Practice question

Ants tending aphids illustrate defensive mutualism. Aphids ingest phloem sap and excrete excess carbohydrate as honeydew, which provides ants with an energy-rich food. Ant workers patrol the colony and attack ladybirds, lacewings, and parasitoid wasps, thereby lowering aphid mortality. Some ants also move aphids to suitable shoots or shelter them during unfavorable conditions. The aphid’s principal service is a resource, whereas the ant’s principal service is defense, so the association is classified by the protective function emphasized. It is not commensalism because both partners can benefit, and it is neither dispersive mutualism nor inquilinism because transport of gametes or simple use of shelter is not central. Benefits remain context dependent: ants may prey on aphids when honeydew is scarce, and attendance can limit aphid dispersal or increase crowding. Mutualism denotes positive net fitness effects under the observed conditions, not permanent cooperation or conscious exchange. Experimental ant exclusion commonly reveals the defensive contribution through increased aphid predation or parasitism.