Practice question

The ant–aphid association is classed as defensive mutualism because ants protect aphids from predators and parasitoids, while aphids provide carbohydrate-rich honeydew. Aphids excrete honeydew after feeding on sugar-rich phloem sap; attending ants collect it and may stimulate release by antennation. In return, ants attack ladybird beetles, lacewing larvae, or parasitoid wasps, remove dead aphids, and sometimes shelter aphid colonies. Both partners can therefore gain increased fitness: ants acquire an accessible energy source, and aphids experience lower enemy mortality. The interaction is not purely beneficial under all conditions. Ant attendance may constrain aphid dispersal, alter feeding, or promote dense colonies, and benefits depend on ant species, predator pressure, and alternative sugar sources. It is termed defensive rather than simply resource-based because the service emphasized is enemy protection, although a resource exchange accompanies it. Commensalism would require one partner to be unaffected, whereas parasitism would require one to benefit at the other’s net expense.