A binary presence-absence matrix in which rows are species and columns
are sites.
Value
Returns the number of unique species pairs that never co-occur.
Details
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.
References
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.