Functional Diversity Indices in Semi-Arid Grassland Ecosystems

Abstract

Functional diversity indices — metrics that quantify the diversity of ecological traits within biological communities —provide superior predictors of ecosystem functioning and resilience compared to species richness alone, yet their application to animal communities in semi-arid grasslands remains limited relative to plant-focused studies. This study quantified functional diversity of arthropod and small mammal communities across a grazing intensity gradient (ungrazed,
lightly grazed, moderately grazed, heavily grazed) at 36 semi-arid grassland sites in Sweden, France, and Austria, using 14 functional traits spanning body size, trophic guild, mobility, reproductive strategy, and stress tolerance. A total of 4,284 arthropod individuals representing 312 morphospecies and 186 small mammal individuals representing 18 species were sampled using pitfall traps, sweep nets, and live-capture grid trapping. Four functional diversity indices were computed:
functional richness (FRic), functional evenness (FEve), functional divergence (FDiv), and functional dispersion (FDis). Grazing intensity had significant effects on all four indices (PERMANOVA: F = 12.4, R2  = 0.38, p < 0.001). Light grazing maximised FRic (mean 0.68 ± 0.06) and FDis (mean 0.54 ± 0.05) relative to ungrazed (FRic: 0.52 ± 0.07; FDis: 0.42 ± 0.06) and heavily grazed sites (FRic: 0.38 ± 0.08; FDis: 0.31 ± 0.07). Aboveground net primary productivity (ANPP) was best predicted by FDis (R2
 = 0.62, p < 0.001) rather than species richness (R2  = 0.34). Trait-environment relationships identified body size, trophic position, and desiccation tolerance as the traits most strongly filtered by aridity and grazing
intensity gradients. These findings support the intermediate disturbance hypothesis for functional diversity and highlight the importance of light grazing management for maintaining functional diversity and ecosystem productivity in semi-arid grasslands.