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 larger scales. Environmental niches explained more (39%-64%) of the slope at larger scales, exceeding 50% at scales > 30 m, and these niches always captured the majority of the structured slopes. Environmental position (environmental mean values) effects were steady in absolute strength across scales and explained most (98%-68%) of the niche effect, but this proportion decreased with increasing scale. The effect of environmental heterogeneity increased with spatial scales, starting to rise at the 30 m scale after controlling for environmental position. Excluding soil properties from analyses strongly reduced these niche effects, highlighting the importance of soils for structuring the small-scale SAR. There was also substantial stochasticity in the SAR slopes, which was only partially explained by passive sampling.

Synthesis: Our results show that the small-scale SAR in the studied subtropical forest follows a power-law, exhibits a scale shift in slope at 30 m, and is strongly shaped by niche effects that are dominated by environmental position relative to heterogeneity. However, soil heterogeneity controls the increase of niche effect and shift in the SAR slope with increasing spatial scales. Hence, edaphic factors can be responsible for scale-dependence in small-scale SARs, thereby linking small-scale and large-scale SARs. 

Methods
See the associated Open Access paper for methodology