Infer species-association network

Description

Infers a species association network by determining which co-occurrence patterns between species are more or less likely than expected under a null model of community assembly. Defaults to estimation of association using a robust shrinkage estimator for inverse covariance matrices.

Usage

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make_netassoc_network(obs, nul=vegan::permatfull(obs)$perm[[1]], 
  method="partial_correlation", args=list(method="shrinkage",verbose=FALSE),
  p.method="fdr", alpha=0.05, numnulls=1000, 
  plot=TRUE,plot.legend=TRUE, plot.title=TRUE, verbose=TRUE)

Arguments

obs

A m x n community matrix describing the abundance or presence/absence of m species at n sites. Represents the observed data.

nul

A m x n community matrix describing the abundance or presence/absence of m species at n sites. Represents the regional null expectation data. The default value is a resampling of the observed data that preserves row and column sums, but this default method is not recommended.

method

The name of a function used to calculate relationships between species. The function must accept at least the arguments mat, a m x n (species x site) matrix. Defaults to partial_correlation.

args

A list of additional arguments to be passed to the method function.

p.method

The method used to correct p-values for multiple comparisons. See p.adjust for options.

alpha

Analysis-wide Type I error rate, controlled via the argument p.method.

numnulls

Number of resamples of the nul matrix used to assemble null communities. Larger values produce more accurate results.

plot

If TRUE, plots all intermediate matrices calculated by the algorithm. Can be used to visualize input and output.

plot.title

If TRUE, adds titles to diagnostic plots.

plot.legend

If TRUE, adds legends to diagnostic plots.

verbose

If TRUE, prints status updates and progress bars during calculations.

Details

Steps taken are:

1) obtaining input data and trimming to eliminate species that do not occur in any site 2) resampling a set of null community matrices from the expectation with the same richness and abundance as the observed community 3) calculating species co-occurrence scores for each pair of species within the observed matrix and all resampled null matrices 4) calculating standardized effect sizes and p-values for species' co-occurrence scores 5) thresholding effect sizes to retain only significant associations 6) converting matrix of scores to association network

The resulting network can be analyzed using functions from the igraph network package.

The user should specify a nul matrix of the same dimensionality as obs based on some regional distribution modeling approach (e.g. MaxEnt). The default reshuffling method is not recommended but provided to allow immediate output from the function.

This process by default builds a Gaussian graphical model via estimating an inverse covariance matrix (precision matrix, which can be used to calculate partial correlation coefficients) for all species pairs. This graph is then compared to a distribution of null graphs, such that the final output is a graph with edge weights corresponding to standardized effect sizes after correction for multiple comparisons.

A range of different methods are provided in partial_correlation for estimating relationships between species. Note that while a method is provided for the graphical lasso (L1-regularization) its use is not recommended, as it will produce very sparse null networks and then a narrow (or singular) distribution of null edge weights.

The inverse covariance methods implemented in partial_correlation result in symmetric association metrics. Non-symmetric metrics (e.g. describing predation or commensalism) are possible mathematically but their usage is not well-established. For an example of how to implement these, see pairwise_association.

Value

A list with the following components:

matrix_spsite_obs

Trimmed obs matrix

matrix_spsite_nul

Trimmed nul matrix

matrix_spsp_obs

Observed co-occurrence scores for all species

matrix_spsp_ses_thresholded

Observed co-occurrence scores for all species after removing those with non-significant p-values

matrix_spsp_pvalue

P-values for all species after correction for multiple comparisons

network_all

An igraph object representing the association network

network_pos

An igraph object representing an association network including only positive associations

network_pos

An igraph object representing an association network including only negative associations

See Also

vegan::permat

Examples

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set.seed(1)
nsp <- 10
nsi <- 50
m_obs <- floor(matrix(rpois(nsp*nsi,lambda=5),ncol=nsi,nrow=nsp))
m_nul <- floor(matrix(rpois(nsp*nsi,lambda=5),ncol=nsi,nrow=nsp))

m_obs[1,1:(nsi/2)] <- rpois(n=nsi/2,lambda=20)
m_obs[2,1:(nsi/2)] <- rpois(n=nsi/2,lambda=20)

n <- make_netassoc_network(m_obs, m_nul,
  method="partial_correlation",args=list(method="shrinkage"),
  p.method='fdr', 
  numnulls=100, plot=TRUE,alpha=0.05)
  
# experimental demonstration of non-symmetric metrics  
#n <- make_netassoc_network(m_obs, m_nul,
#  method="pairwise_association",args=list(method="condentropy"),
#  p.method='fdr', 
#  numnulls=100, plot=TRUE,alpha=0.05)

n$network_all