The world’s first farmers were not human beings but the leaf-cutter ants of the genus Atta. These ants do not eat the leaves they carry off. Instead, they bring them down into their nests, where they process the leaves into a pulp and then spread the pulp onto a fungus that grows only in their nests. The fungus then feeds on the food that has been brought to it and grows, after which the ants harvest the knob-like stalks of the fungus and consume them. What is the mode of interaction between the ants and the fungus in this community?