Ant dominance may mask effects of tree diversity on carcass decomposition

Tree diversity promotes the diversity of organisms at higher trophic levels and associated ecosystem functions. Tree diversity may thus also influence communities of insect decomposers and thereby accelerate the decomposition of animal carcasses. To test this, we placed 864 dead mice and measured their decomposition over two days across 96 plots in a forest biodiversity experiment (BEF-China) during the monsoon season in subtropical China in 2024. We also captured carcass-associated arthropods using mouse-baited traps subsequently. We sampled 8,916 decomposer invertebrates (≥ 60 species, twelve orders). The abundance, richness, and composition of decomposers (ants, flies and other) were indirectly linked to tree diversity through associated changes in canopy cover. Ants were most abundant in traps (55.5% of individuals) and dominated carcasses in over half of cases (55.9%), whereas flies, although common (33.1%), rarely dominated decomposition (7.0%). Carcass decomposition was 30.8% faster in closed forests but was not additionally influenced by tree or decomposer species richness. Decomposition was instead strongly related to ant-fly interactions. It was 66.5% faster when flies, rather than ants, were the primary decomposers. When both groups co-occurred, ants often outcompeted flies, resulting in slower decomposition. This study shows that, in subtropical forests during the monsoon season, ants dominate small vertebrate carcasses, suppress other decomposers and respond little to tree diversity. Consequently, decomposition rates are only weakly linked to the richness of decomposer or tree species. These findings highlight that ecosystem functions mediated by higher trophic interactions can depend more on the identity and interactions of dominant species than on overall biodiversity.