Carrion decomposition in a subtropical forest biodiversity experiment

Tree species richness promotes the diversity of higher trophic levels and ecosystem functioning. Tree species richness may thus also affect communities of insect decomposers, and through this, accelerate the decomposition of animal carrion. However, these effects might be masked by other factors driving decomposition, such as forest structure, topography, and competition between different decomposer groups. We placed 1728 dead mice and observed their decomposition for up to seven days, and captured carrion decomposers with mouse-baited traps across 96 plots in the worldwide largest forest biodiversity experiment (BEF-China) in subtropical China in May 2023 and July 2024. We sampled 30,975 decomposer invertebrates from at least 65 species of nine orders. The abundance, species richness, and composition of decomposer groups (flies, ants, other arthropods) was related to sampling year, canopy cover and slope steepness, but not to tree species richness. Flies (Calliphoridae, Sarcophagidae and Muscidae) were nine times more abundant and were more often the primary decomposer of carrion in 2023 than in 2024, especially in closed forests. In contrast, when flies were rare in 2024, ants primarily decomposed carrion, especially in areas with more ants or fewer flies, independently of environmental factors or tree species richness. Carrion decomposition was accelerated in 2023 compared to 2024, in closed forests and partially on steeper slopes, but was not influenced by tree species richness. Carrion decomposition was faster when flies instead of ants were the primary decomposers. When both insect groups co-occurred on carrion, ants typically outcompeted flies, resulting in slower decomposition. This study shows that carrion decomposers were largely unaffected by tree species richness, and consequently, carrion decomposition also appeared to be insensitive to bottom-up effects of tree species richness. Instead, our results highlight the interactive effects of temporal and environmental factors on decomposer communities of small vertebrate carrion, the competitive interactions between decomposer groups, and decomposition rates. Our findings thus challenge the general expectation that tree diversity promotes higher-trophic diversity and ecosystem functioning.