Integrating intraspecific trait variability in functional diversity: An overview of methods and a guide for ecologists

Variability in traits within species (intraspecific trait variability; ITV) has attracted increased interest in functional ecology, as it can profoundly influence the detection of functional trait patterns, calculations of functional diversity (FD), and assessments of ecosystem functioning. This interest stems from the recognition that species are not homogeneous entities but rather mosaics of individuals with varying trait values. Since multiple methods have emerged to explicitly incorporate ITV into FD calculations, accurate estimates and meaningful interpretations of FD would benefit from a more explicit methodological framework to account for ITV. Some methods treat individuals as the unit of analysis, while others characterize trait distributions around species means. Ecologists navigating this landscape of methods may face challenges in selecting the most appropriate approach to address their research questions, which also depend on data availability. Here, we synthesize the current literature to provide guidelines regarding how and when to use the various available methods to quantify ITV in biological systems and integrate it within FD. We also provide a toolbox to calculate the presented metrics in the form of implemented R code. As a case study, we computed correlations between FD metrics 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, although the type of data available might also impose some limitations. Simulations revealed strong correlations among FD metrics that account for ITV, particularly those indicating the size of the occupied functional trait space. Furthermore, ITV seems to be more important for increasing the functional volume than between-species variability, while regularity metrics (how even species abundances are distributed in the functional trait space) were nearly insensitive to changes in between- or within-species variability. As evidence accumulates and shows how ITV is key to shaping species' fitness and distributions as well as affecting ecosystem functioning, this synthesis will serve as a conceptual and practical tool ideally inspiring and guiding researchers to integrate ITV in FD analyses.