Hitting the moving target: modelling ontogenetic shifts with stable isotopes reveals the importance of isotopic turnover

Abstract

Summary1. Ontogenetic niche shifts are widely prevalent in nature and are important in shaping the structure and dynamics of ecosystems. Stable isotope analysis is a powerful tool to assess these shifts, with d 15 N providing a measure of trophic level and d 13 C a measure of energy source.2. Previous applications of stable isotopes to study ontogenetic niche shifts have not considered the appreciable time lag between diet and consumer tissue associated with isotopic turnover. These time lags introduce significant complexity into field studies of ontogenetic niche shifts.3. Juvenile Chinook salmon (Oncorhynchus tshawytscha) migrate from freshwater to marine ecosystems and shift their diet from feeding primarily on invertebrates to feeding primarily on fish. This dual ontogenetic habitat and diet shift, in addition to the long time lag associated with isotopic turnover, suggests that there is potential for a disconnect between the prey sources that juvenile salmon are consuming, and the inferred prey sources from stable isotopes. 4. We developed a model that considered ontogenetic niche shifts and time lags associated with isotopic turnover, and compared this 'ontogeny' model to one that considered only isotopic turnover. We used a Bayesian framework to explicitly account for parameter uncertainty. 5. Data showed overwhelming support for the ontogeny model relative to the isotopic turnover model. Estimated variables from best model fits indicate that the ontogeny model predicts a much greater reliance on fish prey than does the stomach content data. Overall, we found that this method of quantifying ontogenetic niche shifts effectively accounted for both isotopic turnover and ontogenetic diet shifts; a finding that could be widely applicable to a variety of systems.

Integrated Diet Analyses Reveal Contrasting Trophic Niches for Wild and Hatchery Juvenile Chinook Salmon in a Large River Delta

“…; Hertz et al. ). A lampara net was used to sample these fish from eight additional NS stations, which were farther from shore and too deep to sample by beach seine (Figure ).…”

“…We also collected muscle and liver samples from 28 juvenile Chinook Salmon captured in nearshore intertidal (NS) habitat ( Figure 1). These samples were used to test for differences in diet integration rates among tissues and to account for potential temporal mismatches in habitat use and stable isotope signals due to rapid ontogenetic changes in habitat and diet (Heady and Moore 2013;Hussey et al 2014;Hertz et al 2016). A lampara net was used to sample these fish from eight additional NS stations, which were farther from shore and too deep to sample by beach seine (Figure 1).…”

Causes and consequences of ontogenetic dietary shifts: a global synthesis using fish models

“…Thus, in both marine and freshwater systems, many prey taxa frequently have specific habitat requirements (Chapman, 1999;Tachet et al, 2010) and, consequently, ontogenetic changes in habitat use by a predator may lead to unavoidable changes in diet. This is particularly evident in diadromous species (that migrate between freshwater and marine ecosystems; Dixon et al, 2012;Hertz et al, 2016) and lacustrine migrants (moving between littoral and pelagic or profundal habitats; Werner & Hall, 1988;Knudsen et al, 2006). The habitat preferences of fishes commonly change during development (e.g.…”

Detrital nutrient content and leaf species differentially affect growth and nutritional regulation of detritivores

“…One notable framework, ecological stoichiometry, focuses on mass constraints of elements like nitrogen (N), and phosphorus (P) on organismal growth (Sterner and Elser 2002). As central tools of ecological stoichiometry, organismal N and P budgets explain growth limitation by N or P availability but remain rare because they are difficult to construct, and recent studies have raised challenges for budget implementation among consumers, including problems of selective feeding, heterogeneous turnover of nutrients in consumers and resources, and dynamic nutrient demands during ontogeny (Hood et al 2014, Dodds et al 2014, Hertz et al 2016. Though challenging to construct, nutrient budgets are powerful because they provide mechanistic bases for growth and fitness, and simultaneously bridge levels of organization by placing organisms within ecosystem-level processes like nutrient cycling (Bouchard and Bjorndal 2000, Mischler et al 2016, Sperfeld et al 2016.…”