Conversion of subtropical natural forests to plantations reduces ant taxonomic and functional diversity

The widespread conversion of natural forests to monoculture plantations poses a major threat to soil biodiversity, yet its impacts on the taxonomic and functional dimensions of keystone species remain poorly understood. Focusing on ants, social insects highly sensitive to environmental changes, we investigated their diversity responses to forest conversion in a mountain ecosystem. Using natural forests as a baseline, we quantified shifts in ant community structure in adjacent broadleaved and fir plantations. Forest conversion induced severe biotic homogenization, with a single genus (Pheidole) accounting for 62%–68% of total ant abundance. Conversion to fir plantations, in particular, led to pronounced declines in taxonomic diversity, significantly reducing Shannon–Wiener diversity and Pielou evenness, underscoring the disruptive effects of coniferous monocultures on natural species assemblages. Functional diversity declined even more sharply after conversion. Functional dispersion (FDis) and Rao's quadratic entropy (RaoQ) decreased by 68%–74% in broadleaved plantations and by 61%–69% in fir plantations, with the strongest reductions observed in summer. Notably, taxonomic and functional diversity declines were decoupled in response to forest conversion. In plantations, ant diversity variation was primarily associated with moisture and substrate quality (lignin and cellulose), whereas in natural forests, it was more closely linked to litter inputs (litter quantity). Our findings demonstrate that forest conversion profoundly compromises both the structural composition and functional organization of ant communities, potentially triggering cascading effects on critical ecosystem processes. To mitigate biodiversity loss in mountain forest ecosystems, management practices should avoid replacing natural forests with monoculture plantations and instead maintain habitat conditions that support both taxonomic and functional diversity of ants.