Bringing the Eltonian niche into functional diversity

Studies of functional diversity (FD) have gained immense interest because they promise a more mechanistic understanding of the relationship between diversity and ecosystem functioning. However, the concept of FD is currently not used to its full potential because of several biases and shortcomings. For instance, while the analysis of traits related to species' Grinnellian niches, i.e. traits influencing species' fitness under different environmental conditions, is well-advanced, there is a lack of studies on the functional diversity related to species' Eltonian niches, i.e. species' functional roles in ecological processes and their effects on other species, a discrepancy known as the Eltonian Shortfall. Most importantly, the current indirect approach of measuring FD via species traits restricts analyses to species with similar traits, usually a taxon. Consequently, FD is generally measured for only one taxon but across different ecological processes, when it would ideally be measured for one ecological process but across all taxa that contribute to that process. These discrepancies hinder advances of our understanding of the relationship between diversity and ecosystem functions and services. We introduce a new approach to measuring functional diversity that is designed to overcome these shortcomings by bridging research on networks, species niches, and functional traits. Instead of characterizing functional roles indirectly via species' traits, we propose to characterize functional roles directly via the traits of species' resources and interaction partners in a given ecological process. Critically, this shift in perspective for the first time allows comparisons of the functional roles of all taxa that participate in an ecological process regardless of their own morphology. We illustrate our new approach with a study on functional roles of frugivores in seed-dispersal systems. Our approach is an important addition to existing approaches to studying FD, and it facilitates new studies in a vastly unexplored field of functional diversity research.