Scale-dependent species-area relationship: niche-based versus stochastic processes in a typical subtropical forest

Determining the patterns and drivers of the small-scale species-area relationship (SAR) is crucial for improving our understanding of community assembly and biodiversity patterns. Niche-based and stochastic processes are two principal categories of mechanisms potentially driving SARs. However, their relative importance has rarely been quantified rigorously owing to scale-dependence and the simplified niche volumes often used.
In a fully mapped, 24-ha plot of a typical subtropical forest, we built the SARs and well-defined niche-hyper-volumes of a broad range of environmental variables at scales of 10 - 70 m (cell sizes). We then simulated passive sampling and partitioned the variances of the SAR slopes to disentangle the two contrasting mechanisms.
We found that the small-scale SAR best followed a power-law relationship, consistent with large-scale SARs. The SAR slope declined with increasing scale; it was lower than expected under passive sampling at scales below 30 m and higher at la...