| Summary: | Background/Question/Methods Currently two distinct models of community assembly and diversity patterns dominate the ecological literature. In niche-based models species are sorted by environmental filters, whereas in neutral models community diversity and assembly is determined by connectivity and heterogeneity in regional sources pools. The relative roles of niche and neutral influences over the organization of a metacommunity can be context dependent. Ecological context is defined by both the characteristics of the habitat (e.g., scale of observation, provincial history, gradients in physiological constraints, resource availability, predation pressure) and the traits of the organisms that make up the metacommunity (e.g., trophic status, dispersal ability, tolerance of physiological challenges). Here we focus on the different contexts in which water-availability gradients influence metacommunity dynamics and diversity patterns. Water-availability gradients can influence community composition by creating environmental filters related to water availability/stress, or can alter the balance between environmental-filtering and ecological drift. We describe a general theoretical model to predict diversity patterns based on the balance of niche and neutral influences over community assembly. We used diversity patterns predicted under different community assembly scenarios to interpret diversity patterns observed for aquatic macroinvertebrates (Everglades), fishes (Everglades), meiofauna (Antarctic soils), and bacteria (Antarctic soils). Results/Conclusions We compared community composition and beta-diversity patterns for assemblages of fishes and aquatic macroinvertebrates collected from 26 locations spanning a distance of 36 km in Everglades National Park from 2004-2008 (FCE-LTER). Sites were classified as short (inundated < 1 yr) and long-hydroperiod (inundated > 1 yr). For macroinvertebrates, community composition was more stochastic in space and time in the short hydroperiod habitat, and converged upon a “long hydroperiod” type as time since re-wetting approached 360 days. A comparison against simulated patterns suggests water permanence is negatively associated with niche-based community assembly at a regional scale in a relatively young (< 6,000 yrs) ecosystem. Small fishes (better dispersers and higher trophic level) showed no difference between habitat types. We also compiled data sets to assess bacterial and meiofaunal communities in Antarctic soils from the MCM-LTER, for which sampling locations spanned 1,500 km (latitudes 72°S - 85°S). Water-availability gradients were strongly correlated with meiofauna community composition (partial R2 = 0.36, p < 0.001). OTUs for soil bacteria identified using TRFLPs from 16S ribosomal DNA showed similar patterns. These relationships between environmental gradients and community composition demonstrate the importance of niche-based processes over diversity patterns at large scales in a relatively old (~20,000 - 2,500,000 yrs) ecosystem.
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