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

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 m(2) to 1,024 m(2) 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 m(2). 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.

Automated summary

Based on data from the GrassPlot database, we found that z-values for rooted presence showed a strong monotonous increase with decreasing grain size below 1 m(2), while for shoot presence, the scale dependence was weaker and exhibited hump-shaped curves. Environmental variables such as latitude, vegetation type, naturalness, and land use were found to have strong effects on z-values, resulting in secondary peaks at smaller grain sizes.