Does assessment of taxonomic completeness protect threatened species of river macroinvertebrates?

A central objective of environmental management is to preserve and maintain biodiversity, including populations of threatened species. However, consideration whether standard assessment of the condition of biota is also sufficient to protect species with conservation status has received little attention in freshwaters. We used data from 225 reference and impacted river sites in Finland to study whether the occurrence and abundance of threatened macroinvertebrate species (TS, Red-Listed by IUCN) are associated with assessed condition of macroinvertebrate assemblages (O/E-ratio of taxonomic completeness based on RIVPACS-type modelling). We suggested that a minimal acceptable condition, equal to e.g. 'good' ecological status described by EU Water Framework Directive, should also ensure the viability of TS populations. We defined an O/E between the 10th percentile of reference O/E-distribution and 10th percentile * 3/4 to show 'good' status; larger and smaller values indicating 'high' and poorer status, respectively. The number and abundance of TS, and occurrence of individual TS showed positive relationships with the O/E. However, TS were concentrated to 'high' status with only few occurrences and low abundance in the suggested 'good' status. The results imply that conventional class boundaries may not be efficient enough for preservation of threatened river macroinvertebrates.