Species Removal Dampens the Scale Dependency of Ecological Determinism and Stochasticity in Coastal Communities.

Deterministic and stochastic processes control community dynamics. However, the responses of both processes to the loss of foundation species, which strongly influence community dynamics across spatial scales, are unclear. We experimentally examined how spatial extent and foundation species removal affect rocky-intertidal community dynamics over 3 years in eight field sites spanning ~1000 km along the southeastern Pacific. The normalised stochasticity ratio (NST), which distinguishes between stochastic (> 50%) and deterministic (< 50%) community dynamics, decreased with spatial extent for sessile and mobile species, with consistently lower values under foundation species removal for sessile communities. The effect of foundation species removal on NST was strongest in smaller sessile communities and diminished as spatial extent increased, while mobile communities showed no significant response to the disturbance. Our experimental results demonstrate that the loss of foundation species disrupts the scale dependency of ecological mechanisms, highlighting its negative implications for biodiversity conservation and ecosystem functioning.