New insights into small-scale species-area relationships and beta diversity due to the GrassPlot database

The post provided by Jürgen Dengler, Iwona Dembicz, Jinghui Zhang & Idoia Biurrun

This post refers to the articles “Fine-grain beta diversity of Palaearctic grassland vegetation” by Dembicz et al. (https://doi.org/10.1111/jvs.13045) and “Scale dependence of species-area relationships is widespread but generally weak in Palaearctic grasslands” by Zhang et al. (https://doi.org/10.1111/jvs.13044), both published in “Macroecology of vegetation” Special Feature in the Journal of Vegetation Science.

It is about one century that plant ecologists have become interested in the principles how the number of species (species richness) increases with area. Following the first mathematical model of species-area relationships (SARs) proposed by Arrhenius (1921), the power function, numerous other models have been suggested (see review by Dengler 2009). While for coarse-grain SARs, such as those of islands, several comprehensive studies have demonstrated that the power law is on average the best model (e.g. Triantis et al. 2012), comparable studies for fine-grain SARs were lacking up to recently. Moreover, in vegetation ecological literature, there is still a widespread belief that SARs of plant communities either follow a saturation function (i.e. do not increase beyond a certain area called minimum area) or they adhere to the Gleason model (i.e. a logarithmic function).

Jürgen Dengler became interested in SARs of grassland vegetation in the early years of the 21st century. At that time, he supervised the Diplom theses of Swantje Löbel and Steffen Boch at the University of Lüneburg, who studied the dry grassland vegetation of the Baltic islands of Öland and Estonia, respectively, using, among others, nested-plot sampling to analyse SARs (e.g. Löbel et al. 2006; Dengler & Boch 2008). At the IAVS Symposium 2007 in Swansea, UK, Jürgen Dengler and co-workers presented a talk on a comparative analysis of SARs in grasslands, based on 50 nested-plot series from various regions in Europe (Dengler et al. 2007). These initial studies, however, made it clear that much larger and more representative datasets were needed to reach conclusive results. Thus, Jürgen Dengler in 2009 started international research expeditions with the aim of sampling high-quality multi-scale and multi-taxon data of grassland vegetation, which later became the EDGG Field Workshops (see Dengler et al. 2016a), using a standardised methodology (Dengler et al. 2016b), which is now also widely applied elsewhere. These annual Field Workshops are conducted annually (or sometimes twice a year) in numerous different regions and grassland types across the Palaearctic (e.g. https://vegsciblog.org/2020/09/04/high-diversity-in-armenian-grasslands-edgg-field-workshop/; https://vegsciblog.org/2020/11/12/sampling-multi-scale-and-multi-taxon-plant-diversity-data-edgg-field-workshop/). Many EDGG members became “addicted” to these events and attended them whenever possible, including Iwona Dembicz and Idoia Biurrun, who have meanwhile overtaken the coordination from Jürgen Dengler. Finally, in spring 2017, the GrassPlot database was founded with an international workshop at the University of Bayreuth, Germany (Dengler et al. 2018). Meanwhile, GrassPlot contains 225 datasets from 49 countries, totalling 202,579 plots and 6,664 nested-plot series with at least four grain sizes (https://edgg.org/databases/GrassPlot).

Using this wealth of data, Dengler et al. (2020) tested which function describes the shape of fine-grain SARs in continuous vegetation best. Across a wide range of non-forest habitat types of the whole Palaearctic biogeographic realm, we found that the power function, in general, is the best model, while logarithmic and saturation functions (as often suggested in vegetation ecological literature) generally provide a poor fit to the data.

This strong support for the power function allowed us in a next step to focus solely on this model and ask which variables influence the slope parameter z of this function (Dembicz et al. 2021). It is important to note that z is not only a SAR parameter but also a particularly suitable measure of multiplicative beta diversity as we show in our paper. Before our study, little was known about the behaviour of z, and no theory of drivers of fine-grain beta diversity existed. Therefore, we used the multitude of in situ measured environmental variables available in GrassPlot as well as further variables derived via the plot coordinates to test whether z depends on them. We found that z-values of the three big taxonomic groups of vegetation differed systematically with lichens > vascular plants > bryophytes. Land use generally had a negative effect on z-values. Among all tested potential predictors, unexpectedly elevation (positive) and latitude (u-shaped with a minimum around 50° N) had a particularly strong influence. While some of our results contradict naïve assumptions on beta diversity, they are fully consistent with what is to be expected from the fact that beta diversity is mathematically linked to mean occupancy. We summarized all our findings in a conceptual figure that illustrates how the different groups of predictors could influence mean occupancy and beta diversity, respectively; thus, providing a range of testable theories for future beta diversity studies in continuous vegetation.

In the third paper on SARs based on GrassPlot data, Zhang et al. (2021) analysed whether the small deviations from the power law found by Dengler et al. (2020) follow any principles. For that, we used the concept of local z-values, i.e. we cut the SARs into many segments and calculated the slope parameter between two subsequent grain sizes in each nested-plot series, to test whether these local z-values show any type of scale dependence. We found that it makes a big difference whether vegetation is recorded with the shoot presence system or the rooted presence system (Williamson 2003; Dengler 2008). Both approaches to record vegetation are widely used, but many vegetation ecologists seemingly are not aware that two approaches exist and have strong effects on alpha and beta diversity. In the herbaceous vegetation covered by GrassPlot we found that z-values recorded with the rooted presence system strongly increase below 1 m² towards the smallest grain sizes, just as expected theoretically (Williamson 2003). For the shoot-presence system, by contrast, we found widespread but rather weak scale dependence, usually with a local maximum around 0.03 m².

