Convergence and Community Assembly of Stream Fishes: An Intercontinental Analysis

Abstract

A mechanistic understanding of general community assembly is crucial for predicting how species will respond to future environmental changes. My dissertation explores general ecological patterns of stream fish assemblages based on intercontinental comparisons using new quantitative approaches. In this study, I tested for convergent trait-environment patterns across regional faunas in response to consistent sets of environmental filters acting on functional traits. I found similar patterns of trait-environment patterns across all five regions, implying common environmental filters acted on local community assembly. The congruent trait-environment patterns implied that water velocity and habitat structural complexity act as universal environmental filters. These universal filters appear to produce similar trait distributions of fish assemblages in streams across all regions. In my study, I also tested for convergence of species traits within similar microhabitats. This study revealed a high prevalence of convergence of functional traits among fish species occupying similar microhabitats of small, low-gradient streams. The prevalence of convergent suites of functional traits implies that adaptation to similar environmental conditions resulted in repeated patterns of evolution along multiple niche dimensions. Fishes occupying areas with relatively fast water velocity or little structural complexity generally occupied a restricted morphological space and exhibited the highest degrees of convergence. Finally, I test for similar patterns of functional trait and phylogenetic dispersion across regions and along environmental gradients. Here, under-dispersion was consistently more prevalent than over-dispersion in all regions regardless of null model or functional metric. Functional metrics tended to decrease with high water velocity, shallow water depth, and non-structured substrates microhabitats. Together, these results emphasize that environmental filtering plays an important role in structure stream fish assemblages. Furthermore, I detected more instances of functional trait under-dispersion coupled with phylogenetic under-dispersion, which may reflect a signal of phylogenetic niche conservation or stabilizing selection acting on species’ traits and ecology at the local scale. Overall, the results of my study implied that steam fish assemblages were structured by relatively consistent deterministic mechanisms.