Function to calculate number of unique pairs of species that never co-occur and form a "checkerboard pair".
A binary presence-absence matrix in which rows are species and columns are sites.
In Diamond's (1975) assembly rules model, pairs of species that never co-occur in any site are interpreted as examples of interspecific competition. A set of communities structured this way should contain more checkerboard pairs than expected by chance.
Returns the number of unique species pairs that never co-occur.
Diamond, J.M. 1975. Assembly of species communities. p. 342-444 in: Ecology and Evolution of Communities. M.L. Cody and J.M. Diamond (eds.). Harvard University Press, Cambridge.
Connor, E.F. and D. Simberloff. 1979. The assembly of species communities: chance or competition? Ecology 60: 1132-1140.
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