DISCUSSION
Soil invertebrates contribute to forest litter decomposition across the globe
Soil invertebrates make considerable contributions to the decomposition of forest litter. Our synthesis shows 31% of litter decomposition, globally, is mediated by invertebrates. Our estimate is in line with the previous estimates ranging from 27% to 35% of invertebrate effects on decomposition (Kampichler & Bruckner 2009; García-Palacios et al. 2013; Xu et al. 2020). Based on the overall estimates, our analysis further shows a regional pattern of invertebrate effects on litter decomposition and particular invertebrate groups driven the regional difference. We have shown that the invertebrate contribution to litter decomposition in tropical forests is 1.5 times higher than in forests outside the tropics. This latitudinal pattern was observed consistently across all zoogeographic realms except the Ethiopian (Afrotropical) and all biomes. The pattern is supported both historically and ecologically. The presence of particular decomposer groups (e.g. termites) may explain the strong effects of soil invertebrate on decomposition in the tropics. The so-called ‘tropical conservatism hypothesis′ suggests that, in the past, when the Earth was warmer, tropical lineages had wider distributions. Long-term cooling since the late Eocene has restricted distributions of tropical fauna and driven their extinctions in now non-tropical regions (Wiens & Donoghue 2004; Guénard et al. 2015). This history over deep time may well underpin current ecological functions. Major biogeographic differences in soil invertebrate species pools must be considered when discussing the major drivers of decomposition globally. Plant community and environmental conditions also drive soil invertebrate activities and abundance in tropical forests. Diverse plant communities create rich food resources for soil invertebrates and is the basis for maintaining their abundance and activities (Cebrian 1999; Kurokawa et al. 2010; Lavelle et al. 2022). Humid and warm environment in tropical regions are generally favorable for the activities of specific decomposer invertebrates, which are crucial for their corresponding ecological functions, which includes litter decomposition (Tan et al. 2020).