LEAF-CUTTING ants are one of Brazil’s biggest agricultural pests, eating their way through crops at an astonishing rate. Not surprisingly, research on the ants has focused almost exclusively on ways to kill them. But the findings of a study in a deforested area of Amazonian rainforest suggest that leaf-cutting ants are not all bad.
Researchers from Pará Federal University (UFPA) in Belém and the Woods Hole Research Center in Massachusetts say that one species of leaf-cutting ant, Atta sexdens, can make the soil more fertile in places where the primary forest has been cleared.
The research in Paragominas, a region of the Amazon basin where a steady stream of incoming farmers has destroyed the forest to create pastures for cattle, suggests that the insects change the physical and chemical structure of the soil so that it retains more nutrients. The study is one of several in the region investigating the impact of land use on biodiversity, and the recovery process in damaged rainforest.
Leaf-cutting ants are found all over Brazil and are particularly destructive in eucalyptus and citrus plantations. Their nests can be enormous, occupying up to 50 square metres and containing three million ants.
Paulo Moutinho and his colleagues at UFPA studied an area of forest that was cleared 16 years ago. They dug holes 5 metres deep in the middle of ants’ nests, and compared the soil with samples from a nearby area of similar forest where there were no leaf-cutters. “We are discovering that in some cases during the recovery of the forest, this species can bring benefits,” says Moutinho.
The soil from the nests was much crumblier, making it easier for plant roots to push through. The team found that the density of roots in the soil inside the nest was ten times greater than in soil farther away. The roots were especially dense in the chambers where the insects dump dead ants and unwanted leaves.
The decomposition of plant material stored by the ants increases the pH of the soil and increases what soil scientists call the exchange capacity – the ability of the soil to hold onto nutrients so that they are not washed out of reach of plants.
Moutinho says that the next step will be to measure the nutrients in the two types of soil to confirm this. “Possibly in the future, it will be important to manage the population of leaf-cutting ants, making the most of their benefits for agricultural production systems, rather than simply exterminating them,” he says.