Detecting indicator species: Some extensions of the IndVal measure

The indicator value (IndVal) of a species has long been the most popular measure to express species importance in community classifications. Nevertheless, a few problems concerning the original definition of IndVal still require clarification and some modifications are also in order so as to exploit the capabilities of the method more fully. In particular, we propose novel component terms (specificity, concentration and fidelity) that may be incorporated in the calculation of IndVal and also suggest some minor, although important terminological amendments. We argue that the choice among these terms should largely depend on whether the target classification is based on abundance or presence-absence data. The expanded capabilities of the approach and the sensitivity of IndVal variants to the sharpness of classifications are illustrated by actual examples coming from a benthic macroinvertebrate survey along the Danube River and a study of dolomite grassland communities in the Buda Hills, Hungary. We found that analyses by the original IndVal plus the new variants may give a more complete picture on any classification than a particular selection among component terms of IndVal. The use of several indices simultaneously is particularly recommended when selection of indicator species is the primary objective of the study.