by Frederik De Laender, Andrew Gonzalez, Olivia Bleeckx, Dieter Ebert, György BarabásWe provide a general theoretical explanation for a longstanding result in spatial ecology: weak dispersal among habitat patches promotes local species richness. Using analytical approximations of spatial Lotka–Volterra competition models, we show that local species persistence in heterogeneous landscapes can be expressed as the balance between the total density across patches and the local exclusion rate. We further demonstrate that local multispecies coexistence is governed by the feasibility domain, linking spatial coexistence to a structural property of nonspatial competitive systems. Together, these results explain why weak dispersal increases local species richness and why this effect strengthens with the number of available patches, i.e., landscape size. We test these predictions using numerical simulations and find that the theory breaks down only when both dispersal and competitive interactions are very strong, in which case dispersal has a unimodal effect on coexistence. In contrast, landscape size retains a positive effect on coexistence whenever an effect is detectable. We then apply the theory to long-term data from a natural metacommunity of microcrustaceans. We detect strong competition among species and find no detectable effect of dispersal rate on local coexistence, whereas species co-occurrence increases with local landscape size, as predicted by theory. Together, our results identify how dispersal, interaction strength, and landscape size jointly regulate local species richness in competitive systems.