-roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.

Microbes are trophic analogs of animals

“…Specifically, the consistent TDF glu-phe among all consumers reflected predictable patterns of 15 N-discrimination in glutamic acid and phenylalanine, the two amino acids previously shown to be critically important to accurate TP estimation among animals (17,20,26,33). The degree of intertrophic 15 N-discrimination in glu was relatively high, whereas that of phe was characteristically low, which contrasted with the more variable patterns observed in the remaining amino 15 N-discrimination within specific amino acids, across a broad diversity of heterotrophic organisms.…”

“…Among the microbiota, we cultured ascomycete and basidiomycete fungi, as well as proteobacteria and filamentous bacteria (actinomycetes). When a consumer had developed to maturity on a given diet, the entire organism was homogenized and the δ 15 N values of amino acids within its tissues were measured using established analytical protocols (17,26,34). Cultures of fungus gardens were maintained within a controlled-environment laboratory setting (24-26°C, 16:8 photoperiod).…”

“…Altogether, we show that microbes can be integrated with plants and animals in a food chain, thereby unifying the macro-and microbiome in studies of trophic ecology. intertrophic 15 N-discrimination within amino acids extracted from consumer taxa, focusing on two key compounds: glutamic acid and phenylalanine (17,19,20,26). For microbial organisms to be trophically equivalent to animals, their respective patterns of intertrophic 15 N-discrimination in glutamic acid and phenylalanine must mirror those of animals.…”

Opportunism on the High Seas: Foraging Ecology of Olive Ridley Turtles in the Eastern Pacific Ocean

“…We compared two approaches to estimate trophic position (TP) according to Chikaraishi et al (2009Chikaraishi et al ( , 2010 and Bradley et al (2015). We used δ 15 N values for either phenylalanine (Phe) or the weighted mean of three source amino acids (glycine, lysine, and phenylalanine), and glutamic acid (Glu) or the weighted mean of three trophic amino acids (alanine, leucine, and glutamic acid) to estimate olive ridley TP in each sub-region.…”

“…Source amino acids reflect the isotopic composition at the base of the food web, whereas trophic amino acids reflect the trophic level of the consumer. Examining δ 15 N values of both types of amino acids can thus yield insights that cannot be gained with bulk-tissue analysis alone (Chikaraishi et al, 2009(Chikaraishi et al, , 2010.…”