Stefano Mammola, Carlos P. Carmona, Thomas Guillerme, Pedro Cardoso
Organisms, species, and sets of species interact and perform functions in the environment in ways that we are just starting to unravel. Quantifying how much such functions and interactions contribute to our own well-being through so-called ecosystem services is growing in importance. Scientists are doing so by measuring species traits, defined as any characteristic of a species (morphology, physiology, behaviour, etc.) measured at the individual level. Species traits can be as diverse as the body size of species, their role in the food web (carnivores, herbivores, pollinators, etc.), or their physiological tolerance to temperature variations. By combining all these traits one can measure the hypothetical functional space of a given habitat or ecosystem, and infer the mechanisms and processes behind how it functions. As researchers have realized how important it is to do such quantification, the number of statistics that reflect in one way or another different facets of this functional diversity have begun to grow exponentially. This growth has made it difficult for any single researcher to grasp the field in its entirety. We here summarize and put into context a plethora of frameworks and measures for the quantification of different aspects of functional diversity and advise on when and how to use them. We do so by organizing the different methods and metrics in a simple unifying scheme that considers the type of functional process under investigation and the level of organisation at which it is measured. Our hope is that this study will help researchers approaching functional diversity analyses for the first time, but also experienced researchers, in navigating the current jungles of statistical tools and mathematical indexes.