Scale dependence of species–area relationships is widespread but generally weak in Palaearctic grasslands

Abstract

Questions Species–area relationships (SARs) are fundamental for understanding biodiversity patterns and are generally well described by a power law with a constant exponent z. However, z‐values sometimes vary across spatial scales. We asked whether there is a general scale dependence of z‐values at fine spatial grains and which potential drivers influence it. Location Palaearctic biogeographic realm. Methods We used 6,696 nested‐plot series of vascular plants, bryophytes and lichens from the GrassPlot database with two or more grain sizes, ranging from 0.0001 m² to 1,024 m² and covering diverse open habitats. The plots were recorded with two widespread sampling approaches (rooted presence = species “rooting” inside the plot; shoot presence = species with aerial parts inside). Using Generalized Additive Models, we tested for scale dependence of z‐values by evaluating if the z‐values differ with gran size and tested for differences between the sampling approaches. The response shapes of z‐values to grain were classified by fitting Generalized Linear Models with logit link to each series. We tested whether the grain size where the maximum z‐value occurred is driven by taxonomic group, biogeographic or ecological variables. Results For rooted presence, we found a strong monotonous increase of z‐values with grain sizes for all grain sizes below 1 m². For shoot presence, the scale dependence was much weaker, with hump‐shaped curves prevailing. Among the environmental variables studied, latitude, vegetation type, naturalness and land use had strong effects, with z‐values of secondary peaking at smaller grain sizes. Conclusions The overall weak scale dependence of z‐values underlines that the power function generally is appropriate to describe SARs within the studied grain sizes in continuous open vegetation, if recorded with the shoot presence method. When clear peaks of z‐values occur, this can be seen as an expression of granularity of species composition, partly driven by abiotic environment.

Fine-grain beta diversity in Palaearctic open vegetation: variability within and between biomes and vegetation types

“…Third, we found a clear decrease in explained variance (or in other words, in distinctness of the patterns) from vascular plants via complete vegetation to bryophytes and lichens. This is consistent with findings of two previous GrassPlot studies that looked at other aspects of fine-grain z-values Zhang et al 2021). Partially, this might be explained by the fact that, due to the generally low replication of smaller plots within a nested plot series Figure 5.…”

“…A recent study using GrassPlot (Dengler et al 2020a) found that nested SARs at the mentioned spatial scales can be well described with a power law, S = c A z , where S is species richness, A area and c and z modelled parameters (Arrhenius 1921;Dengler 2009). The exponent z only exhibits a small amount of scale dependence (Zhang et al 2021), meaning that for most purposes it is adequate to assume z to be constant within the range of grain sizes included in GrassPlot. As well as being a parameter of the power-law SAR, the z-value, when calculated using nested-plot data, is also a measure of β-diversity (Jurasinski et al 2009;.…”

Macroecology of vegetation — Lessons learnt from the Virtual Special Issue

“…Fine‐grain beta diversity was the topic of three studies based on the GrassPlot database. Based on the previous finding that the power function is generally the best approximation of the species–area relationship (SAR) even at very fine grains (10 −4 –10 3 m 2 ; Dengler et al, 2020); two studies used the modelled exponent of the power function ( z ‐value) as a measure of multiplicative beta diversity within nested‐plot series (Dembicz et al, 2021; Zhang et al, 2021). Dembicz et al (2021) found consistent differences in z ‐values between three studied taxonomic groups in relation to elevation and to land‐use intensity and used their findings to propose a new conceptual model for separating causes of fine‐grain beta diversity.…”

“…Dembicz et al (2021) found consistent differences in z ‐values between three studied taxonomic groups in relation to elevation and to land‐use intensity and used their findings to propose a new conceptual model for separating causes of fine‐grain beta diversity. Zhang et al (2021) went a step further and asked whether the small deviations from the power law show any regularities. They dissected the SAR into segments between two subsequent grain sizes to test for potential scale dependences between the respective ‘local’ z ‐values and found that the result depends on the way vegetation was sampled in the field.…”

“…A total of 15 contributions used fine‐grain plant community data to address macroecological questions at various extents: global (Kusumoto et al, 2021; Testolin et al, 2021), across the whole Palaearctic (Biurrun et al, 2021; Dembicz et al, 2021; Zhang et al, 2021), across Europe (Axmanová et al, 2021; Boonman et al, 2021; Padullés Cubino et al, 2021; Sporbert et al, 2021; Večera et al, 2021), larger parts of Europe (Cao Pinna et al, 2021; Wagner et al, 2021) or at state level (Bourgeois et al, 2021; Craven et al, 2021). Most of these studies rely on two large vegetation‐plot databases established and maintained by two working groups of the International Association for Vegetation Science (IAVS), the European Vegetation Archive (EVA; Chytrý et al, 2016) by the European Vegetation Survey (Axmanová et al, 2021; Boonman et al, 2021; Cao Pinna et al, 2021; Padullés Cubino et al, 2021; Sporbert et al, 2021; Večera et al, 2021; Wagner et al, 2021) and the GrassPlot database (Dengler et al, 2018) by the Eurasian Dry Grassland Group (Biurrun et al, 2021; Dembicz et al, 2021; Zhang et al, 2021). Testolin et al (2021) used data from the global vegetation‐plot database sPlot (Bruelheide et al, 2019), and four relied on regional data compilations (Bourgeois et al, 2021; Craven et al, 2021; Kusumoto et al, 2021; Tordoni et al, 2021).…”