An ecological perspective on functional redundancy and robustness in microbial communities

Microbial communities often maintain ecosystem functions despite environmental fluctuations and compositional turnover. This robustness is frequently attributed to functions being encoded and provided by multiple members of the community, a phenomenon referred to as functional redundancy. Yet functional redundancy does not by itself explain whether a function will be maintained after a perturbation. A central challenge is therefore to understand when such functional overlap among taxa can translate into compensation and functional robustness. Here, we argue that addressing this challenge requires moving beyond asking whether redundancy is present to asking under which ecological conditions redundant providers are maintained and can buffer function. This requires focusing on clearly defined functions, identifying the taxa that provide them, and understanding the processes that determine whether these providers persist, are lost, or are replaced under changing conditions. From this perspective, the potential for redundancy to support functional robustness depends on whether providers differ in their responses to environmental perturbations, avoid coupled loss through interaction-network effects, and are maintained over relevant timescales through coexistence, turnover, or immigration. These conditions can be examined through the lens of trait-based ecology, interaction-network theory, and coexistence theory. They are jointly shaped by how a focal function is connected to trait distributions and species interactions, including resource competition. As a result, the relationship between functional redundancy and microbial community dynamics is inherently function-specific. This perspective shifts the focus from quantifying redundancy alone to testing when redundant providers can actually persist, compensate, and realize functions under change.