Inferences to estimate consumer’s diet using stable isotopes: Insights from a dynamic mixing model

Abstract

Stable isotope ratios are used to reconstruct animal diet in trophic ecology via mixing models. Several assumptions of stable isotope mixing models are critical, i.e., constant trophic discrimination factor and isotopic equilibrium between the consumer and its diet. The isotopic turnover rate (λ and its counterpart the half-life) affects the dynamics of isotopic incorporation for an organism and the isotopic equilibrium assumption: λ involves a time lag between the real assimilated diet and the diet estimated by mixing models at the individual scale. Current stable isotope mixing model studies consider neither this time lag nor even the dynamics of isotopic ratios in general. We developed a mechanistic framework using a dynamic mixing model (DMM) to assess the contribution of λ to the dynamics of isotopic incorporation and to estimate the bias induced by neglecting the time lag in diet reconstruction in conventional static mixing models (SMMs). The DMM includes isotope dynamics of sources (denoted δs), λ and frequency of diet-switch (ω). The results showed a significant bias generated by the SMM compared to the DMM (up to 50% of differences). This bias can be strongly reduced in SMMs by averaging the isotopic variations of the food sources over a time window equal to twice the isotopic half-life. However, the bias will persist (∼15%) for intermediate values of the ω/λ ratio. The inferences generated using a case study highlighted that DMM enhanced estimates of consumer’s diet, and this could avoid misinterpretation in ecosystem functioning, food-web structure analysis and underlying biological processes.

How strongly does diet variation explain variation in isotope values of animal consumers?

“…The simplest, proper way to do so is to weight the isotopic or diet values at a past time t − τ by a factor dμ ( τ ) = re − rτ dτ (note that ). A maximal weight is given to present values while values at times earlier than approximately t − 5/ r (which is a somewhat arbitrary value, and is numerically close to and similar in concept to the value t − 2 log(2)/log( r ) suggested in a related formulation [ 24 ], being too far in the past, essentially do not contribute to present observations. In this formulation we assume that turnover rates are the same for all individuals, but of course this is another potential source of variation that has been identified as important in some systems [ 26 ] and may be worthy of further exploration for specific cases outside our more general investigation of the effects of diet variation.…”

“…While we frame our model in the context of variation among individuals, it provides equal insight into how variation in diet over time and/or space might manifest as variation with in a consumer. We show that as expected from the process of elemental incorporation, tissue turnover rates play a key role in determining how sensitive the consumer tissue will be to changes in diet [ 17 , 24 ]; a process that has previously been linked with the relative body masses of consumers and their prey [ 25 ]. While it is intuitive to state that consumers are likely never at equilibrium with their diets [ 11 ], we have shown that one can shift the interpretation such that consumer isotope values do represent an historic equilibrium averaged over a temporal window defined by their tissue turnover rate.…”

Identifying the limits where variation in consumer stable isotope values reflect variation in diet

“…While we frame our model in the context of variation among individuals, it provides equal insight into how variation in diet over time and/or space might manifest as variation with in a consumer. We show that as expected from the process of elemental incorporation, tissue turnover rates play a key role in determining how sensitive the consumer tissue will be to changes in diet (Fry, 2006;Ballutaud et al, 2022). While it is intuitive to state that consumers are likely never at equilibrium with their diets (Phillips et al, 2014), we have shown that one can shift the interpretation such that consumer isotope values do represent an historic equilibrium averaged over a temporal window defined by their tissue turnover rate.…”

Was that my meal? Uncertainty from source sampling period in diet reconstruction based on stable isotopes in a syngnathid fish

“…This study pointed out significant differences for some trophic features in the pipefish Syngnathus acus depending on the time lag applied for the assessment of the resources contributing to their biomass. The use of time lags is extremely important but rarely evaluated or applied in trophic studies (Possamai et al, 2021;Ballutaud et al, 2022). The time lag applied in the present study was based on several assumptions (Planas et al, 2020a).…”