Continental Drivers of Population‐Level Intraspecific Trait Variability in Salamanders, Birds, and Small Mammals

ABSTRACT Aim Intraspecific trait variability (ITV) is a key component of biodiversity because it informs how organisms respond to abiotic and biotic conditions. Understanding how the variability among individuals within a population (population‐level ITV) changes along environmental gradients offers insight into how local processes contribute to broad‐scale biodiversity patterns. Here, we examine how climate and species‐richness interact to drive population‐level ITV across taxa with distinct life‐histories. Location United States and Canada. Time Period Present. Major Taxa Studied Plethodon salamanders, birds, and small mammals. Methods Individual body‐size measurements were compiled from multiple large‐scale biodiversity data sources, and population‐level ITV across populations of the focal taxa was quantified as the coefficient of variation (CV). Using Bayesian phylogenetic generalized linear mixed models (PGLMMs), we tested the effects of mean annual temperature, precipitation, and relative humidity, as well as their interactions with species richness, while accounting for species identity, phylogeny, and site. Results The influence of environmental predictors on population‐level ITV differed across taxonomic groups. Salamander ITV declined with humidity and increased with temperature in species‐rich sites, bird ITV increased with temperature, and small‐mammal ITV decreased with precipitation but increased with humidity and temperature in species‐rich sites. Main Conclusions We found that abiotic and biotic conditions shape trait diversity within populations, and that these factors differ across taxonomic groups according to their life‐histories. Groups with relatively low mobility and that are closely tied to their microhabitats showed increased population‐level ITV in warm, species‐rich environments. These findings challenge the classic niche‐packing expectations by suggesting that responses to climatic stress and interspecific competition can increase individual variation within populations, potentially reflecting differences in the strategies individuals adopt to cope with environmental challenges. Together, these insights underscore the importance of studying intraspecific trait variation for understanding macroecological patterns in biodiversity.