Ex 1: Wolves

Here we show how to do a mixing model analysis using the script version of MixSIAR. For a demonstration of the GUI version, see the MixSIAR Manual. For a clean, runnable .R script, look at mixsiar_script_wolves.R in the example_scripts folder of the MixSIAR package install:

mixsiar.dir <- find.package("MixSIAR")

You can run the wolves example script with:


While the GUI may be convenient for users less familiar with R, we advise using the script version of MixSIAR for several reasons:

  1. Repeatability: You can run different models and have a record of the commands that created each one. There are many reasons you’d want to do this. For example, you may want to compare model results with an uninformative prior vs. an informative prior, one error term option vs. another, grouping sources a priori vs. a posteriori, different MCMC run lengths, etc.

  2. Speed: Clicking through the GUI buttons can get onerous after a while.

  3. Installation ease: Some users aren’t able to install the GTK+ software that the GUI depends on (more issues on Mac). It may be worth figuring out the script version (R skills!) instead of figuring out how to get GTK+ installed.

The basic MixSIAR workflow is the same using a script or the GUI:

  1. Load data files:
  2. Pre-model checks
  3. Choose model structure options
  4. Run model
  5. Use diagnostics to decide if the model has converged
  6. Analyze output

Wolves Example

The “Wolves Example” uses data reconstructed from (not identical to) Semmens et al. 2009. Here, we investigate the diet of 66 wolves in British Columbia with:

Load MixSIAR package

Load mixture data

Load the mixture data, i.e. your:

The wolves consumer data has 2 covariates: Region and Pack, where Pack is nested within Region (fac_nested=c(FALSE,TRUE)). By “nested”, we mean that all wolves in a given pack are in the same region - each pack is entirely within one region. This is an excellent example of hierarchical structure, fit with 2 random effects (fac_random=c(TRUE,TRUE)).

Load source data

Load the source data, i.e. your:

If you look at wolves_sources.csv, you will see that each Region has different isotope values - this is specified with source_factors="Region". We do not have concentration dependence data here, so conc_dep=FALSE. We only have source summary statistics (Mean, SD, and sample size), not the original “raw”" data, so data_type="means". Note that wolves_sources.csv has a column titled “n”" with the sample size of each source estimate. This must be in your source data file when you run your data!

Load discrimination data

Load the discrimination data, i.e. your:

Plot data

This is your chance to check:

You should always look at the isospace plot—this is a good check that the data is loaded correctly, and that the isospace geometry makes sense. If the mixture data are well outside the source polygon, you have a serious violation of mixing model assumptions, and it must be true that either 1) You’re missing a source, or 2) You’re using an incorrect discrimination factor. MixSIAR, like SIAR, fits a residual error term, and thus will always find a solution even if it is nonsensical.

Also note that the MixSIAR isospace plot adds the discrimination means AND SDs to the raw source values. This is because model uses the source + discrimination values to fit the mixture data, calculated as \(\sqrt{\sigma^2_{source} + \sigma^2_{discr}}\), under the assumption of independence. Error bars indicate \(\pm\) 1 SD.

Calculate convex hull area

If 2 isotopes/tracers, calculate normalized surface area of the convex hull polygon(s) as in Brett (2014).

Note 1: discrimination SD is added to the source SD (see ?calc_area for details)

Note 2: If source data are by factor (as in wolf ex), computes area for each polygon (one for each of 3 regions in wolf ex)

## [1]  0.8750158 13.9726701 13.6440547

Plot prior

Define your prior, and then plot using “plot_prior”

Bayesian analyses require priors, and MixSIAR includes a plot_prior function to plot the prior on the mixture (diet) proportions (at the highest hierarchical level, p.global). The prior represents our knowledge about the proportions before we consider the biotracer data. A natural tendency is to want a flat/“uninformative” prior, where all values between 0 and 1 are equally likely. However, because proportions are not independent, there is no truly uninformative prior (e.g. the histograms are not flat). The best we can do with the Dirichlet distribution is set \(\alpha\) = c(1,1,1), which is uninformative on the simplex. In other words, all combinations of the proportions are equally likely. See the section titled “Constructing informative Bayesian priors” in the forthcoming MixSIAR paper.

Because the mean of the “uninformative” prior, \(\alpha\) = c(1,1,1), is \(\frac{1}{n.sources}\), we also call it the generalist prior. This reflects two facts: 1) it is not really uninformative, and 2) for weakly informative data it shifts the posterior towards a generalist diet, \(p_1 = p_2 = p_3 = \frac{1}{3}\). The amount of shift depends on the informativeness (quality and quantity) of the data.

Write JAGS model file

Write the JAGS model file (define model structure). The model will be saved as model_filename (“MixSIAR_model.txt” is default, but you may want to change if you create many different models).

There are 3 error term options available:

  1. Residual * Process (resid_err = TRUE, process_err = TRUE)
  2. Residual only (resid_err = TRUE, process_err = FALSE)
  3. Process only (resid_err = FALSE, process_err = TRUE)

In the Wolves Example we want the “Residual * Process” error option. The differences between “Residual * Process”, “Residual only”, and “Process only” are explained in Stock and Semmens (in revision).

Note: If you have only 1 mix datapoint, you have no information about the mixture/consumer variability. In this case, we use the original MixSIR error model (which does not fit a residual error term). This is the same behavior as siarsolo in SIAR.

Run model

Choose one of the MCMC run options:

run == Chain Length Burn-in Thin # Chains
“test” 1,000 500 1 3
“very short” 10,000 5,000 5 3
“short” 50,000 25,000 25 3
“normal” 100,000 50,000 50 3
“long” 300,000 200,000 100 3
“very long” 1,000,000 500,000 500 3
“extreme” 3,000,000 1,500,000 500 3

You can also set custom MCMC parameters, e.g:

Good idea to use run = "test" first to check if 1) the data are loaded correctly and 2) the model is specified correctly:

After a test run works, increase the MCMC run to a value that may converge

jags.1 will be an rjags object where you can access the MCMC chains for plotting, aggregating sources a posteriori, etc.

Analyze diagnostics and output

First you can set output options like file names, plot file types, etc. (see ?output_JAGS for details).

Then you can call output_JAGS to process diagnostics, summary statistics, and create posterior density plots:

For a thorough explanation of the output from output_JAGS, see the Wolves Example section of the MixSIAR Manual. You will also find examples of accessing the MCMC chains for post hoc plotting and analysis there.