4.2 Metabarcoding without taxonomic identification
Metabarcoding can provide ecological information even if reference databases are not available, as molecular taxonomic units can allow the comparison of communities among sites with environmental differences (Jiet al. 2013; Apothéloz-Perret-Gentil et al. 2017). The taxonomy-free approach allows overcoming the fact that, despite intensive efforts, databases remain incomplete for many taxa (Fig. 3). Primers with high taxonomic coverage and resolution are essential also in this case. High taxonomic coverage is needed to avoid under-representation of some taxa, while resolution allows teasing apart related taxa. Related taxa can have very different ecological properties, and some widespread taxa, tolerant to human disturbance, can be closely related to highly sensitive specialists (Caro et al.2005). Therefore, ecological responses of communities can remain obscured if metabarcodes are not able to resolve related taxa with different ecology. Our study focused on European taxa, where taxonomic knowledge is particularly good (Moustakas & Karakassis 2005; Rodrigueset al. 2010; Brewer et al. 2012) and, with targeted studies, we could envisage an improvement of database completeness in the next years. However, our results on primer performance can be also useful in megadiverse, tropical areas, where taxonomy-free biomonitoring can be a viable option (Andersen et al. 2019).