A functional traits approach was adopted to examine patterns of fish diversity in relation to environmental and spatial variables and for comparison with findings from earlier analyses of fish taxonomic diversity in a nearly pristine river in the Llanos region of Colombia. Fishes were surveyed during the low-water period at 34 sites along the longitudinal fluvial gradient of the Bita River, a clearwater tributary of the Orinoco River. We compiled a matrix of 37 traits associated with locomotion, feeding, and defense for 132 species to compute functional β diversity metrics among four reaches along the entire longitudinal fluvial gradient and also within reaches. Functional redundancy (trait under-dispersion) was found to be common throughout the fluvial gradient. Functional β diversity was high at both spatial scales, with widespread evidence of functional nestedness and functional turnover, especially in upper reaches. In the Bita River, environmental filtering and stochastic processes, such as random dispersal, appear to be the dominant mechanisms influencing fish functional diversity. Inferences derived from analysis of functional versus taxonomic diversity are largely congruent, although analysis of functional β diversity found more evidence of nestedness than turnover at both spatial scales. Both lines of evidence stress the importance of habitat heterogeneity and aquatic habitat connectivity for conservation of high species diversity in this system.

Two core questions in plant community ecology are to what extent the distributions of species are structured by local environmental conditions, and whether taxa differ in this regard. We compared the distributions of trees, Melastomataceae and ferns on soil and topographic gradients in a Costa Rican lowland rain forest (trees and ferns 983 plots, Melastomataceae 277 plots). To test whether these plant groups differed in the prevalence or type of habitat specialization, we calculated species’ environmental optima and tolerances on each gradient. Habitat specialization was defined as a significantly biased optimum, or a narrow tolerance, relative to values obtained under spatially restricted randomizations of species occurrences. Within plant groups, we also asked whether the dispersion of species optima differed from random expectation on each gradient. Fern optima were over-dispersed on multiple gradients, implying considerable interspecific habitat partitioning, and tree optima were over-dispersed in relation to topographic position. Habitat specialization was more prevalent in the two predominantly understorey groups than in trees (75% of Melastomataceae species, 81–87% of ferns, 57–58% of trees). Species optima of Melastomataceae and ferns also tended towards lower landscape positions than did those of trees, perhaps reflecting a higher proportion of drought-sensitive species in these two groups.

Environmental filtering and competitive interactions are important ecological processes in community assembly. The contribution of the two processes to community assembly can be evaluated by shifts in functional diversity patterns. We examined the correlations between functional diversity of six traits (leaf chlorophyll concentration, dry matter content, size, specific leaf area, thickness and wood density) and environmental gradients (topography and soil) for 92 species in the 20-ha Dinghushan forest plot in China. A partial Mantel test showed that most of the community-weighted mean trait values changed with terrain convexity and soil fertility, which implied that environmental filtering was occurring. Functional diversity of many traits significantly increased with increasing terrain convexity and soil fertility, which was associated with increased light and below-ground resources respectively. These results suggest that co-occurring species are functionally convergent in regions of strong abiotic stress under the environmental filtering, but functionally divergent in more benign environments due to resource partitioning and competitive interactions. Single-trait diversity and multivariate functional diversity had different relationships with environmental factors, indicating that traits were related to different niche axes, and associated with different ecological processes, which demonstrated the importance of focusing niche axes in traits selection. Between 9% and 41% of variation in functional diversity of different traits was explained by environmental factors in stepwise multiple regression models. Terrain convexity and soil fertility were the best predictors of functional diversity, which contributed 30.5% and 29.0% of total R2 to the model. These provided essential evidence that different environmental factors had distinguishing impacts on regulating diversity of traits.