Tropical land use alters functional diversity of soil food webs and leads to monopolization of the detrital energy channel

Agricultural expansion is among the main threats to biodiversity and functions of tropical ecosystems. It has been shown that conversion of rainforest into plantations erodes biodiversity, but further consequences for food-web structure and energetics of belowground communities remains little explored. We used a unique combination of stable isotope analysis and food-web energetics to analyze in a comprehensive way consequences of the conversion of rainforest into oil palm and rubber plantations on the structure of and channeling of energy through soil animal food webs in Sumatra, Indonesia. Across the animal groups studied, most of the taxa had lower litter-calibrated & UDelta;C-13 values in plantations than in rainforests, suggesting that they switched to freshly-fixed plant carbon ('fast' energy channeling) in plantations from the detrital C pathway ('slow' energy channeling) in rainforests. These shifts led to changes in isotopic divergence, dispersion, evenness, and uniqueness. However, earthworms as major detritivores stayed unchanged in their trophic niche and monopolized the detrital pathway in plantations, resulting in similar energetic metrics across land-use systems. Functional diversity metrics of soil food webs were associated with reduced amount of litter, tree density, and species richness in plantations, providing guidelines on how to improve the complexity of the structure of and channeling of energy through soil food webs. Our results highlight the strong restructuring of soil food webs with the conversion of rainforest into plantations threatening soil functioning and ecosystem stability in the long term.

Automated summary

Agricultural expansion threatens tropical ecosystems by eroding biodiversity and impacting the structure and energetics of soil food webs. This study in Sumatra, Indonesia, found that converting rainforest into plantations led to a shift in carbon sources for animals, with most taxa choosing freshly-fixed plant carbon in plantations. However, earthworms remained unchanged and monopolized the detrital pathway. The functional diversity of soil food webs was associated with litter amount, tree density, and species richness in plantations. Overall, this restructuring of soil food webs highlights the long-term threat to soil functioning and ecosystem stability.