Functional significance of the leaf-cutting ant Atta cephalotes (Formicidae) in coffee plantations: An enemy or an ally?

Abstract

Leaf-cutter ants are appreciated as ecosystem engineers. Conversely, under agricultural scenarios, they are considered serious pests. Coffee is one of the crops selectively foraged by this species either under traditional shading or unshaded intensified systems. Despite the economic importance of coffee in Neotropical regions, there are few observational field studies on the impact of leaf-cutter ants on coffee plantation soils and how impact may differ in response to shade management. Three hypotheses were tested in coffee plantations in presence of Atta cephalotes: (1) bioturbation activity by leaf-cutter ants depends on the soil properties, which in turn, are affected by agronomic management of the coffee plantations; (2) a relatively higher concentration of resources (i.e., coffee plants) results in greater leaf-cutter ant activity in unshaded plantations than in shaded plantations; and (3) the effect of ant defoliation on fruit production depends on the level of foliar damage. Ten active ant nests were selected from each management system in ten farm plots. Soil samples were collected from nests and from outside the nests (control), physical, chemical and biological properties were analysed, and leaf-cutter ant herbivory was also assessed. No significant differences in microbial activity, evaluated through respiration, were found between nest and control soils. Nest soils had higher and lower percentages of sand and silt, respectively, than the control soils. Nest soils contained significantly higher amounts of sulphur, regardless of shade management. A. cephalotes translocated mineral soils and improved water infiltration in nest soils. The herbivory index and proportion of damage were greater in unshaded coffee, although fruit production tended to increase, possibly due to a compensatory effect as coffee plants responded to low levels of foliar damage. Therefore, the first hypothesis was only partially supported since bioturbation depended on soil properties, and certain soil properties were affected by coffee management, while the second and third hypotheses were supported by the data, suggesting that leaf-cutter ants differentially affect coffee plantations based on their shade management and on the moment the coffee plants are affected. Our results form the basis for the development of future studies that evaluate production impacts and economic damage in coffee varieties by leaf-cutter ants. Depending on the ecological context of the agroecosystem and management practices used, leaf-cutter ants can have positive or negative impacts for coffee production.