Ant suppression experiments have emerged as a powerful method for assessing the role of ants in ecosystems. However, traditional methods have been limited to canopy ants, and not assessed the role of ants on and below ground. Recent advances have enabled whole-ecosystem ant suppression in large plots, but large-scale experiments are not always feasible. Here, we develop a small-scale, whole-ecosystem suppression method. We compare techniques for monitoring suppression experiments, and assess whether habitat complexity in oil palm influences our method’s effectiveness.

We conducted ant suppression experiments in oil palm agroforestry in Sumatra, Indonesia. We used targeted poison baits, a physical barrier, and canopy isolation to suppress ants in 4m-radius arenas around single palms. We sequentially tested three suppression methods that increased in intensity over 18 months. We sampled ant abundance before and after suppression by fogging, using pitfall traps, and extracting soil monoliths. We also monitored ants throughout the experiment by baiting. We tested the soil for residual poison and monitored other invertebrates (Araneae, Coleoptera, Orthoptera, and Chilopoda) to test for cross-contamination. Plots were established under four oil palm management treatments that varied in their habitat complexity: reduced, intermediate, and high understory complexity treatments in mature plantation, and a recently-replanted plantation.

Post-treatment ant abundance was 92% lower in suppression than control plots. Only the most intensive suppression method, which ran for the final nine months, worked. Baiting rarely reflected the other monitoring methods. The treatment negatively affected Orthoptera, but not other taxa. We detected no residual poison in the soil. Coleoptera abundance increased in suppression plots post-treatment, potentially due to reduced competition with ants. Our findings were consistent across management treatments.