Mid Sweden University

Nestedness is a pattern whereby species-poor assemblages are composed

of subsets of the species occurring in richer assemblages. One of the

most commonly used measures of the degree of nestedness for

presence-absence matrices is the 'discrepancy' metric. A hitherto

neglected property of that metric is that it may take several values

for a given site-by-species matrix in the presence of ties in the

marginal totals. This complicates the quantification of nestedness for

the observed presence-absence matrix, as well as the assessment of

statistical significance, which is typically achieved through Monte

Carlo simulations. A solution to the problem is to calculate the

minimum discrepancy using a modified algorithm involving permutations

of columns with tied totals.