Indirect effects of exotic grazers: livestock decreases the nutrient content of refuse dumps of leaf-cutting ants through vegetation impoverishment

1. We present evidence of how exotic grazers can indirectly change the nutrient availability on the soil surface by affecting the quality of refuse dumps of leaf-cutting ants through changes in vegetation. Understanding the mechanisms involved in this process is vital to propose proper control and restoration practices. 2. In the Monte Desert of Argentina, we sampled seven paddocks with similar environmental conditions but different stocking rates (individuals ha⁻¹). In each paddock we measured grazing intensity, plant species number and cover, leaf-cutting ant nest density, leaf-cutting ant diet and the nutrient content of the external ant refuse dumps and of the adjacent non-nest soils. We found higher content of N, C and P in refuse dumps than in non-nest soils, but the strength of this effect decreased with an increase in stocking rates. As stocking rate increased, plant richness and cover, the number of plant species harvested by ants and the nutrient content of their refuse dumps all decreased. Nest density was not affected by stocking rates. 3. These results suggests that livestock reduce plant richness and cover through grazing, affecting ant diet with the subsequent alteration of the nutrient content of their external refuse dumps. A decrease in the nutrient quality of the refuse dumps may affect plants that establish and grow on this substrate and the rate of nutrient cycling. 4. Synthesis and applications. We have shown that negative effects of exotic grazers on plants may spread throughout native insect herbivores to affect soil nutrient availability. These results can provide useful information to prevent soil impoverishment and restore the nutrient content of soils in this managed system. A slight reduction in stocking rates can restore the contribution of ants to soil fertility. Additionally, refuse dumps from paddocks with low grazing levels should be employed as fertilizer to restore soil quality in paddocks with high grazing intensities.