Stable Isotope Mixing Models in R with simmr

Introduction

tl;dr see the Quick Start vignette

simmr is a package designed to solve mixing equations for stable isotopic data within a Bayesian framework. This guide is designed to get researchers up and running with the package and giving them a full list of all the available features. No expertise is required in the use of R.

simmr is designed as an upgrade to the SIAR package and contains many of the same features. This new version contains a slightly more sophisticated mixing model, a simpler user interface, and more advanced plotting features. The key differences between SIAR and simmr are:

• simmr has a slightly richer mixing model based on code from the Parnell et al 2013 Environmetrics paper
• simmr does not have a menu system; all commands must be run through the console or script windows
• simmr uses ggplot2 to create graphs and JAGS to run the mixing model

We assume that you have a sound working knowledge of stable isotopic mixing models, and the assumptions and potential pitfalls associated with these models. A list of required reading is presented in Appendix A of this guide. We strongly recommend reading Philips et al 2015, Canadian Journal of Zoology for the basic assumptions and dos and don’ts of running mixing models.

We assume that if you have got this far you have installed R. We also recommend installing Rstudio as this provides a very neat interface to use R and simmr. The instructions below all assume you are using Rstudio.

If you find bugs in the software, or wish to suggest new features, please add your input to the simmr GitHub issues page.

Installation of the simmr package

The simmr package uses the JAGS (Just Another Gibbs Sampler) programmer to run the stable isotope mixing model. Before you install simmr, visit the JAGS website and download and install JAGS for your operating system.

Next, start Rstudio and find the window with the command prompt (the symbol >). Type

install.packages("simmr")

It may ask you to pick your nearest CRAN mirror (the nearest site which hosts R packages). You will then see some activity on the screen as the simmr package and the other packages it uses are downloaded. The final line should then read:

package 'simmr' successfully unpacked and MD5 sums checked

You then need to load the package. Type

library(simmr)

This will load the simmr package and all the associated packages. You’ll need to type the library(simmr) command every time you start R. If you haven’t installed JAGS properly you will be informed at this point.

Considerations before running simmr

Before getting started there are a couple of points to consider.

How to run simmr

The general structure for running simmr is as follows:

1. Call simmr_load on the data to get it into the right format
2. Plot the data in isotope space (‘iso-space’) using plot
3. Run the mixing model with simmr_mcmc or simmr_ffvb
4. Check the model converged with summary
5. Check the model fit is calibrated with posterior_predictive
6. Explore the results with plot and summary, and prior_viz. If you have multiple groups and want to compare output between them, use the `compare_groups’ function

For the next part of this document, we concentrate on simple examples without grouping structure.

mix <- matrix(c(
  -10.13, -10.72, -11.39, -11.18, -10.81, -10.7, -10.54,
  -10.48, -9.93, -9.37, 11.59, 11.01, 10.59, 10.97, 11.52, 11.89,
  11.73, 10.89, 11.05, 12.3
), ncol = 2, nrow = 10)
colnames(mix) <- c("d13C", "d15N")
s_names <- c("Zostera", "Grass", "U.lactuca", "Enteromorpha")
s_means <- matrix(c(-14, -15.1, -11.03, -14.44, 3.06, 7.05, 13.72, 5.96), ncol = 2, nrow = 4)
s_sds <- matrix(c(0.48, 0.38, 0.48, 0.43, 0.46, 0.39, 0.42, 0.48), ncol = 2, nrow = 4)
c_means <- matrix(c(2.63, 1.59, 3.41, 3.04, 3.28, 2.34, 2.14, 2.36), ncol = 2, nrow = 4)
c_sds <- matrix(c(0.41, 0.44, 0.34, 0.46, 0.46, 0.48, 0.46, 0.66), ncol = 2, nrow = 4)
conc <- matrix(c(0.02, 0.1, 0.12, 0.04, 0.02, 0.1, 0.09, 0.05), ncol = 2, nrow = 4)

simmr_in <- simmr_load(
  mixtures = mix,
  source_names = s_names,
  source_means = s_means,
  source_sds = s_sds,
  correction_means = c_means,
  correction_sds = c_sds,
  concentration_means = conc
)

plot(simmr_in)

plot(simmr_in,
  xlab = expression(paste(delta^13, "C (per mille)",
    sep = ""
  )),
  ylab = expression(paste(delta^15, "N (per mille)",
    sep = ""
  )),
  title = "Isospace plot of example data"
)

simmr_out <- simmr_mcmc(simmr_in)

simmr_out_ffvb <- simmr_ffvb(simmr_in)

summary(simmr_out, type = "diagnostics")

