The Role of the Ecosystem Engineer, the Leaf‐Cutter Ant Atta cephalotes, on Soil CO₂ Dynamics in a Wet Tropical Rainforest

Abstract

Leaf-cutter ants are dominant herbivores that disturb the soil and create biogeochemical hot spots. We studied how leaf-cutter ant Atta cephalotes impacts soil CO 2 dynamics in a wet Neotropical forest. We measured soil CO 2 concentration monthly over 2.5 years at multiple depths in nonnest and nest soils (some of which were abandoned during the study) and assessed CO 2 production. We also measured nest and nonnest soil efflux, nest vent efflux, and vent concentration. Nest soils exhibited lower CO 2 accumulation than nonnest soils for the same precipitation amounts. During wet periods, soil CO 2 concentrations increased across all depths, but were significantly less in nest than in nonnest soils. Differences were nonsignificant during drier periods. Surface efflux was equal across nest and nonnest plots (5 μmol CO 2 m À2 s À1 ), while vent efflux was substantially (10 3 to 10 5 times) greater, a finding attributed to free convection and sporadic forced convection. Vent CO 2 concentrations were less than in soil, suggesting CO 2 efflux from the soil matrix into the nest. Legacy effects in abandoned nests were still observable after more than two years. These findings indicate that leaf-cutter ant nests provide alternative transport pathways to soil CO 2 that increase total emissions and decrease soil CO 2 concentrations, and have a lasting impact. Estimated total nest-soil CO 2 emissions were 15 to 60% more than in nonnest soils, contributing 0.2 to 0.7% to ecosystem-scale soil emissions. The observed CO 2 dynamics illuminate the significant carbon footprint of ecosystem engineer Atta cephalotes and have biogeochemical implications for rainforest ecosystems.Plain Language Summary Leaf-cutter ants modify their habitat to the extent that they are called ecosystem engineers. Living throughout the Americas, they construct massive nests to which they import the vegetation they harvest to feed a fungus they cultivate as their main food source. We studied the most common leaf-cutter ant in Costa Rica to assess the impact of its nests on carbon dioxide (CO 2 ) levels in surrounding soils and on soil CO 2 emissions. In the Costa Rican rainforest, heavy rains easily clog the clayey soils, accumulating CO 2 from microbial and root respiration. During wet periods, we observed lower CO 2 concentrations in nest soils relative to nonnest soils. We attribute this difference to the nest structure, which provides ventilation for both nest CO 2 and the CO 2 originated in the surrounding soil. We also found that soil CO 2 emissions were the same in nest and nonnest soils, but nest openings had emissions 100,000 times greater. Consequently, nests and their surrounding soils emit 15 to 60% more CO 2 than the equivalent nonnest soil areas. This difference, together with the expanding range of leaf-cutter ants, favored by human activities and warmer climate, has implications with respect to the global carbon cycle.

Welcome to the Atta world: A framework for understanding the effects of leaf‐cutter ants on ecosystem functions

“…A recent study found soil CO 2 concentration significantly lower in A. cephalotes nest soils compared with non‐nest soils during wet periods, but this difference was not significant during dry periods. For the same precipitation amounts, nest soils accumulated less CO 2 than non‐nest soils (Fernandez‐Bou et al, ). These findings are attributed to the internal network of tunnels and chambers that act as a preferential flow path for the surrounding CO 2 produced in the soil matrix to find its way out to the atmosphere.…”

“…Normally produced by temperature differences, free convection is a less efficient mechanism for nest ventilation than forced convection. Nevertheless, it can be a significant mechanism for gas transport when cool, dense gas overlies warm, less dense gas (Fernandez‐Bou et al, ). Elevated relative humidity also decreases air density (at constant pressure) since water molecules have lower molecular weights than dry air molecules (moist air rises over dry air).…”

“…Nest turnover for multiple Atta sp. varies from 11% to 27% per year (Fernandez‐Bou et al, ; Meyer et al, ; Perfecto & Vandermeer, ; Wirth et al, ). As nests grow and expand, more chambers and tunnels are excavated, increasing the number of pathways for material movement.…”

“…The rates at which nests move across the landscape may influence temporal as well as spatial variation in soil biogeochemistry, element fluxes, plant community composition and soil structural dynamics. Using new sensor systems, Fernandez‐Bou et al () found a 0.2%–0.7% increase in CO 2 emissions from A. cephalotes nests at the ecosystem scale. This use of prototype devices in a novel approach to measure nest carbon fluxes may underestimate total vent emissions by an order of magnitude (Fernandez‐Bou et al, ).…”

“…Using new sensor systems, Fernandez‐Bou et al () found a 0.2%–0.7% increase in CO 2 emissions from A. cephalotes nests at the ecosystem scale. This use of prototype devices in a novel approach to measure nest carbon fluxes may underestimate total vent emissions by an order of magnitude (Fernandez‐Bou et al, ). Improving tools and methods for tracking nest formation, growth rate, age, migration and turnover will enable us to better quantify the impacts of leaf‐cutter ants on carbon and nutrient dynamics across spatial and temporal scales.…”