Despite growing interest in trait-based approaches to community assembly, little attention has been given to seasonal variation in trait distribution patterns. Mobile animals can rapidly mediate competitive interactions through dispersal, suggesting that the relative importance of assembly mechanisms can vary over short time scales in response to ontogenetic changes in dispersal or seasonal fluctuations in environmental conditions. This study analyzed seasonal changes in functional trait distributions of fishes in the Xingu River, a major tributary of the Amazon River with large, predictable temporal variation in hydrologic conditions and species density. We addressed two questions: 1) are trait distributions of wet-season assemblages underdispersed relative to dry-season assemblages, and 2) are trait distributions of dry- and wet-season assemblages significantly different from patterns derived from random assembly. Focusing on traits related to habitat use and trophic strategy, we directly compared functional diversity of assemblages between seasons and tested whether functional diversity patterns within each season differed from random based on standard effect sizes calculated from two null models.
Results/Conclusions
Comparison of observed functional diversity between dry- and wet-season assemblages revealed that wet-season trait distributions are significantly underdispersed relative to the dry season for metrics related to species packing and overall niche volume occupied. The evenness of trait distributions displayed a pattern opposite of that predicted, with wet-season assemblages being more evenly spaced. Temporal patterns were identical between the two trait categories analyzed. Further, wet-season assemblages were significantly underdispersed compared to random expectations based on two null models. Higher functional richness in dry-season communities is consistent with increased niche complementarity during a time when biotic interactions are stronger; however, non-significant null model results suggest that stochastic factors or a combination of assembly mechanisms operate during the dry season. These results demonstrate that the relative influence of community assembly mechanisms on tropical fishes vary seasonally in response to changing abiotic conditions. Expanded habitat and decreased density of aquatic organisms during the wet season may limit the influence of biotic assembly mechanisms, including competitive exclusion, which might partially explain the extraordinary fish diversity found in this system.