v_ratio: SchlutersVRatio Co-occurrence Metric
In EcoSimR: Null Model Analysis for Ecological Data

Description Usage Arguments Details Value Note References Examples

Takes a binary presence-absence matrix or a matrix of abundances and returns Schluter's (1984) variance ratio.

1	v_ratio(m = matrix(rbinom(100, 1, 0.5), nrow = 10))

`m`	a binary presence-absence matrix in which rows are species and columns are sites. The entries may be either abundances or occurrences of indivdual species.

The variance ratio is the ratio of the variance in species number among sites to the sum of the variance of the species occurrences. If the average covariation in abundance (or occurrence) of each species pair is close to zero, the expected value for this ratio is approximately 1.0. V-ratios larger than 1.0 imply positive average covariation in the abundance of species pairs, whereas V-ratios significantly smaller than 1.0 imply negative average covariation.

Returns the variance ratio of the matrix.

This index is determined exclusively by the row and column sums of the matrix, so it cannot be used with null model algorithms that hold both of those elements fixed. A simple randomization of the rows of the matrix (see sim2) assumes that all sites are equiprobable, so it may generate large values (= positive covariance) that reflect heterogeneity among sites.

Schluter, D. 1984. A variance test for detecting species associations, with some example applications. Ecology 65: 998-1005.

McCulloch, C.E. 1985. Variance tests for species association. Ecology 66: 1676-1681.