## 
## Summary for 1

## R-hat values - these values should all be close to 1.

## If not, try a longer run of simmr_mcmc.

##     deviance      Zostera        Grass    U.lactuca Enteromorpha     sd[d13C] 
##            1            1            1            1            1            1 
##     sd[d15N] 
##            1

post_pred <- posterior_predictive(simmr_out)

print(post_pred)

prior_viz(simmr_out)

summary(simmr_out, type = "statistics")

## 
## Summary for 1

##                mean    sd
## deviance     39.343 4.978
## Zostera       0.232 0.137
## Grass         0.276 0.089
## U.lactuca     0.272 0.039
## Enteromorpha  0.220 0.125
## sd[d13C]      0.744 0.296
## sd[d15N]      0.666 0.302

summary(simmr_out, type = "quantiles")

## 
## Summary for 1

##                2.5%    25%    50%    75%  97.5%
## deviance     32.830 35.584 38.356 41.832 51.811
## Zostera       0.030  0.120  0.213  0.326  0.528
## Grass         0.119  0.216  0.270  0.330  0.477
## U.lactuca     0.202  0.246  0.271  0.296  0.351
## Enteromorpha  0.033  0.119  0.205  0.304  0.490
## sd[d13C]      0.323  0.544  0.695  0.881  1.478
## sd[d15N]      0.243  0.463  0.616  0.799  1.367

plot(simmr_out, type = "density")

plot(simmr_out, type = "matrix")

## Registered S3 method overwritten by 'GGally':
##   method from   
##   +.gg   ggplot2

compare_sources(simmr_out,
  source_names = c("Zostera", "U.lactuca")
)

## Prob (proportion of Zostera > proportion of U.lactuca) = 0.379

compare_sources(simmr_out,
  source_names = c(
    "Zostera",
    "U.lactuca",
    "Enteromorpha"
  )
)

## Most popular orderings are as follows:

##                                    Probability
## Zostera > U.lactuca > Enteromorpha      0.3161
## Enteromorpha > U.lactuca > Zostera      0.2664
## U.lactuca > Enteromorpha > Zostera      0.1967
## U.lactuca > Zostera > Enteromorpha      0.1583
## Zostera > Enteromorpha > U.lactuca      0.0317
## Enteromorpha > Zostera > U.lactuca      0.0308

How to run simmr on multiple groups

In many cases we have data from different sampling locations, or different types of individuals (e.g. male/female) and we to compare between these groups. simmr can handle these data sets provided they all share the same sources, corrections and concentration dependence values.

A useful data set is given by Inger et al 2006 and provided as part of the original SIAR package. These data concern Brent Geese observed on 8 separate sampling periods.

These data are included in the package, and we can load these data into R with:

data(geese_data)

…and into simmr with:

simmr_groups <- with(
  geese_data,
  simmr_load(
    mixtures = mixtures,
    source_names = source_names,
    source_means = source_means,
    source_sds = source_sds,
    correction_means = correction_means,
    correction_sds = correction_sds,
    concentration_means = concentration_means,
    group = groups
  )
)

Note that the group object above is specified to be a factor but can also be an integer, the levels of which will appear in plots. However, when specifying groups in later commands you should use the integer values to reference which groups to plot

Next it is a matter of following the simmr commands as before to load in, with an extra argument specifying the groups:

When we create the isospace plot we can specify which groups we wish to plot:

plot(simmr_groups,
  group = 1:8,
  xlab = expression(paste(delta^13, "C (per mille)",
    sep = ""
  )),
  ylab = expression(paste(delta^15, "N (per mille)",
    sep = ""
  )),
  title = "Isospace plot of Inger et al Geese data",
  mix_name = "Geese"
)

In the above code group = 1:8 can be changed specify any of the available groups. For example group = 2 would plot just sampling period 2, or group = c(1,3:7) would plot just sampling period 1 and 3 to 7.

The command for running the simmr model is identical to before:

simmr_groups_out <- simmr_mcmc(simmr_groups)

or

simmr_groups_out_ffvb <- simmr_ffvb(simmr_groups)

simmr will automatically run the model for each group in turn. This may take slightly longer than a standard single group run.