With the completion of this series of three papers based on thousands of nested plots from a wide range of vegetation types across the whole Palaearctic biogeographic realm, a long scientific journey is completed, 20 years after the first steps. However, this is not the end of GrassPlot. Currently, half a dozen further studies on various aspects of vegetation diversity and ecology are in preparation, and more are planned for the future. If you have suitable data, it is therefore worthwhile to contribute them to EDGG’s collaborative database and thus gain the options to become a co-author of future papers and/or get access to the whole database for your projects. Likewise, EDGG, after two years of interruption due to the Covid-19 pandemic, will resume its annual Field Workshops in May 2022 with a latitudinal transect through the Ukrainian steppes, to be followed by many more exciting venues in subsequent years.

References

• Arrhenius, O. 1921. Species and area. Journal of Ecology 9: 95–99.
• Dembicz, I., Dengler, J., Steinbauer, M.J., Matthews, T.J., Bartha, S., Burrascano, S., et al. 2021. Fine-grain beta diversity of Palaearctic grassland vegetation. Journal of Vegetation Science 31: e13045. https://doi.org/10.1111/jvs.13045
• Dengler, J. 2008. Pitfalls in small-scale species-area sampling and analysis. Folia Geobotanica 43: 269–287.
• Dengler, J. 2009. Which function describes the species-area relationship best? – A review and empirical evaluation. Journal of Biogeography 36: 728–744. https://doi.org/10.1111/j.1365-2699.2008.02038.x
• Dengler, J. & Boch, S. 2008. Sampling-design effects on properties of species-area curves – a case study from Estonian dry grassland communities. Folia Geobotanica 43: 289–304.
• Dengler, J., Boch, S., Dolnik, C., Jeschke, M., Kiehl, K. & Löbel, S. 2007. Comparative analysis of species-area relationships in European dry grasslands – across regions, taxa, and scales. Oral presentation at the 50th IAVS Symposium, Swansea. DOI: https://doi.org/10.13140/RG.2.2.16654.89926
• Dengler, J., Biurrun, I., Apostolova, I., Baumann, E., Becker, T., Berastegi, A., et al. 2016a. Scale-dependent plant diversity in Palaearctic grasslands: a comparative overview. Bulletin of the Eurasian Dry Grassland Group 31: 12−26.
• Dengler, J., Boch, S., Filibeck, G., Chiarucci, A., Dembicz, I., Guarino, R., et al. 2016b. Assessing plant diversity and composition in grasslands across spatial scales: the standardised EDGG sampling methodology. Bulletin of the Eurasian Dry Grassland Group 32: 13−30.
• Dengler, J., Wagner, V., Dembicz, I., García-Mijangos, I., Naqinezhad, A., Boch, S., et al. 2018. GrassPlot – a database of multi-scale plant diversity in Palaearctic grasslands. Phytocoenologia 48: 331–347.
• Dengler, J., Matthews, T.J., Steinbauer, M.J., Wolfrum, S., Boch, S., Chiarucci, A., et al. 2020. Species-area relationships in continuous vegetation: Evidence from Palaearctic grasslands. Journal of Biogeography 60: 72–86. https://doi.org/10.1111/jbi.13697
• Löbel, S., Dengler, J. & Hobohm, C. 2006. Species richness of vascular plants, bryophytes and lichens in dry grasslands: The effects of environment, landscape structure and competition. Folia Geobotanica 41: 377–393.
• Triantis, K.A., Guilhaumon, F. & Whittaker, R.J. 2012. The island species-area relationship: biology and statistics. Journal of Biogeography 39: 215–239.
• Williamson, M. 2003. Species-area relationships at small scales in continuum vegetation. Journal of Ecology 91: 904–907.
• Zhang, J., Gillet, F., Bartha, S., Alatalo, J.M., Biurrun, I., Dembicz, I., et al. 2021. Scale dependence of species-area relationships is widespread but generally weak in Palaearctic grasslands. Journal of Vegetation Science 32: e13044. https://doi.org/10.1111/jvs.13044

Brief personal summaries:

Jürgen Dengler is Professor of Vegetation Ecology at the Zurich University of Applied Sciences (ZHAW) in Switzerland. He is co-founder of EDGG and GrassPlot and deeply interested in all aspects of Palaearctic grassland vegetation, particularly its biodiversity and underlying drivers.

Iwona Dembicz is Assistant Professor at the Institute of Environmental Biology of the University of Warsaw in Poland. Her scientific interests include ecology of grassland ecosystems, biodiversity patterns and nature conservation. She is involved in the management and development of GrassPlot.

Jinghui Zhang is Associate Professor at the Inner Mongolia University in China. He completed his postdoctoral project on species-area relationships (SARs) under the supervision of Jürgen Dengler in 2020. Subsequently, he became very interested in the maintenance mechanism of Palaearctic grassland biodiversity.

Idoia Biurrun is Associate Professor at the University of the Basque Country UPV/EHU in Spain and the GrassPlot database manager.

GrassPlot is the collaborative, multi-scale vegetation-plot database of EDGG, the IAVS Working Group dealing with biodiversity, ecology and conservation of Palaearctic grasslands