Soil fauna community distributions
In this study, the comparative analysis was conducted on the order composition similarities of 14 forest sites in East Asia. As expected, smaller distances between the sites tended to increase similarity in species composition (Figure 2). Order richness was observed to be decreased with increasing latitude (Figure 1). This was consistent with most research findings that lower latitudes provide more available resources (Convey, 2013; Johnston & Sibly, 2018). Decaëns (2010) pointed to an enhanced efficiency of mutualism under tropical climates as a possible reason for a latitudinal gradient in soil animal communities. However, the results of this study found that the total richness of soil animals in temperate zones was significantly higher than that in tropical and subtropical zones, which was in line with the results of the non-linear shifts in soil animal community with latitude studied by Petersen and Luxton (1982). Total soil animal biomass declined from temperate ecosystems (forests and grass-lands) towards both arctic and tropical ecosystems and were accompanied by shifts in soil animal community composition (Alice et al., 2020). For instance, the biomass of smaller soil animal groups (Nematoda, Collembola, Enchytraeidae, and Acari) decreased in tropical ecosystems comparing to temperate ecosystems (Petersen 1982; Johnston & Sibly, 2020). This could be resulted from different tolerance of the diverse soil faunas to climate, especially temperature.
Based on the 14 sites we studied, the patterns of order turnover increased significantly with increasing latitude differences for βj and βs of overall orders, phytophage, and predacity faunas, respectively (Figure 4). These results were consistent with those obtained in previous studies (Abrahamczyk et al., 2011; Ulrich et al., 2012; Andersen et al., 2015). The reasons for the increase in order turnover rate with increasing latitude differences could be complicated. Among the reasons, latitudinal gradients in climatic tolerance and sampling effect of the species pool have been mostly reported (Dunck et al., 2016; Kent et al., 2020). The hypothesis of latitudinal gradients in climatic tolerance claims that species are more climatically tolerant in high than in low latitudes (Chen et al., 2016). Lower climatic tolerance may further lead to narrower niche breadths in tropical than in temperate mountains, thereby decreasing the likelihood of co-occurrence of different species and increasing the species turnover rate (Janion-Scheepers et al., 2019; Kent et al., 2020).