Abstract
Ecological models suggest that high diversity can be generated by purely niche‐based, purely neutral or by a mixture of niche‐based and neutral ecological processes. Here, we compare the degree to which four contrasting hypotheses for coexistence, ranging from niche‐based to neutral, explain species richness along a body mass niche axis. We derive predictions from these hypotheses and confront them with species body‐mass patterns in a highly sampled marine phytoplankton community. We find that these patterns are consistent only with a mechanism that combines niche and neutral processes, such as the emergent neutrality mechanism. In this work, we provide the first empirical evidence that a niche‐neutral model can explain niche space occupancy pattern in a natural species‐rich community. We suggest this class of model may be a useful hypothesis for the generation and maintenance of species diversity in other size‐structured communities.