Vascular plant species richness and bioindication predict multi-taxon species richness

Plants regulate soils and microclimate, provide substrate for heterotrophic taxa, are easy to observe and identify and have a stable taxonomy, which strongly justifies their use as indicators in monitoring and conservation. However, there is no consensus as to whether plants are strong predictors of total multi-taxon species richness. In this study, we investigate if general terrestrial species richness can be predicted by vascular plant richness and bioindication. To answer this question, we collected an extensive dataset on species richness of vascular plants, bryophytes, macrofungi, lichens, plant-galling arthropods, gastropods, spiders, carabid beetles, hoverflies, and genetic richness (operational taxonomic units = OTUs) from environmental DNA metabarcoding. We also constructed a Conservation Index based on threatened red list species. Besides using richness of vascular plants for prediction of other taxonomic groups, we also used plant-derived calibration of the abiotic environment (moisture, soil fertility and light conditions) as well as the degree of anthropogenic impact. Bivariate relationships between plant species richness and other species groups showed no consistent pattern. After taking environmental calibration by bioindication into account, we found a consistent, and for most groups significant, positive effect of plant richness. Plant species richness was also important for richness of fungal OTUs, Malaise OTUs and for the Conservation Index. Our multiple regression analyses revealed (a) a consistently positive effect of plant richness on other taxa, (b) prediction of 12%-55% of variation in other taxa and 48% of variation in the total species richness when bioindication and plant richness were used as predictors. Our results justify that vascular plants are strong indicators of total biodiversity across environmental gradients and broad taxonomic realms and therefore a natural first choice for biodiversity monitoring and conservation planning.