An extension of presence/absence coefficients to abundance data: a new look at absence

Alternative community analyses, based on quantitative and presence/absence data, are comparable logically if the data type is the only factor responsi ble for differences among results. For presence/absence indices that consid er mutual absences, no quantitative alternatives are known. To facilitate s uch comparisons, a new family of similarity coefficients is proposed for ab undance data. Formally, this extension is achieved by generalizing the four cells of the usual 2 x 2 contingency table to the quantitative case. This implies an expanded meaning of absence: for a given species at a given site it is understood as the difference between the actual value and the maximu m detected in the entire study. The correspondence between 10 presence/abse nce coefficients and their quantitative counter-parts is evaluated by graph ical comparisons based on artificial data. The behaviour of the new functio ns is also examined using field data representing post-fire regeneration pr ocesses in grasslands and a chronosequence pertaining to forest regeneratio n after clear-cut. The examples suggest that the new coefficients are most informative for data sets with low beta-diversity and temporal back-ground changes.