run.mar(data, variables=NULL, restrictions=NULL, search=c("random","exhaustive",
"fwdstep","exhaustive.true"), boot=500, ntop=10, export=FALSE)data indicating how each taxon column should be assigned
for the analysis (first two values for the time-block and date columns should be 0):
FALSE to skip bootstrappingsearch="random", "exhaustive", or "exhaustive.true", the number of top best-fit models from the random search to be returned for potential comparison to the selected best-fit modelexport.MAR is executed at the end of the analysis. export.MAR creates a new directory and saves all components of the MAR model object in that directory as csv filMAR containing:variables argumentrestrictions argumentsearch argument$bestfit and $bootstrap:mean means of variates' stationary distributions
covariance covariance matrix of stationary distributionresiduals E
covariance sigma
corrmatrix correlation matrixeigB eigenvalues of the B matrix
detB determinant of the B matrix
maxeigB max eigenvalue of B matrix
maxeigkrB max eigenvalue of B matrix kronecker productssigma.over.Vinf -tr(sigma)/tr(Vinf)
maxeigBxB max eigenvalue of B'B matrix ("worst-case" reactivity)$bootstrap will be NULL. Otherwise, in addition to the statistics above, $bootstrap will also contain a $limits list with the upper and lower 95% confidence limits of the best-fit model elements.
If search="random", "exhaustive", or "exhaustive.true", the result will also contain $top.bestfit, an array of the top best-fit models tested during the model search (the first of which is the best-fit model that was selected). The number of top models returned may be less than the value set for the ntop argument if ntop exceeds the potential number of model configurations that can be tested for the selected variables and search method. The dimension of the array in which each top best-fit model is stored is named by its respective AIC value.variables or restrictions arguments are not provided, the function creates
windows that allow the user to pick which column variables in data should be included in the MAR model as variates or covariates and to set restrictions on potential interactions between model variables. Unless the variables argument is provided in the function call, the restrictions argument must be NULL.
Search Types:
If search="random", which is the default, a random search is performed to find the best-fit model (as determined by AIC) for the included variate time-series. For each variate, 100 random models are constructed according to the restrictions that were set, and the model with the lowest AIC of these models is retained. This process is repeated 100 times, resulting in 100 "best-of-100" models. If any variable occurs in less than 15 of the 100 "best-of-100" models, that variable is discarded (i.e., the probability of that variable occurring in the random search is set to 0) and the search is repeated until the number of variables in successive searches remains constant (resulting in at least 2 search iterations per variate unless all variables are retained in the first iteration). The model with the lowest AIC of the final 100 "best-of-100" models is retained.
If search="exhaustive", a search through possible models for each variate with respect to restrictions is performed using a leap and bound algorithm (Furnival and Wilson,1974) to find the "best-fit" (lowest AIC) model of all potential variable combinations without explicitly examining all possible subsets.
If search="fwdstep", the best-fit model for each variate is built up from the NULL intercept model by sequentially adding whichever variable most improves the model AIC from the pool of potential variables. The model from the series with the lowest AIC is retained.
If search="exhaustive.true", a true exhaustive search through all potential variable combinations with respect to restrictions is performed.
Statistics:
The coefficients of the final "best-fit" MAR model for all variates are attained using least-squares estimation. The coefficients of the B- and C-matrices represent interaction strengths of the column variables on the row variables. If the data were z-scored prior to analysis (see prepare.data), the A intercept values will not be significantly different from 0. Estimates of the stationary distribution mean and covariance for each variate, of the process errors, and of community stability (resilience and reactivity) are calculated following Ives (2003). If bootstrap is not set to FALSE, these statistics are also calculated for the bootstrapped model.prepare.data, plot.MAR, export.MAR
Packages used for exhaustive search methods:
leaps, bestglm## These examples take 1-2 minutes to run
## construct a MAR model using 'run.mar' arguments to set variables and restrictions ##
data(L4.mar)
myvar<-c(0,0,0,1,1,0,0,0,1,1,1,1,0,0,1,1,0,0,2,2,2) # 8 variates, 3 covariates
myres<-matrix(0.5,nrow=length(which(myvar==1)),
ncol=length(which(myvar!=0))) # no restrictions (all 0.5)
run1<-run.mar(L4.mar, variables=myvar, restrictions=myres, search="exhaustive")
run1 # only some elements of the object are printed
str(run1) # to see all elements
summary(run1) # some summary statistics for the model
plot(run1)
# set a few restrictions on taxa interactions
myres[1,c(1,6,9)]<-c(1,0,0) # included, not included, not included
# re-run the analysis with same variates as 'run1' and new restrictions
run1b<-run.mar(L4.mar,run1,myres,"exhaustive")
plot(run1,run1b)
# 'run1' variables and restrictions with a different search method
run1c<-run.mar(L4.mar,run1,run1,"fwdstep")
plot(run1,run1c,legend=TRUE) # plot with legend
## construct a MAR model using windows to select variables and restrictions ##
run2<-run.mar(L4.mar,search="exhaustive")
run2
summary(run2)
plot(run2)Run the code above in your browser using DataLab