Isotopic niche reflects stress-induced variability in physiological status

Abstract

The isotopic niche has become an established concept in trophic ecology. However, the assumptions behind this approach have rarely been evaluated. Evidence is accumulating that physiological stress can affect both magnitude and inter-individual variability of the isotopic signature in consumers via alterations in metabolic pathways. We hypothesized that stress factors (inadequate nutrition, parasite infestations, and exposure to toxic substances or varying oxygen conditions) might lead to suboptimal physiological performance and altered stable isotope signatures. The latter can be misinterpreted as alterations in isotopic niche. This hypothesis was tested by inducing physiological stress in the deposit-feeding amphipod Monoporeia affinis exposed to either different feeding regimes or contaminated sediments. In the amphipods, we measured body condition indices or reproductive output to assess growth status and δ13C and δ15N values to derive isotope niche metrics. As hypothesized, greater isotopic niche estimates were derived for the stressed animals compared to the control groups. Moreover, the δ15N values were influenced by body size, reproductive status and parasite infestations, while δ13C values were influenced by body size, oxygen conditions and survival. Using regression analysis with isotope composition and growth variables as predictors, we were able to discriminate between the amphipods exposed to nutritionally or chemically stressful conditions and those in the control groups. Thus, interpretation of isotopic niche can be confounded by natural or anthropogenic stressors that may induce an apparent change in isotopic niche. These findings stress the importance of including measures of growth and health status when evaluating stable isotope data in food web studies.

Trophic niche segregation allows range‐extending coral reef fishes to co‐exist with temperate species under climate change

“…It is important to note that stable isotope signatures may be confounded by environmental stressors unrelated to changes in diet, potentially leading to inaccurate depictions of trophic niche space (Karlson, Reutgard, Garbaras, & Gorokhova, 2018). A study of freshwater snails showed that high temperatures, such as those experienced by temperate species along their trailing range edge, may enrich δ 15 N and δ 13 C values (Ek, Karlson, Hansson, Garbaras, & Gorokhova, 2015).…”

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

“…Isotopic niche size is also related to nutritional status, typically expanding when animals encounter stressful environments such as limited resource availability, and retracting in benign environments [120,121,124]. For example, Karlson et al [125] demonstrated that nutritional stress, parasite infestation, and toxin contamination all resulted in larger carbon-nitrogen isotope niches in the marine amphipod Monoporeia affinis, with nutritional stress producing the largest absolute change in isotopic niche space.…”

Trophodynamics of Southern Ocean pteropods on the southern Kerguelen Plateau

“…Among the numerous studies examining how a range of nutritional stresses affect isotopic compositions of various animals, enrichment in 15 N is the response most likely to confound interpretations of isotopic niches, particularly given that isotopic niche metrics rely on the assumption that interindividual variability in enrichment between consumer and prey is insignificant (Karlson, Reutgard, Garbaras, & Gorokhova, ; McCue & Pollock, ). One experiment‐based study on the amphipod Monoporeia affinis revealed wider niche widths among specimens exposed to nutrient and chemical stress relative to control animals (Karlson et al, ). It is understood that increases in isotopic niche areas are a function of interindividual variability in the responses of consumers to stress exposure through changes in their metabolic pathways.…”

“…One experiment-based study on the amphipod Monoporeia affinis revealed wider niche widths among specimens exposed to nutrient and chemical stress relative to control animals (Karlson et al, 2018).…”

“…However, no studies have tested how these changes affect C. limacina isotopically. Among the numerous studies examining how a range of nutritional stresses affect isotopic compositions of various animals, enrichment in 15 N is the response most likely to confound interpretations of isotopic niches, particularly given that isotopic niche metrics rely on the assumption that interindividual variability in enrichment between consumer and prey is insignificant (Karlson, Reutgard, Garbaras, & Gorokhova, 2018;McCue & Pollock, 2008).…”