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).