Rainfall increases conformity and strength of species–area relationships

The positive relationship between species richness and area is regarded as one of the few laws in ecology. Therefore, deviations from predictable species–area scaling, evident as high residual variance in species–area curves, are often interpreted as anomalous behaviour. Small‐island systems often do not conform to species–area relationships, yet the high stochasticity in their species–area curves is frequently treated as unexplainable noise or attributed to idiosyncratic extinction rates. Here, we introduce a statistical framework that incorporates the degree of stochasticity in species–area relationships as an explicit, interpretable model parameter. Using a global island plant dataset for atolls (378 islands across 19 atolls) – prototypical examples for small‐island dynamics – we show that the degree of residual variance in species–area curves can be captured, modelled, and linked to environmental conditions. Our heteroscedastic modelling approach demonstrates that apparent stochasticity in species–area relationships is not random but predictable through environmental drivers. Specifically, we found that increased rainfall reduces the residual variance around the species–area curve, indicating that resource availability is a critical factor enabling conformity to species–area scaling. Cyclone disturbance frequency did not drive stochasticity, challenging the prevailing view that disturbance regimes drive the stochasticity in species–area scaling on small islands. By treating residual variance as an explicit model parameter in species–area relationships rather than unexplainable noise, our approach provides new insights into the conditions enabling biological communities to conform to species–area scaling. Shifting the focus in species–area studies on the residual variance as an interpretable model parameter that captures the degree of conformity to species–area scaling offers novel perspectives into the environmental factors prerequisite for species–area scaling. This contributes to unifying the apparent anomalous, stochastic nature of small‐island systems with the general law of linear species–area scaling.