Functional diversity (FD) aims to describe the diversity of species' functional roles in an ecosystem. It is commonly measured as the diversity of trait combinations that reflect species' adaptations to their functional roles. The concept of FD, however, is currently not used to its full potential because of a number of shortcomings. For instance, FD is currently usually measured without defining the ecological process of interest and without selecting traits relevant for this specific process. Consequently, FD is measured from a mix of traits that represent species’ adaptations to different, yet unspecified, ecological processes. This makes comparisons between studies currently impossible. Most importantly, the indirect approach of measuring FD via species’ traits restricts analyses to species with the same morphology, usually a specific taxon, ignoring the functional roles of species from other taxa. Consequently, FD is often measured for one taxonomic group at a time but across different ecological processes (e.g., "the functional diversity of birds"), when it should be measured for one distinct ecological process but across all taxonomic groups that contribute to that process (e.g., "the functional diversity of pollinators"). This discrepancy hinders advances of our understanding of the relationship between diversity and ecosystem functions and services.
We introduce a new concept of measuring functional diversity. Instead of indirectly inferring species' functional roles from their trait adaptations, we propose to directly assess functional roles from species’ resource use and their interactions with other species in a given ecological process. This definition of the functional role is based on the concept of the Eltonian niche. Accordingly, the functional role of a species is defined by the traits of its interaction partners. This allows the inclusion of all species that contribute to an ecological process, regardless of their own morphology. We present our new concept together with a complete methodological framework that offers new ways to describe the functional roles of species in ecological processes, including several new ways of describing the specialization of species. For instance, our approach is the first that allows quantifying the contribution of individual species to FD. We show how our approach can be extended to compare functional roles in different ecological processes and to incorporate both the Eltonian (“resource use”) and Grinnellian (“environmental requirements”) aspects of a species’ niche. We present the application of the framework with an empirical dataset of a seed-dispersal system from the tropical Andes.