Taxonomic and functional homogenization of soil communities along the gradient of forest conversion

Background/Question/Methods

Human dominance substantially alters biotic and abiotic conditions in terrestrial ecosystems, and often reduces the environmental heterogeneity, which is fundamental in providing various habitat conditions and thus contributes to sustaining different taxa. An increasing line of evidence shows that ecological communities can be homogenized as a result of land-use intensification at various spatial scales. Although such biotic homogenization has been reported to be significant at the larger spatial scales, it could happen at the local scale by changing biotic interactions such as competition. In evaluating this possibility of altered community processes, this study relied on the trait-based approach. Evaluating structures of ecological assemblages based on functional traits has been shown to be powerful to detect the processes underlying community assembly such as limiting similarity and habitat filtering. Here, we aimed to quantify how processes shaping local biodiversity have been modified, resulting in the biotic homogenization. In this study, we focused on oribatid mite communities in natural, semi-natural and larch plantation forests in central Japan. By quantifying changes in taxonomic and functional β-diversity along the gradient of forest conversion, we evaluated the processes of taxonomic and functional homogenization in soil communities.

Results/Conclusions

In the larch-dominated stands, litter diversity was lowered, and the within-stand variations of litter thickness significantly decreased, leading to the habitat homogenization for litter-dwelling taxa. As a consequence, taxonomic ß-diversity of litter communities was lower in the larch plantations (i.e., taxonomic homogenization). Interestingly, the standardized taxonomic ß-diversity representing the magnitude of deviation from the expectation of a random assembly process showed the opposite trend. That is, it was higher in the larch-dominated plantations, suggesting intraspecific aggregation. On the other hand, this clustering was weaker in the natural forest, probably resulting from the higher litter diversity and the associated higher habitat heterogeneity. When the focus was given to functional ß-diversity, functional turnover among local communities in each stand decreased with increasing the larch dominance (i.e., functional homogenization). Especially, functional traits were significantly diverged in the larch-dominated stands due to the decreased functional turnover (i.e., local communities are functionally similar each other). The opposite pattern was observed in the natural forest (i.e., local communities are functionally less similar), likely resulting from the fine-scale habitat filtering. In sum, our study explains underlying community processes behind the observed patterns of biological organization, which can be potentially useful in giving some implications for restoring biodiversity.