Long-term analyses of biodiversity data highlight a ‘biodiversity conservation paradox’: biological communities show substantial species turnover over the past century1,2, but changes in species richness are marginal1,3,4,5. Most studies, however, have focused only on the incidence of species, and have not considered changes in local abundance. Here we asked whether analysing changes in the cover of plant species could reveal previously unrecognized patterns of biodiversity change and provide insights into the underlying mechanisms. We compiled and analysed a dataset of 7,738 permanent and semi-permanent vegetation plots from Germany that were surveyed between 2 and 54 times from 1927 to 2020, in total comprising 1,794 species of vascular plants. We found that decrements in cover, averaged across all species and plots, occurred more often than increments; that the number of species that decreased in cover was higher than the number of species that increased; and that decrements were more equally distributed among losers than were gains among winners. Null model simulations confirmed that these trends do not emerge by chance, but are the consequence of species-specific negative effects of environmental changes. In the long run, these trends might result in substantial losses of species at both local and regional scales. Summarizing the changes by decade shows that the inequality in the mean change in species cover of losers and winners diverged as early as the 1960s. We conclude that changes in species cover in communities represent an important but understudied dimension of biodiversity change that should more routinely be considered in time-series analyses.
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The R code for retrieving resurvey ID × species × time interval combinations and that was used to calculate the results presented in this paper is provided in Supplementary Code 1 and is available at https://github.com/idiv-biodiversity/ReSurveyGermany_Analysis. The R code that was used to produce the null models in Supplementary Code 2 is available at https://github.com/idiv-biodiversity/ReSurveyGermany_null_models.
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We are grateful to the surveyors who recorded vegetation in the field and provided these data. We acknowledge the data contributors who made their data available to us or helped in recording these data: T. Dittmann, A. Erfmeier, B. Gerken, K. Günther, S. Heinz, W. Hakes, H. Heklau, A. Henrichfreise, E. Hüllbusch, A. Huwer, A. Immoor, S. L. Kühn, B. Krause, S. Leonhardt, J. Reinecke, U. Scheidel, I. Vollmer and E. Wagner. We thank D. Bowler for her analysis of spatial representativeness; V. Hahn and S. Bernhard for their advice on Figs. 2 and 3; and T. Muer, the Regensburgische Botanische Gesellschaft and the Haupt Verlag for the permit to use the photographs from Floraweb.de for Fig. 4. We appreciate the support for the strategic project sMon by the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, funded by the German Research Foundation (DFG-FZT 118, 202548816).
The authors declare no competing interests.
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Extended data figures and tables
The coloured lines indicate the start and the end of a project, black diamonds show in which years surveys were made. Resurvey type refers to either studies that were repeated within a particular community across a site without attempts to match plots (community comparison), or were carried out on matched plots, which were either permanently marked or relocated from exact descriptions (semi-permanent). The lower graph shows the number of times a particular year was included in any of the time series.
The temporal change of species richness (SR) in plot records (a–c) and mean cover change of species (d–f) is shown separately for short (≤ 2 years), medium (> 2 and ≤ 10 years) and long observation intervals (> 10 years). The black dashed line shows zero change, while the red solid line in a)–c) shows the mean change of richness and in d)–f) the species’ median change in cover in percentage points. According to a mixed effects model estimated mean overall effect size was in a) +0.025 (p = 3.9 x 10−9, df = 4,142), b) +0.007 (p = 0.093, df = 3,903) and c) −0.150 (p < 2 x 10−16, df = 8,612). In d)–f) plot Interval comparisons of the mean of all cover changes per species between time points Y1 and Y2 of the start and end year, respectively, are shown on an axis with a sign*square root-transformation. According to an exact binomial test estimated overall median of cover change was in d) 0 (95 per cent confidence interval 0 and 0.007), e) −0.02 (CI −0.02 and 0) and f) −0.26 (CI −0.53 and 0.002).
Interval comparisons of species richness (SR) in plot records between time points Y1 and Y2 of the start and end year, respectively, and divided by the length of the interval in decades ((Y2-Y1)*10) (n = 13,987). Estimated overall effect size was +0.062 according to a mixed effects model (p = 1.8 x 10−7) with a 95% confidence interval between +0.039 and +0.086.
