Environmental complexity shapes maintenance of bacterial diversity through context-dependent interactions among niche axes.

Microbial communities are often more species-rich than predicted from classical ecological models. The high levels of coexistence observed in nature are typically attributed to forces that modulate niche availability and stabilize communities. Specific drivers of niche partitioning are often tested in isolation, and the interactive effects of niche variation across resources, space, and time have not been tested together experimentally to determine how they affect community responses. Here, we used 26 bacterial strains previously isolated from carnivorous pitcher plant (Sarracenia purpurea) aquatic pools to construct and expose species-rich synthetic communities to four factors that alter environmental complexity in a fully factorial design, creating combinations of resource complexity, spatial niche structure, and temporal fluctuations. Across treatments, increased niche complexity generally, but not always, promoted the long-term retention of more species, with a saturating effect at the highest levels of complexity. Resource complexity emerged as a primary driver of diversity, with its effects also depending on other niche axes. Interactions among factors frequently deviated from additive expectations, with both synergistic and antagonistic effects observed depending on the combination of conditions. Together, these results show that environmental complexity shapes bacterial diversity through context-dependent, nonlinear interactions among niche dimensions, highlighting that the relationship between niche dimensionality and diversity is contingent on how environmental factors combine.