fit_ssm {bsam}

Fit Bayesian state-space models to animal movement data

Description

Fits state-space models to animal tracking data. User can choose between a first difference correlated random walk (DCRW) model, a switching model (DCRWS) for estimating location and behavioural states, and their hierarchical versions (hDCRW, hDCRWS). The models are structured for Argos satellite data but options exist for fitting to other tracking data types.

Usage

fit_ssm(
  data,
  model = "DCRW",
  tstep = 1,
  adapt = 10000,
  samples = 5000,
  thin = 5,
  span = 0.2
)

Arguments

data

A data frame containing the following columns, "id","date", "lc", "lon", "lat". "id" is a unique identifier for the tracking dataset. "date" is the GMT date-time of each observation with the following format "2001-11-13 07:59:59". "lc" is the Argos location quality class of each observation, values in ascending order of quality are "Z", "B", "A", "0", "1", "2", "3". "lon" is the observed longitude in decimal degrees. "lat" is the observed latitude in decimal degrees. The Z-class locations are assumed to have the same error distributions as B-class locations.

Optionally, the input data.frame can specify the error standard deviations for longitude and latitude (in units of degrees) in the last 2 columns, named "lonerr" and "laterr", respectively. These errors are assumed to be normally distributed. When specifying errors in the input data, all "lc" values must be equal to "G". This approach allows the models to be fit to data types other than Argos satellite data, e.g. geolocation data. See dat4jags for other options for specifying error parameters.

WARNING: there is no guarantee that invoking these options will yield sensible results! For GPS data, similar models can be fit via the moveHMM package.

model

name of state-space model to be fit to data. This can be one of "DCRW", "DCRWS", "hDCRW", or "hDCRWS"

tstep

time step as fraction of a day, default is 1 (24 hours).

adapt

number of samples during the adaptation and update (burn-in) phase, adaptation and updates are fixed at adapt/2

samples

number of posterior samples to generate after burn-in

thin

amount of thinning of to be applied to the posterior samples to minimize within-chain sample autocorrelation

span

parameter that controls the degree of smoothing by stats::loess, used to obtain initial values for the location states. Smaller values = less smoothing. Values > 0.2 may be required for sparse datasets

Details

The models are fit using JAGS 4.2.0 (Just Another Gibbs Sampler, created and maintained by Martyn Plummer; http://martynplummer.wordpress.com/; http://mcmc-jags.sourceforge.net). fit_ssm is a wrapper that first calls dat4jags, which prepares the input data, then calls ssm or hssm, which fit the specified state-space model to the data, returning a list of results.

Value

For DCRW and DCRWS models, a list is returned with each outer list elements corresponding to each unique individual id in the input data Within these outer elements are a "summary" data.frame of posterior mean and median state estimates (locations or locations and behavioural states), the name of the "model" fit, the "timestep" used, the input location "data", the number of location state estimates ("N"), and the full set of "mcmc" samples. For the hDCRW and hDCRWS models, a list is returned where results, etc are combined amongst the individuals

Author(s)

Ian Jonsen

References

Jonsen ID, Mills Flemming J, Myers RA (2005) Robust state-space modeling of animal movement data. Ecology 86:2874-2880

Block et al. (2011) Tracking apex marine predator movements in a dynamic ocean. Nature 475:86-90

Jonsen et al. (2013) State-space models for biologgers: a methodological road map. Deep Sea Research II DOI: 10.1016/j.dsr2.2012.07.008

Jonsen (2016) Joint estimation over multiple individuals improves behavioural state inference from animal movement data. Scientific Reports 6:20625

Examples

# Fit DCRW model for state filtering and regularization - 
# using trivial adapt & samples values for speed
data(ellie1)
fit <- fit_ssm(ellie1, model = "DCRW", tstep = 4, adapt = 10, samples = 100, 
              thin = 1, span = 0.2)


# Fit DCRWS model for state filtering, regularization and behavioural state estimation - 
# using trivial adapt & samples values for speed
 fit.s <- fit_ssm(ellie1, model = "DCRWS", tstep = 2, adapt = 10, samples = 100, 
                thin = 1, span = 0.2)
 diag_ssm(fit.s)
 map_ssm(fit.s)
 plot_fit(fit.s)
 result.s <- get_summary(fit.s)

# fit hDCRWS model to > 1 tracks simultaneously
# this may provide better parameter and behavioural state estimation 
# by borrowing strength across multiple track datasets - 
# using trivial adapt & samples values for speed
data(ellie2)
hfit.s <- fit_ssm(ellie2, model = "hDCRWS", tstep = 2, adapt = 10, samples = 100, 
                thin = 1, span = 0.2)
diag_ssm(hfit.s)
map_ssm(hfit.s)
plot_fit(hfit.s)
result.hs <- get_summary(hfit.s)