Understanding the mechanisms that drive complementarity niche partitioning among species is pivotal to the progress of biodiversity and ecosystem functioning debate. Here, we provided an empirical test on how the physical space associated with species’ niche affect the biodiversity effects of benthic ecosystem engineers on cross-habitat nutrient regeneration. In laboratory, we orthogonally manipulated the number and composition of 3 benthic invertebrate bioturbator species that differ in their vertical distribution in sediment over 3 different sediment depths/volume (i.e. physical bioturbating space).
NH4 flux from the sediment to the water was positively related to bioturbator species richness only in the deepest/greatest sediment depth/volume. Furthermore, net positive biodiversity effects of benthic bioturbators species richness on benthic-pelagic NH4 flux increased linearly with sediment depth/volume. Such results suggest that sediment bioturbating space mediates the occurrence and the magnitude of complementarity effects among the 3 invertebrate species, and sheds light on the importance of physical space to modulate positive effects of biodiversity on ecosystem functioning.