Ohio is an eastern USA state that historically was >70% covered in upland and mixed coniferous forest; about 60% of it glaciated by the Wisconsinan glacial e

The tropical niche conservatism hypothesis suggests that most groups should be most phylogenetically clustered in cold, dry environments. This idea has been well‐tested in plants and some animal groups, but not for fishes. We assess the geographic patterns of freshwater fish phylogenetic structure and investigate the relationships between these patterns and environmental variables across North America and within two biogeographic realms. Phylogenetic relatedness and diversity of 360 freshwater fish assemblages across North America were quantified with three metrics based on a well‐dated phylogeny, and were related to 15 environmental variables using correlation and regression analyses. Geographically, the data were analyzed for North America as well as for separate biogeographic realms. We found that cold temperatures are the strongest determinant of phylogenetic clustering overall. However, in the arid west, clustering is most pronounced in the driest regions. In eastern North America, phylogenetic clustering increases at higher latitudes, while the reverse is true in western North America. The strongest phylogenetic clustering for freshwater fish assemblages on the continent is found in the most arid, rather than the coldest, climate in North America. Our results highlight that patterns of phylogenetic structure of freshwater fishes in North America are driven by both ecological and evolutionary processes that differ regionally.

The effective number of species (ENS) has been proposed as a robust measure of species diversity that overcomes several limitations in terms of both diversity indices and species richness (SR). However, it is not yet clear if ENS improves interpretation and comparison of biodiversity monitoring data, and ultimately resource management decisions. We used simulations of five stream macroinvertebrate assemblages and spatially extensive field data of stream fishes and mussels to show (a) how different ENS formulations respond to stress and (b) how diversity–environment relationships change with values of q, which weight ENS measures by species abundances. Values of ENS derived from whole simulated assemblages with all species weighted equally (true SR) steadily decreased as stress increased, and ENS‐stress relationships became weaker and more different among assemblages with increased weighting. The amount of variation in ENS across the fish and mussel assemblages that was associated with environmental gradients decreased with increasing q. Synthesis and applications. Species diversity is valued by many human societies, which often have policies designed to protect and restore it. Natural resources managers and policy makers may use species richness and diversity indices to describe the status of ecological communities. However, these traditional diversity measures are known subject to limitations that hinder their interpretation and comparability. The effective number of species (ENS) was proposed to overcome the limitations. Unfortunately, our analyses show that ENS does not improve interpretability of how species diversity responds to either stress or natural environmental gradients. Moreover, incorporating the relative abundance of individuals in different species (evenness) into diversity measures as implemented in ENS can actually weaken detection of diversity responses. Natural resources managers and policy makers therefore need to be cautious when interpreting diversity measures, including ENS, whose values are jointly influenced by richness and evenness. We suggest that both researchers and practitioners measure and report three aspects of diversity (species richness, evenness, and composition) separately when assessing and monitoring the diversity of ecological communities.