Function to calculate number of unique species combinations in a matrix
a binary presence-absence matrix in which rows are species and columns are sites.
In Diamond's (1975) assembly rules model, species interactions lead to certain "forbidden combinations" of species. A set of communities structured this way should contain fewer species combinations than expected by chance.
Returns the number of unique species combinations represented by the different columns (= sites) in the matrix.
This metric is most useful when the number of sites (= columns) is relatively large compared to the number of species (= rows). Empty sites are excluded from the matrix and are not counted as a unique species combination.
Diamond, J.M. 1975. Assembly of species communities. p. 342-444 in: Ecology and Evolutoin of Communities. M.L. Cody and J.M. Diamond (eds.). Harvard University Press, Cambridge.
Pielou, D.P. and E.C. Pielou. 1968. Association among species of infrequent occurrence: the insect and spider fauna of Polyporus betulinus (Bulliard) Fries. Journal of Theoretical Biology 21: 202-216.
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