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The niche concept is a central paradigm in ecology, but empirical evidence on how the niche structure of belowground communities is affected by ecosystem disturbance is lacking. Niche differentiation evolves due to stable coexistence of species in a community, suggesting that in unstable communities, the niches of species should largely overlap. Here, we test this prediction using empirical data on the trophic‐niche structure of several belowground collembolan communities of natural (forest and meadow) and disturbed (pasture and urban lawn) habitats. Scaled stable isotope compositions of carbon and nitrogen were used to reveal the trophic specialization of species. In full agreement with the theoretical predictions, trophic‐niche structure collapsed in disturbed habitats, which was illustrated by decreased interspecific and increased intraspecific variation in the stable isotope compositions. In stable natural habitats, different trophic niches are occupied by different collembolan species, but in disturbed habitats, poor niche specialization is partly compensated for by intraspecific trophic flexibility. The simplification of food‐web structure was in line with the evidence coming from disturbed aquatic ecosystems, suggesting that the collapse of trophic‐niche structure may be a common feature of disturbed communities across biomes.

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