A central goal in ecology is to understand what maintains species diversity in local communities. Classic ecological theory1,2posits that niches dictate the maximum number of species that can coexist in a community and that the richness of observed species will be below this maximum only where immigration is very low. A new alternative theory3,4is that niches, instead, dictate the minimum number of coexisting species and that the richness of observed species will usually be well above this because of ongoing immigration. We conducted an experimental test to discriminate between these two unified theories using a manipulative field experiment with tropical intertidal communities. We found, consistent with the new theory, that the relationship of species richness to immigration rate stabilized at a low value at low immigration rates and did not saturate at high immigration rates. Our results suggest that tropical intertidal communities have low niche diversity and are typically in a dispersal-assembled regime where immigration is high enough to overfill the niches. Observational data from other studies3,5suggest that these conclusions may generalize to other ecological systems. Our new experimental approach can be adapted for other systems and be used as a ‘niche detector’ and a tool for assessing when communities are niche versus dispersal assembled.