Facundo Xavier Palacio, Gianluigi Ottaviani, Stefano Mammola , Caio Graco-Roza, Francesco de Bello, Carlos Carmona

The variability in traits within species (intraspecific trait variability; ITV) has attracted an increased interest in functional ecology, as it can profoundly influence the detection of functional trait patterns, calculation of functional diversity (FD), and assessments of ecosystem functioning. This renewed focus stems from the recognition that species are not homogeneous entities but rather mosaics of individuals with varying traits. Researchers dealing with FD have increasingly recognized this issue, and consequently, multiple methods have emerged to explicitly incorporate ITV into FD calculations. Some methods treat individuals as the unit of analysis, while others characterize trait distributions around their means. Ecologists navigating this landscape of methods may face challenges in selecting the most appropriate approach to address their research questions. This synthesis provides an overview and guidelines on how and when to use the different methods available to quantify ITV in biological assemblages and integrate it into FD. As a case study, we computed correlations on simulated assemblages with varying degrees of trait variability. Our findings suggest that the choice of FD metric should be guided primarily by the ecological question being addressed and, to a lesser extent, by the number and types of traits. Simulations revealed strong correlations among FD metrics that account for ITV, particularly those indicating the size of the occupied functional trait space. As evidence accumulates and shows how ITV is key to shaping species’ fitness and distribution as well as affecting ecosystem functioning, this synthesis will serve as a conceptual and practical tool inspiring and guiding researchers to integrate ITV in functional diversity analyses.