Assigning indicator taxa based on assemblage patterns: Beware of the effort and the objective!

Abstract

Environmental managers often use indicator taxa to monitor full biodiversity and hidden environmental factors. For identifying practical indicators from assemblage data collected in the field, it is crucial to remove irrelevant variation, which unfortunately is not a common practice. We demonstrate, based on field data from Estonian forests, (i) how an attractive indicator group of macrofungi, perennial polypores, loses its apparent indicator value when variation in study effort and conspicuous environmental factors have been reduced; (ii) that simply including survey effort variation is sufficient to create significant covariation between species richness of taxon groups, which has often been taken as a justification for indicator assignment. These results imply that standardizing study effort should become a requirement for any field study that reports indicator taxa based on covariance patterns. We encourage researchers to be explicit and critical about the practical value of indicator taxa when compared with direct measurement of habitat conditions.