FR                      Forage Ratio (Strauss 1979).
ivlev                   Ivlev's electivity (Ivlev 1961). Returns
                        Ivlev's electivity index from vectors of
                        consumed and available food items.
manlysalpha             Manly's alpha feeding preference (Chesson
                        1978). Returns Manly'as alpha index from
                        vectors of initial and final food item counts.
