Body Mass–Biomass Scaling Modulates Species Keystone-Ness to Press Perturbations

Identifying species with disproportionate effects on other species under press perturbations is essential, yet how species traits and community context drive their ‘keystone-ness’ remain unclear. We quantified keystone-ness as linearly approximated per capita net effect derived from normalised inverse community matrices and as non-linear per capita community biomass change from simulated perturbations in food webs with varying biomass structure. In bottom-heavy webs (negative relationship between species' body mass and their biomass within the web), larger species at higher trophic levels tended to be keystone species, whereas in top-heavy webs (positive body mass to biomass relationship), the opposite was true and the relationships between species' energetic traits and keystone-ness were weakened or reversed compared to bottom-heavy webs. Linear approximations aligned well with non-linear responses in bottom-heavy webs, but were less consistent in top-heavy webs. These findings highlight the importance of community context in shaping species' keystone-ness and informing effective conservation actions.