Disturbance increases diversity by weakening competition rather than reducing intrinsic population growth
Physical disturbance is pervasive across ecosystems, yet how it shapes diversity remains unresolved. For over five decades, two major hypotheses offer contrasting explanations. One proposes that disturbance weakens heterospecific competition, whereas the other argues that it reduces intrinsic demographic rates thereby delaying competitive exclusion. We established 210 experimental plant communities, applied early- or late-season clipping, and used a Bayesian model with counterfactual simulations to separate these pathways. Only early clipping increased diversity across timescales; late clipping reduced it during the experiment and had no effect at equilibrium. The logic of the demographic hypothesis was supported: higher intrinsic demographic rates reduced diversity. Surprisingly, disturbance generally increased rather than reduced these rates, causing the demographic pathway to decrease diversity. In accordance with the competition hypothesis, disturbance weakened heterospecific competition across the interaction matrix, increasing diversity under both regimes. Yet nearly all this competition-mediated diversity increase was captured by weakened competitive effects of the two dominant grasses on the rest of the community. Together with disturbance-induced changes in the grasses' own dynamics, these effects nearly reproduced the full diversity response. Diversity increased only under early clipping because competitive release outweighed opposing changes in dominant-grass dynamics; under late clipping it did not. More broadly, disturbance effects on diversity emerge from opposing changes in intrinsic demography and competition, with their balance determining whether diversity increases or declines.