The underestimated role of leaf-cutting ants in soil and geomorphological development in neotropical America

Abstract

Leaf-cutter ants belonging to the genus Atta are important ecosystem engineers and conspicuous bioturbators in Neotropical regions acting as superorganisms. The Atta ants excavate impressive and long-lived nests with complex subterranean systems that provide shelter from predation and climatic extremes and allow reproduction and fungus-growing. Despite their importance and the large body of literature on the ecology of Atta ants, their role in geological processes is still poorly studied and underestimated compared to other soil invertebrates such as termites and earthworms. In this work, based on interdisciplinary literature and examples from South America, we provide a systematic review of the main mechanisms and processes by which the leaf-cutter ants direct and indirectly affect the soil characteristics and properties, and also their impact on surficial geological and geomorphological processes at different scales. We also highlight how specific digging and transport processes, such as biomixing, biosorting, biotransfer, and biodeposition affect pedogenesis and landform development. We highlight how the soil turnover and mounding processes create different surficial landforms that are easily eroded and reworked along the slopes by runoff. In the subsurface, the digging behavior results in soil sorting and biogenic microaggregates development that increase soil porosity, with implications for water-holding capacity. The subterranean system creates preferential flux pathways that increase the leaching processes and the complexity of weathering fronts in Atta-bioturbated soils. The impact of the bioturbation of leaf-cutting ants can reach >5 m deep, composed mainly of biofabrics, such as biopores with loose or dense infillings, clay linings, mamillated vugs, excrements, oval granular and rounded microaggregates with biotic origin at great depths (> 2 m). These impacts indicate long-term bioturbation by ants that can result in different soil development pathways. Long-term Atta bioturbation can result in soil homogenization (proisotropic soil development) or create textural contrasts between the soil horizons associated with stone-layers (proanisotropic soil development) through fine material transport from the subsurface. Besides the physical modifications, the Atta nests are biogeochemical hot spots or ‘islands of fertility’ that increase the habitat quality and growth and affect vegetation patterns through changes in the biotic and abiotic components of soils via nutrient concentrations. The Atta nests affect chemical weathering patterns due to presence of fungal and waste chambers that change the soil pH and CO2 concentration. The impact of leaf-cutting ants in landscape development is an expression of the long-term bioturbation activities and their cumulative impacts (multiple generations) on earth surface processes.