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We used simulations of five stream macroinvertebrate assemblages and spatially extensive field data of stream fishes and mussels to show (a) how differentENSformulations respond to stress and (b) how diversity–environment relationships change with values ofq, which weightENSmeasures by species abundances.

Values ofENSderived from whole simulated assemblages with all species weighted equally (true SR) steadily decreased as stress increased, andENS‐stress relationships became weaker and more different among assemblages with increased weighting.

The amount of variation inENSacross the fish and mussel assemblages that was associated with environmental gradients decreased with increasingq.

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 thatENSdoes 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 inENScan actually weaken detection of diversity responses. Natural resources managers and policy makers therefore need to be cautious when interpreting diversity measures, includingENS, 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.