Vascular plants are strong predictors of 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 the use of plants as bioindicators in monitoring and conservation. However, insects and fungi make up the vast majority of species. Surprisingly, it remains untested whether plants are strong predictors of total multi-taxon species richness. To answer this question, we collected an extensive data set on species richness of vascular plants, bryophytes, macrofungi, lichens, plant-galling arthropods, gastropods, spiders, carabid beetles, hoverflies and OTU richness from environmental DNA metabarcoding. Plant species richness per se was a moderate predictor of richness of other taxa. Taking an ecospace approach to modelling, the addition of plant-derived bioindicators revealed 1) a consistently positive effect of plant richness on other taxa, 2) prediction of 12-55% of variation in other taxa and 48 % of variation in the total species richness.


INTRODUCTION
for more details on site selection and stratification. The field inventory aimed at an unbiased and representative assessment of the multi-taxon bryophytes. For the remaining taxa, which are more demanding to find, catch, and identify, we The bioinformatic processing of the sequence data followed the strategy outlined in widely acknowledged (e.g., Bálint et al. 2016) that species richness is difficult to estimate from 1 8 4 sequencing data of environmental DNA, Frøslev et al. (2017) showed that careful bioinformatics 1 8 5 processing can produce richness measures based on OTU data with strong correlation to richness 1 8 6 metrics based on survey data. For this study, a simple OTU count was used as a DNA based 1 8 7 richness metric, after ensuring that variation in sequencing depth between samples only had a  A metric of conservation value was produced to test if plants can predict the richness of species 1 9 1 of conservation concern. For vascular plants, macrofungi, lichens, gastropods, spiders and 1 9 2 arthropods we used the national red list for Denmark (Wind & Pihl 2004). For taxonomic groups 1 9 3 lacking a national red list (bryophytes and galling arthropods) an expert-based red listing was 1 9 4 created for this project using the same criteria as the official red lists (bryophyte expert: Irina 1 9 5 Goldberg, galling arthropod expert: Hans Henrik Bruun). Each red listed species contributed to a 1 9 6 weighted score of threatened species per site (the Conservation Index) as follows: red list status 1 9 7 RE (regionally extinct) and CR (critically endangered) = 4 points, red list status EN (moderately 1 9 8 endangered) = 3 points, red list status VU (vulnerable) = 2 points, and red list status NT (near 1 9 9 threatened) and DD (data deficient) = 1 point. We used field-measured abiotic variables to validate the plant-based environmental calibration. Environmental recordings and estimates included soil pH, soil C, N and P, soil moisture, leaf N 2 0 3 1 1 and P concentrations, air temperature, light intensity, soil surface temperature, and humidity, 2 0 4 number of trees >40dbh, dead wood volume, and vapor pressure deficit (VPD). For further 2 0 5 details on methods for collection of the abiotic data see (Brunbjerg et al. 2017a).  etc.). For each species in the dataset a natural habitat score was calculated as: Where f () = frequency of species in the mentioned habitat category. The species level score is thus a number between 0 and 1, where 0 implies that the species only occurs in agricultural biotopes and 1 implies that the species only occurs in habitats of 2 2 0 conservation concern. The natural habitat index was calculated for each site as the mean of 2 2 1 species scores. It reflects land use and land use history under the assumption that protected 2 2 2 natural areas have been less intensively managed than farmland habitats. We used Spearman rank correlation to test for correlations between species richness of vascular vapor pressure deficit (VPD)).

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We also grouped the 130 study sites into five different land use intensity categories from 2 3 3 protected Annex 1 habitats, over other uncultivated areas, plantation forest and extensively 2 3 4 farmed habitats to intensive farmland. ANOVA followed by Tukey's post hoc tests was used to 2 3 5 test for differences in mean natural habitat index value between the five habitat types.

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To assess the efficiency of plants as indicators for other taxonomic groups, we performed GAM smoothers fitted to the residuals of the models were conservatively significant (p < 0.01) 2 4 6 Fig. 1). In all cases, except carabid beetles, the relationship between plant species richness and 2 9 8 other groups was positive (Fig. 1).

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Multiple regression of species richness of the selected taxa varied in percent explained and Conservation Index and 10-16% for fungal OTU richness, malaise OUT richness and galling 3 0 5 insects (Fig 2). eukaryote OTU richness and carabid species richness (Fig. 1).

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In monitoring programs, plants are often used as general indicators of conservation status of European flora. Our approach may still be applicable in other parts of the world because species  Since an estimated > 85 % of the World's terrestrial species remain undescribed (Mora et al.  surrogacy. It has long been acknowledged that plants serve as mutualistic partners for other  Pennisi, E. (2005). What determines species diversity? Science, 309, 90. The biodiversity of species and their rates of extinction, distribution, and protection. Science, 5 6 0 344, 1246752. diversity predicts beta but not alpha diversity of soil microbes across grasslands worldwide. Ecol. Lett., 18, 85-95.  richness of other taxonomic groups and OTU richness.