A central goal in ecology is to understand what maintains species diversity in local communities. Classic ecological theory^1,2 posits 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 theory^3,4 is 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 studies^3,5 suggest 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.

生态学的一个中心目标是了解是什么维持了当地社区的物种多样性。经典生态理论^1,2认为生态位决定了一个群落中可以共存的物种的最大数量，并且只有在移民非常低的情况下，观察到的物种的丰富度才会低于这个最大值。一种新的替代理论^3,4相反，生态位决定了共存物种的最低数量，并且由于持续的移民，观察到的物种的丰富度通常会远高于此。我们通过对热带潮间带群落进行操作性现场实验，进行了一项实验测试，以区分这两种统一理论。我们发现，与新理论一致，物种丰富度与移民率的关系在低移民率时稳定在较低值，并且在高移民率时不会饱和。我们的结果表明，热带潮间带群落的生态位多样性较低，并且通常处于分散聚集的状态，其中移民数量足以填满生态位。其他研究的观察数据^3,5表明这些结论可以推广到其他生态系统。我们的新实验方法可以适用于其他系统，并可用作“生态位检测器”和评估社区何时处于生态位与分散聚集的工具。