Stochastic vs. deterministic assembly of aquatic invertebrate communities in permanently flooded and intermittently dry wetlands

Background/Question/Methods

A current debate is whether community composition is primarly structured by deterministic (e.g. Niche theory) or stochastic factors (e.g. Neutral theory). Niche theory suggests individuals with given traits for survival in a habitat with certain environmental conditions and interspecific interactions will determine community composition, while Neutral theory suggests individuals of similar trophic status do not differ in birth, death and dispersal rates and will stochastically determine community composition. Some have suggested that the importance of deterministic vs. stochastic factors varies due to the harshness of environmental conditions. Communities in harsh conditions are theorized to assemble deterministically producing similar species compositions among sites, while communities in benign conditions should assemble stochastically producing dissimilar compositions. We tested this by sampling aquatic invertebrates in ten wetland mesocosms (10 m X 20 m in size and 1 m deep). Twice in the summer, we dewatered five mesocosms (Harsh treatment) and did not dewater the other five (Benign treatment). We assumed that only species adapted to survive dry periods would persist in Harsh treatments, and these communities would have a greater similarity among mesocosms than in Benign treatments.

Results/Conclusions

Hierarchal clustering and non-metric dimensional scaling detected distinct communities between Harsh and Benign treatments. Common taxa in Harsh treatments were Cladocera, Physidae, Coenagrionidae, and Hydrachnidia, while Copepoda, Ostracoda, Ceratopogonidae, and Chironomidae were common in Benign treatments. Furthermore, communities were strongly similar among mesocosms in Harsh than Benign treatments. These results suggest that communities assembled deterministically in Harsh environments and stochastically in Benign environments, which supports both niche and neutral models of community assembly, respectively.