The Importance of Isotopic Turnover for Understanding Key Aspects of Animal Ecology and Nutrition

Abstract

Stable isotope-based methods have proved to be immensely valuable for ecological studies ranging in focus from animal movements to species interactions and community structure. Nevertheless, the use of these methods is dependent on assumptions about the incorporation and turnover of isotopes within animal tissues, which are oftentimes not explicitly acknowledged and vetted. Thus, the purpose of this review is to provide an overview of the estimation of stable isotope turnover rates in animals, and to highlight the importance of these estimates for ecological studies in terrestrial, freshwater, and marine systems that may use a wide range of stable isotopes. Specifically, we discuss 1) the factors that contribute to variation in turnover among individuals and across species, which influences the use of stable isotopes for diet reconstructions, 2) the differences in turnover among tissues that underlie so-called ‘isotopic clocks’, which are used to estimate the timing of dietary shifts, and 3) the use of turnover rates to estimate nutritional requirements and reconstruct histories of nutritional stress from tissue isotope signatures. As we discuss these topics, we highlight recent works that have effectively used estimates of turnover to design and execute informative ecological studies. Our concluding remarks suggest several steps that will improve our understanding of isotopic turnover and support its integration into a wider range of ecological studies.

Novel tri‐isotope ellipsoid approach reveals dietary variation in sympatric predators

“…Carbon (δ 13 C) and sulfur (δ 34 S) isotope data help elucidate the production sources responsible for the energy flow in the food web, while nitrogen (δ 15 N) suggests the relative trophic position at which an animal is feeding (Connolly, Guest, Melville, & Oakes, 2004;Croisetière, Hare, Tessier, & Cabana, 2009;Minagawa & Wada, 1984;Pinnegar & Polunin, 1999). Different animal tissues have different turnover rates (Tieszen, Boutton, Tesdahl, & Slade, 1983) with fast turnover tissues (e.g., plasma or liver) representing short-term diet while slow turnover tissues (e.g., muscle) represent long-term diet (Carter, Bauchinger, & McWilliams, 2019). Consequently, muscle tissue can help identify consistent patterns in predator resource use (Carter et al, 2019;Vander Zanden, Clayton, Moody, Solomon, & Weidel, 2015).…”

“…Different animal tissues have different turnover rates (Tieszen, Boutton, Tesdahl, & Slade, 1983) with fast turnover tissues (e.g., plasma or liver) representing short-term diet while slow turnover tissues (e.g., muscle) represent long-term diet (Carter, Bauchinger, & McWilliams, 2019). Consequently, muscle tissue can help identify consistent patterns in predator resource use (Carter et al, 2019;Vander Zanden, Clayton, Moody, Solomon, & Weidel, 2015).…”

Evaluating mucus exudation dynamics through isotopic enrichment and turnover of skin mucus fractions in a marine fish model

“…Both plasma glucose and plasma 13 C allocation showed the expected pattern of one marked peak 6 h post-prandial, in agreement with that reported by Felip et al (2013 , 2015 ) using a stable isotope post-feeding trial, or by Montoya et al (2010) and Gómez-Milán et al (2011) , who analysed plasma glycaemia performance after ingesta. Moreover, both lower levels (<1%) of total stable isotope allocation per g of ingested isotopes confirmed that plasma does not act as a final fate but rather a transitory pathway with a fast turnover ( Carter et al, 2019 ). 15 N allocation to the liver also corresponded to reports in previous studies by Beltrán et al (2009 ) and Felip et al (2012 , 2015) for gilthead sea bream after a force-fed meal.…”

Exploring source differences on diet-tissue discrimination factors in the analysis of stable isotope mixing models

“…It has generally been reported that in addition to the inherent differences in between animal- and plant-protein based diets 25 , animal protein diets tend to show low DTDFs and high assimilation rates, whereas plant protein diets are usually associated with high DTDFs 29 , 50 . This is because animal protein diets are assumed to readily satisfy the amino acid requirements of consumers, resulting in both the protein and lipid content of consumer tissues being closely matched to those of their diets 64 , 66 . In contrast, the amino acids from plant protein diets are often different, and usually undergo different metabolic pathways, such as the transamination of keto acids from the carbohydrates in order to satisfy the specific amino acid requirements for consumer animals 67 .…”