The summary command works the same as before. By default they will produce output for all groups, or you can specify the groups individually, e.g.:

summary(simmr_groups_out,
  type = "quantiles",
  group = 1
)
summary(simmr_groups_out,
  type = "quantiles",
  group = c(1, 3)
)
summary(simmr_groups_out,
  type = c("quantiles", "statistics"),
  group = c(1, 3)
)

For plotting output with multiple groups you can only specify a single group to plot. This is so that you are not overwhelmed with plots:

plot(simmr_groups_out,
  type = "boxplot",
  group = 2,
  title = "simmr output group 2"
)
plot(simmr_groups_out,
  type = c("density", "matrix"),
  group = 6,
  title = "simmr output group 6"
)

Whilst you can use the compare_sources function for multi-group data, there is also an extra function for comparing a single source between groups via the compare_groups function. This allows for probabilistic output and plots comparing a single source across different groups. The simplest use is where you want to compare just two groups:

compare_groups(simmr_groups_out,
  source = "Zostera",
  groups = 1:2
)

This produces the probability of the group 1 dietary proportion of Zostera being greater than that of group 2. It also produces a boxplot of the difference between the dietary proportions and will save this into a new object if specified. You can turn the plot off by adding the argument plot = FALSE.

If you specify more than two groups simmr will produce the most likely probabilistic orderings of the groups as well as the boxplot as before:

compare_groups(simmr_groups_out,
  source = "Zostera",
  groups = 1:3
)

Combining sources

A common request is that of combining sources. We would recommend always doing this after running simmr, known as a-posteriori combining. Suppose for example, you wish to combine the U.lactuca and Enteromorpha sources which lie in a similar region in the isospace plot of the Geese data. To proceed, we can create a new simmr object using the combine_sources function:

simmr_out_combine <- combine_sources(simmr_out,
  to_combine = c(
    "Grass",
    "Enteromorpha"
  ),
  new_source_name = "Grass+Ent"
)
plot(simmr_out_combine$input)

plot(simmr_out_combine,
  type = "boxplot",
  title = "simmr output: combined sources"
)

This will also work with multiple sources and/or multiple groups:

simmr_groups_out_combine <- combine_sources(simmr_groups_out,
  to_combine = c(
    "Zostera",
    "U.lactuca",
    "Enteromorpha"
  ),
  new_source_name = "U.Lac+Ent+Zos"
)
plot(simmr_groups_out_combine$input,
  group = 1:8
)
plot(simmr_groups_out_combine,
  type = "boxplot",
  title = "simmr output: combined sources",
  group = 8
)
plot(simmr_groups_out_combine,
  type = "matrix",
  title = "simmr output: combined sources",
  group = 8
)

# And we can now compare sources across groups on this new data set
compare_groups(simmr_groups_out_combine,
  source = "U.Lac+Ent+Zos",
  group = 1:3
)

Running simmr with only one isotope

simmr will run fine with only one tracer, and no changes should be required to any of the functions. Here is an example with only one isotope:

mix <- matrix(c(
  -10.13, -10.72, -11.39, -11.18, -10.81, -10.7, -10.54,
  -10.48, -9.93, -9.37
), ncol = 1, nrow = 10)
colnames(mix) <- c("d13C")
s_names <- c("Zostera", "Grass", "U.lactuca", "Enteromorpha")
s_means <- matrix(c(-14, -15.1, -11.03, -14.44), ncol = 1, nrow = 4)
s_sds <- matrix(c(0.48, 0.38, 0.48, 0.43), ncol = 1, nrow = 4)
c_means <- matrix(c(2.63, 1.59, 3.41, 3.04), ncol = 1, nrow = 4)
c_sds <- matrix(c(0.41, 0.44, 0.34, 0.46), ncol = 1, nrow = 4)
conc <- matrix(c(0.02, 0.1, 0.12, 0.04), ncol = 1, nrow = 4)

Now load in with simmr_load:

simmr_in_1D <- simmr_load(
  mixtures = mix,
  source_names = s_names,
  source_means = s_means,
  source_sds = s_sds,
  correction_means = c_means,
  correction_sds = c_sds,
  concentration_means = conc
)

Create a plot. plot.simmr_input automatically creates a 1D version of these plots:

plot(simmr_in_1D)

Now run simmr:

simmr_run_1D <- simmr_mcmc(simmr_in_1D)

## Compiling model graph
##    Resolving undeclared variables
##    Allocating nodes
## Graph information:
##    Observed stochastic nodes: 10
##    Unobserved stochastic nodes: 5
##    Total graph size: 83
## 
## Initializing model

or

simmr_run_1D_ffvb <- simmr_ffvb(simmr_in_1D)

Plot output

plot(simmr_run_1D, type = "boxplot")

The other summary, compare and plot functions should all work the same.

Setting up your own prior distributions

Most Bayesian models will work better when you include informative prior distributions on the dietary proportions. For arguments as to why you should use prior informative information (and where you could get it from), see here. simmr tries to make this easier for practitioners by including a specific function (`simmr_elicit’) for including prior information.

