The BITS model is a Bayesian hierarchical model that simulates the processes of isotope incorporation, turnover, output into incremental tissues, and data averaging during sampling. The modeling approach is demonstrated using the 87Sr/86Sr system, and built with a two-compartment isotope exchange mechanism, with the central pool being a rapidly-exchanging but relatively small pool, and the peripheral pool being a slowly-exchanging but relatively large pool. A steady state is assumed so that pool sizes are constant. No fractionation is assumed between the pools (for 87Sr/86Sr). The model comprises of 4 components: the model input, process model, data model for tissue growth, and data model for 87Sr/86Sr measurements. The model is demonstrated using 3 model studies explained below.
Model study 1 is the Calibration, which is used to estimate turnover parameters using high-resolution 87Sr/86Sr measurements on the ivory from an African Savanna Elephant (Loxodonta africana) that was relocated to a different 87Sr/86Sr baseline.
Model study 2 is the Fidelity test, which uses the same turnover parameters in the Calibration to recover the 87Sr/86Sr history associated with the known relocation.
Model study 3 is the Case study, which is used to estimate possible 87Sr/86Sr intake history from published ivory measurements from an Alaskan Woolly Mammoth (Wooller et al., 2021).
The "data" folder contains both data used in this study, and published data from Wooller et al., (2021) for the case studies.
The code was developed in R (ver. 4.0.5) calling the standalone JAGS (Just Another Gibbs Sampler) program, which is required before the code can be run. The JAGS program can be downloaded via this link. Please make sure to download the version that is appropriate for your operating system.
To call JAGS from RStudio, the R packages "rjags" and "R2jags" are also required.
Other R packages specified in the file headers help to visualize data.
Yang et al., (in revision), BITS: a Bayesian Isotope Turnover and Sampling model for strontium isotopes in proboscideans and its potential utility in movement ecology, Methods in Ecology and Evolution.
Yang, D. (2023), BITS: a Bayesian Isotope Turnover and Sampling model. https://github.com/SPATIAL-Lab/BITS