Species turnover regulates leaf‐cutter ant densities in environmental gradients across the Brazilian Cerrado

Environmental changes resulting from Atta leaf-cutter ant activities characterize them as ecosystem engineers of ecological and economic importance in multiple habitats. Although Atta effects depend strongly on colony density, there has been limited and inconsistent information available on their abundance and interspecific dominance in natural areas and across environmental gradients in the Brazilian Cerrado. Our study bridges this gap by testing the hypothesis that Atta nest density at genus level is high and relatively constant across locations regardless of vegetation type due to strong effects of dominant species turnover. To do so, we conducted a natural experiment surveying Atta nest abundance in five different vegetation types representing the environmental gradient from forest to savannah habitats found in the Cerrado. Our linear transect surveys covered 48.9km in four well-preserved areas across two Brazilian states in which we recorded 124 colonies. Of these, 84 nests belonged to A.laevigata and 40 to A.sexdens, the two dominant Atta species in the region. We also found no nests in 30% of the sampled area, which when combined with strong variation in density measured per transect indicates colonies tend to be aggregated in the landscape. Furthermore, we observed a strong dominant species turnover from forest habitats where 90% of nests belonged to A.sexdens to savannah habitats where 92% of nests were A.laevigata. Turnover effect was reflected in the absence of a significant difference in density between locations and vegetation types, indicating that density at genus level remains high and relatively constant regardless of nest aggregation. Our findings allow larger scale inferences about Atta effects and reveal new insights of their dynamics into natural areas that could affect plant species distribution and contribute to spatial heterogeneity of vegetation, having important implications for Cerrado conservation.