Beyond biomass: how interactions shape species’ importance for ecosystem functioning

Assessing the functional role of species in a changing world is critical for effectively preserving ecosystems. Intuitively, this role should relate to how the loss of a species impacts a function. We develop a general approach to define the functional contribution of a species that interacts with others. Using experimental data from a ciliate microcosm experiment, we infer species dynamic contribution based on the observed functional impact of their removal. Using both model simulations and empirical data, we demonstrate that dynamic contributions are generically unrelated to their biomasses and traits associated with the function, but rather reflect a species’ overall effect on others, and the level of functioning in the community without that species. Our novel characterization of the functional contribution of species reveals that, due to biotic interactions, rare species that do not possess unique functional traits are just as likely as any other species to be functionally important.