The degree to which species in communities differ ecologically and thus niche and neutral processes govern community assembly dynamics remains contentious. We used a combined approach of experiments and field observations to test whether ecological differences among species structure Enallagma damselfly (Odonata) assemblages found in lakes with fish.
Enallagma mortality rates increased and growth rates decreased in the presence of fish when total abundance was experimentally increased, but there was no response to relative abundance manipulations when total abundance was held constant. Across 20 lakes spanning a range of lake environmental conditions, individual Enallagma species demographic rates and relative abundances were uncorrelated across species or with environmental features, suggesting each species is not regulated by different density-dependent ecological factors. However, consistent with our field experiments total mortality rates increased with increasing total Enallagma abundance, and increased total mortality was positively associated with increased fish densities. Collectively, these results indicate that Enallagma species in lakes with fish are ecologically nearly identical, so that neutral dynamics govern their community assembly. In combination with other studies our results also suggest an important conceptual link between niche theory and neutral dynamics. Namely, that mechanisms of coexistence appear to promote the persistence of functional groups (here different genera of damselflies) and regulate the total abundance of all species in each functional group, but ecological drift dominates the dynamics of species’ relative abundances within each group. Finally, our results also indicate that community assembly processes need to be contextualized with speciation dynamics of a clade.