Quasi-neutral community assembly: Evidence from niche overlap, phylogenetic, and trait distribution analyses of a subtropical forest in South America

In spite of recent advances, the relative roles of deterministic and stochastic processes in community assembly remains disputed. We tested community assembly hypotheses in a disturbance gradient in a logged mixed conifer-hardwood forest complex in southern Brazil. If abiotic heterogeneity was strong enough (Selection hypothesis), we expected niche clustering and reduced niche overlap between species and between ecological groups, phylogenetic clustering, significant β-diversity departure from a random expectation after controlling for α-diversity variation, and non-random trait distribution due to abiotic filtering and limiting similarity. Null expectations were predicted under the Neutral hypothesis. Under the Quasi-Neutral hypothesis, we expected niche clustering and reduced niche overlap as in the Selection hypothesis, but random phylogenetic distribution, random β-diversity variation, and random trait distribution due to niche-based specialization but weak habitat matching resulting from dispersal limitation and drift. We used a series of null models to provide an integrative evaluation of community assembly including species abundances, environmental preferences, phylogenetic relatedness, and trait distribution. Results supported the Quasi-Neutral hypothesis due to a combination of reduced niche overlap, niche clustering along soil and disturbance gradients, evidence for abiotic filtering of functional traits but absent phylogenetic structure and random β-diversity variation. Support to the Quasi-Neutral hypothesis provides evidence for niche-based species selection along environmental gradients, coupled with poor matches between species distributions and environmental factors due to stochastic processes. It also highlights the importance of an integrative approach to the evaluation of community patterns indicative of assembly processes. Had only a subset of the approaches used been employed and different conclusions would have been reached: niche overlap and trait distribution analyses would have indicated a stronger role for selection, while phylogenetic and abundance analyses would have indicated stochastic community assembly. Future research should include different disturbance regimes along with environmental variation in order to assess the interplay of the different drivers that shape the community assembly process, as well as the potential of the Quasi-Neutral hypothesis as an explanation for the assembly process of complex and species-rich subtropical forests.