Species based on three defining characteristics. First, they are numerically abundant and account for most of the biomass in an ecosystem. Second, they are at or near the base of the directional interaction networks that characterize ecosystems. Third, their abundant connections to other species in an ecological network mostly reflect non-trophic or mutualistic interactions, including providing structural support for other species, significantly altering ecosystem properties to [dis]favor other species, altering metabolic rates of associated species, and modulating fluxes of energy and nutrient flow through the system. Source: Ellison (2019) [https://doi.org/10.1016/j.isci.2019.02.020]