DISSERTATION RESEARCH: Global patterns of fish functional diversity and trait convergence along species richness and environmental gradients

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Community assembly processes (CAP), such as environmental filtering, limiting similarity, and neutral processes, explain species coexistence and community composition. Environmental filtering posits that environmental conditions allow species with certain traits to establish and persist, causing species traits to cluster. Alternatively, limiting similarity is the idea that species with similar traits will competitively exclude each other, producing an assemblage with an overdispersion of traits. Trait dispersion patterns are used to determine the importance of these scale-dependent, niche-based (i.e., biological, chemical, and physical factors in the environment) processes in structuring assemblages relative to neutral processes. The search for general patterns of these processes has been hindered by several factors including: lack of large-scale studies, inconsistent and erroneous methodologies, and changes in CAP along environmental gradients and species richness. Convergent evolution of functional groups within independent zoogeographic regions would provide additional evidence for ecological mechanisms influencing assemblage structure. A major challenge for ecology is to understand how assemblages are functionally structured and how CAP, at varying spatial scales, influence biodiversity patterns. This dissertation research project has three aims that address these issues: (1) examine knowledge gaps regarding functional diversity and convergence patterns of fish assemblages at a global scale; (2) determine relative contribution of limiting similarity and environmental filtering to neutral processes at three scales using recently proposed analytical methods; and (3) assess changes in functional diversity and community assemblage processes across environment and species richness gradients. Understanding the relative role of niche-based and stochastic processes at various scales is a central goal in community ecology and crucial for not only conservation and management of assemblages but for understanding the effect of the ever-changing environment on communities as a whole. To effectively address these aims, fish assemblages will be sampled in forty wadable streams in five different zoogeographic regions of varying species richness. Traits with well-known relationships to trophic ecology, habitat use, and life history will be measured for each assemblage. The relative contribution of community assembly processes will be tested at three spatial scales using new methods for analysis of functional trait dispersion. Functional diversity indices will be compared across gradients of stream size and species richness. Lastly, patterns of convergence of functional structure and community assembly processes in different zoogeographic systems will be analyzed using multivariate statistical methods. Field research will involve collaborations with scientists and students from several countries.