The temporal change of species richness (SR) in plot records (a–c) and mean cover change of species (d–f) is shown shown separately for small (> 25 m2), medium-size (25 m2) and large plots (>25 m2). The black dashed line shows zero change, while the red solid line in a)–c) shows the mean change of richness and in d)–f) the species’ median change in cover in percentage points. According to a mixed effects model estimated mean overall effect size was in a) −0.03 (p = 0.064, df = 487), b) −0.031 (p = 1.55 x 10−13, df = 4,204) and c) −0.095 (p < 2 x 10−16, df = 9,124). In d)–f) plot Interval comparisons of the mean of all cover changes per species between time points Y1 and Y2 of the start and end year, respectively, are shown on an axis with a sign*square root-transformation. According to an exact binomial test estimated overall median of cover change was in d) −0.017 (95 per cent confidence interval −0.065 and −0.001), e) −0.019 (CI −0.043 and −0.006) and f) −0.26 (CI −0.134 and −0.050).
The histograms show the interval comparisons of plot records between time points Y1 and Y2 of the start and end year, respectively. The black dashed line shows the zero change, while the red solid line shows the mean change as predicted from a mixed effects model. a) Change in Shannon’s index of diversity (H). Estimated mean effect size for H −0.055 (p = 2.2 x 10−16, df = 5,462, 95% confidence interval −0.064 and −0.047). b) Change in Pielou’s index of evenness (E). Estimated mean effect size for E −0.019 (p = 2.6 x 10−16, 95% confidence interval −0.024 and −0.015). c) Difference in the area under the rank abundance curves. Estimated mean difference −0.143 (p = 0.00211, 95% confidence interval −0.194 and −0.091). d) Difference in the number of cover gains and losses. Estimated mean difference −0.407 (p = 7.9 x 10−7, 95% confidence interval −0.569 and −0.246). e) Change in mean cover of all the species in a plot (in per cent covered ground). Estimated mean effect size for mean cover +0.025 (p = 1.0 x 10−10, 95% confidence interval +0.018 and +0.033). f) Change in median cover of all the species in a plot (per cent of covered ground). Estimated mean effect size for median cover −0.007 (p = 0.2984, 95% confidence interval −0.021 and +0.007).
Plot Interval comparisons of the mean of all cover changes per species in percentage points between time points Y1 and Y2 of the start and end year, respectively, shown on an axis with a sign*square root-transformation. The black dashed line shows the zero change, while the red solid line shows the median change in cover across all species. All species in the dataset were included (n = 1,794). Estimated overall median of cover change was −0.0625 (95 per cent confidence interval −0.089 and −0.035) and significantly different from zero according to an exact binomial test (p < 0.001).
One or several of the total of n = 23,641 plot records are summarized under the same plot resurvey ID (n = 7,738). Note that the more complete coverage of Bavaria resulted from including the grassland monitoring Bavaria which started in 200275. The map was produced using rnaturalearthdata (free vector and raster map data at naturalearthdata.com).
Each time series was assigned to the habitat type by using the earliest plot record that matched with the level 3 EUNIS classification. The classification was based on the EUNIS-ESy expert system56 using the R code implementation57. ?: plots not assigned to any level 3 EUNIS habitat type, +: assigned to more than one level 3 EUNIS habitat type, A: Marine habitats, C: Inland surface waters, H: Inland sparsely vegetated habitats or devoid of vegetation, N: Coastal habitats, Q: Wetlands, R: Grasslands and lands dominated by forbs, mosses or lichens, S: Heathlands, scrub and tundra, T: Forests and other wooded land, V: Vegetated man-made habitats, including arable land. Labels for EUNIS habitats were only printed at the top of the corresponding bar section when the number of assigned records was ≥ 150.
Supplementary Methods 1 and 2. Supplementary Methods 1 shows the steps of data preparation and analysis and Supplementary Methods 2 contains an illustration of the null model scenarios.
A list of all projects included in the study.
A list of all taxa that were harmonized across all projects.
A list of all taxon names that were adapted within projects.
The R code to retrieve resurvey ID x species x time interval combinations and to calculate the results.
The R code to produce the null models.
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Jandt, U., Bruelheide, H., Jansen, F. et al. More losses than gains during one century of plant biodiversity change in Germany. Nature 611, 512–518 (2022). https://doi.org/10.1038/s41586-022-05320-w
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