How to compute rarefaction on very small floating point abundances

[dmcglinn]

A common method in invertebrate sampling is to report a relative abundance distribution and the total number of individuals. From this one can reconstruct the community matrix but often many species have very small fractional abundances (i.e., less than 1). Our methods are currently not very sensitive to computing diversities at these fractional abundance levels but in theory the rarefaction method should still apply as defined by the lgamma functions which can tolerate non-integer abundances.