Ants indirectly reduce the reproductive performance of a leafless shrub by benefiting aphids through predator deterrence

Ant–aphid mutualisms can generate cas- cade effects on the host plants, but these impacts depend on the ecological context. We studied the consequences of ant–aphid interactions on the repro- ductive performance of a Mediterranean leafless shrub (Retama sphaerocarpa), through direct and indirect effects on the arthropod community. By manipulating the presence of ants and aphids in the field, we found that ants increased aphid abundance and their persis- tence on the plant and reduced aphid predators by nearly half. However, the presence of ants did not affect the abundance of other plant herbivores, which were relatively scarce in the studied plants. Aphids, and particularly those tended by ants, had a negative impact on the plant reproductive performance by significantly reducing the number of fruits produced. However, fruit and seed traits were not changed by the presence of aphids or those tended by ants. We show that ants favoured aphids by protecting them from their natural enemies but did not indirectly benefit plants through herbivory suppression, resulting in a net negative impact on the plant reproductive perfor- mance. Our study suggests that the benefits obtained by plants from hosting ant–aphid mutualisms are dependent on the arthropod community and plant traits.