We will use the simmr_out object created above. A reminder about the posterior values

summary(simmr_out, type = "quantiles")

All these dietary proportions are very similar. We now suppose we had prior information (e.g. from stomach or fecal contents) that the mean dietary proportions were

proportion_means <- c(0.4, 0.3, 0.2, 0.1)

…and proportion standard deviations:

proportion_sds <- c(0.08, 0.02, 0.01, 0.02)

We put this into the simmr_elicit function as follows:

prior <- simmr_elicit(
  4, proportion_means,
  proportion_sds
)

This may take a few moments to run as the code tries to optimise the parameters of a prior distribution which matches these means and standard deviations, which sometimes may not be exactly possible.

When finished, the model can be run using these prior distributions:

simmr_out_informative <- simmr_mcmc(simmr_in,
  prior_control =
    list(
      means = prior$mean,
      sd = prior$sd
    )
)

The new quantiles are:

summary(simmr_out_informative,
  type = "quantiles"
)

We can plot these priors with their posteriors

prior_viz(simmr_out_informative)

Customising plots

Many of the plots in simmr can be customised by adding on extra options just like a standard ggplot. For example:

plot(simmr_in) + xlim(-100, 100) + ylim(-100, 100)

or even on the output:

plot(simmr_groups_out,
  type = "boxplot",
  group = 2,
  title = "simmr output group 2"
) +
  ylim(0, 0.5)

Note that the above actually changes the x-axis and not the y-axis despite the above command. This is because the coordinates are flipped in the ggplot.

Other options you might like to customise include labels on axes, titles/subtitles, etc. More complicated changes can be made but these involve changing the ggplot commands that simmr uses in the background.

Here is an example where I change the colours of the boxplots:

# First extract the dietary proportions
simmr_out2 <- simmr_out$output[[1]]$BUGSoutput$sims.list$p
colnames(simmr_out2) <- simmr_out$input$source_names

# Now turn into a proper data frame
df <- reshape2::melt(simmr_out2)
colnames(df) <- c("Num", "Source", "Proportion")

# Finally create the new variable that you want to colour by
df$new_colour <- "Type 2"
df$new_colour[df$Source == "Zostera"] <- "Type 1"

# And create the plot
ggplot(df, aes_string(
  y = "Proportion", x = "Source",
  fill = "new_colour", alpha = 0, 5
)) +
  geom_boxplot(notch = TRUE, outlier.size = 0) +
  theme_bw() +
  ggtitle("simmr output boxplot with changed colours") +
  theme(legend.position = "none") +
  coord_flip()

Other advanced use of simmr

Whilst the above gives an introduction to the basic functions of simmr, the package is open source and all code is open to editing. The two objects created as part of this vignette simmr_in and simmr_out are R lists. They can be explored with e.g.

str(simmr_in)

which will show their contents. The simmr_out object in particular allows for full access to all of the posterior dietary proportion samples. We can calculate for example the mean of the Zostera dietary proportion on the first (or only) group:

mean(simmr_out$output$`1`$BUGSoutput$sims.list$p[, "Zostera"])

## [1] 0.2319473

The backquotes around the 1 are required above because they specify which group (the first). We can thus find the probability that the posterior dietary proportion for Zostera is bigger than for Grass:

mean(simmr_out$output$`1`$BUGSoutput$sims.list$p[, "Zostera"]
> simmr_out$output$`1`$BUGSoutput$sims.list$p[, "Grass"])

## [1] 0.3919444

With more detailed R knowledge, it is possible to create scripts which run multiple data sets in richer fashions than the default simmr functions. See the help file help(simmr_mcmc) for a full list of examples.

Appendix - suggested reading

For the maths on the original SIAR model:
Andrew C Parnell, Richard Inger, Stuart Bearhop, and Andrew L Jackson. Source partitioning using stable isotopes: coping with too much variation. PLoS ONE, 5(3):5, 2010.

For the geese data:
Inger, R., Ruxton, G. D., Newton, J., Colhoun, K., Robinson, J. A., Jackson, A. L., & Bearhop, S. (2006). Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis. Journal of Animal Ecology, 75, 1190–1200.

For the maths behind the more advanced JAGS models:
Andrew C. Parnell, Donald L. Phillips, Stuart Bearhop, Brice X. Semmens, Eric J. Ward, Jonathan W. Moore, Andrew L. Jackson, Jonathan Grey, David J. Kelly, and Richard Inger. Bayesian stable isotope mixing models. Environmetrics, 24(6):387–399, 2013.

For some good advice about mixing models:
Donald L Phillips, Richard Inger, Stuart Bearhop, Andrew L Jackson, Jonathan W Moore, Andrew C Parnell, Brice X Semmens, and Eric J Ward. Best practices for use of stable isotope mixing models in food-web studies. Canadian Journal of Zoology, 92(10):823–835, 2014.