So far, we have run models with the spades function.

spades() only has a few arguments:

- debug
- .plotInitialTime
- .saveInitialTime
- progress
- cache

What if we want to do more stuff?

What if we have a stochastic model and need many independent (Monte Carlo) runs?

The experiment function does this.

?experiment # for details and examples

Generally, however, this creates problems because of the computational time required …

The replicates argument

It is as easy as replicates = 2 to do two replicates:

?experiment # for details

# See Example 5
#  ... run mySim from top of help file
sims <- experiment(mySim, replicates = 2)
attr(sims, "experiment")$expDesign # shows 2 reps of same expt

What does it return?

What is the attr?

Replication is necessary but it multiplies the time it takes to run any model.

Fortunately, replication can use parallel processing:

• This is the simplest, yet most effective, parallel processing;
• No communication between individual simulations, until the end;
• So they can be spread across threads on a single computer or among computers;
• Requires a cluser (cl) argument.

• SpaDES functions that are parallel aware:

  • experiment, POM, splitRaster

• In each case, there are two ways:

## option 1:
raster::beginCluster() # make a cluster silently in background

## option 2:
cl <- parallel::makeCluster() # need to explicitly pass the
                              # cluster object into functions
                              # using cl argument

Try it!

?experiment
## see example 1
experimentParams <- list(fireSpread = 
                           list(spreadprob = list(0.2, 0.23),
                                nFires = list(20, 10)),
                         caribouMovement = 
                           list(N = list(100, 1000)))
sims <- experiment(mySim, params = experimentParams)
attr(sims, "experiment")

Often, a spades call will generate output files. Where do they go when using experiment?

?experiment
## see example 4
sims <- experiment(mySim, 
                   params = experimentParams, 
                   dirPrefix = c("expt", "simNum"))
attr(sims, "experiment")$expVals # shows 8 alternative
               #experiment levels, 24 unique parameter values

dir(outputPath(mySim))

Exercise:

Pick a few examples and try to understand them. (NOTE: they get more complicated as you get towards the bottom.)

?experiment

Clearly, using experiment can take a lot of resources, but there are a few options:

• Use your own computer, a local cluster, a server, cloud and more;

• Access Compute Canada for free (though sometimes the queues are long):

  • Blog post on using Compute Canada

  • There is a different example here: ssh among Linux machines

    • Shows how to use a Linux cluster connected via SSH;
    • Can be very effective because is interactive (unlike normal compute clusters).

• Ecological models use equations and functions
• These have parameters

• Parameters can be estimated from:

  • directly from data (e.g., regression, machine learning)
  • indirectly using (e.g., pattern oriented modeling) (e.g., Grimm et al. Science 2005, Marucco & McIntire J Appl Ecol 2010)

• (Just a brief discussion of this here)
• Remember, we are using R!
• So you can use any statistical method that R has
• Mixed models, non-linear models, machine learning, bayesian

nTrees <- 50
diamCM <- rlnorm(nTrees, meanlog = 2, sdlog = 0.7)
height <- rnorm(nTrees, sqrt(diamCM)*2 + rnorm(nTrees))
glmObj <- glm(height ~ diamCM)
plot(diamCM, height)
summary(glmObj)

• These methods often have predict methods associated with them
• So, instead of using "fixed" parameters in simulation models

• We can use these methods which give more accurate prediction:

  • given uncertainty,
  • random effects,
  • covariance matrix

# predict with new data
predVals <- predict(glmObj, newdata = data.frame(
  diamCM = rlnorm(nTrees, meanlog = 2, sdlog = 0.7)
))

• In R, everything is an object
• The glmObj and predictVals are objects

• These have many features that can be used in simulations

  • plot methods
  • standard errors, covariance matrices
  • random effects terms
• For the first 25 years of simulation modeling in ecology, parameters were copied into simulation models and hard coded
• This made it difficult to update the simulation model if there are new data and the regression needs to be re-fit

• Using SpaDES we can now fit the data and use the parameters all inside the simulation

Pattern Oriented Modeling (POM)

• What does this mean?

• Optimization is the process of minimizing some objective function
• Heuristic optimization is when there is no derivative-based solution
• Requires simulation-based approaches
• In R, currently, DEoptim seems to be the best optimization package for heuristic optimization
• POM function in SpaDES uses this package

There are several examples in ?POM, which we will go through:

## example 1
# What is example 1 doing?
# Run it
# What does the result mean?

• What would be next?

• What would be next?

## example 2
# What is example 2 doing?
# Run it
# What does the result mean?

• What would be next?

## example 3 
# What is example 3 doing?
# Run it
# What does the result mean?