Taxonomic resolution matters when studying stress responses of indicator taxa

Most freshwater ecosystems are subject to multiple anthropogenic stressors, which commonly reduce biodiversity across all levels. Existing freshwater bioassessment programmes aim at identifying responses of aquatic biota to stressors. For practical reasons, higher-level taxonomic groups (e.g. genus or family) are often used in these programmes. This approach, however, may bias assessment results as different species can differ substantially in their biological traits, thus emphasising the need for species-level data. 

Animal species have been used as indicators for decades to collect information about particular regions and/or habitats. In freshwater environments many taxa are strongly associated with natural habitats with good water quality and are therefore used as indicator species as opposed to other species which can thrive in highly modified waterbodies with poor water quality. 

For example in New Zealand Single Gill Mayflies of the genus Deleatidium indicate good habitat and water quality conditions, especially if other mayfly or stonefly groups are abundant as well. Sixteen species are known, however, the distribution range of many species is unknown and identification of both adults and larvae is anything but easy. This is clearly a case where molecular methods such as DNA Barcoding could be of great help. In fact there are a number of colleagues who claim that species identification using DNA Barcodes may allow assessing biodiversity and degradation of freshwater ecosystems in greater detail than classical approaches.

A new study by researchers from Germany and New Zealand represents a very good example for the validity of this claim. The colleagues compared stress–response patterns of bioindicator taxa (Deleatidium spp.) using both traditional morphology-based methods and DNA Barcoding.

As it turns out their Deleatidium sample consisted of twelve distinct clades that likely represent distinct cryptic species. Morphological analysis did not allow for further identification than genus. Finally, In a subsequent step they compared stressor responses assessed at genus and species level. 

While overall Deleatidium abundance was unrelated to stressor levels, some of the individual clades differed clearly in the magnitude and direction of their responses to nutrient and sediment stress...These contrasting patterns indicate that individual freshwater invertebrate species, often merged to a higher taxonomic level for biomonitoring purposes, can differ substantially in their tolerance to stressors and respond in more complex ways than observed at genus level.