Functional Vulnerability Arises From Unsteady Trait-Based Networks Despite High Redundancy

Theoretical models suggest that community functional redundancy buffers functional vulnerability because functions of lost species may be compensated by others, yet empirical evidence demonstrates this buffering effect to be weak. We hypothesised that this weak effect arises from differences in the structural arrangement of redundancy within communities. Therefore, we developed 'community functional structural steadiness', a network-based metric characterising structural arrangements by quantifying variability in the impacts of species loss on community functional structure. We calculated functional redundancy, vulnerability and structural steadiness for fish communities across the Lancang-Mekong River basin in Southeast Asia and analysed their interrelationships using linear regressions, mixed-effects models and piecewise structural equation modelling. Structural steadiness improved predictions of vulnerability beyond redundancy alone, and communities with unsteady structure exhibited high vulnerability despite high redundancy. Conservation efforts should seek to identify structurally unsteady communities and prioritise protection of functionally unique species, particularly in biodiversity-rich regions.