The complex dynamics of ant-plant mutualisms: exploring the roles of plant-provided resources and bird predation on ants in shaping plant performance.

Background and aims: Protection mutualisms involve plants receiving defence against herbivores from predators, such as ants and insectivorous birds, in exchange for food resources. Ants can reduce herbivory by actively patrolling plants with extrafloral nectaries (EFNs) or by tending trophobiotic insects that provide them with food rewards. Insectivorous birds also contribute to herbivore suppression but might simultaneously reduce ant activity through predation. Although both ants and birds can enhance plant performance, few studies have explored how the availability of multiple ant-associated resources influences herbivore suppression, or how bird predation on ants affects these ant-plant mutualisms. These gaps limit our understanding of the dynamics in complex multitrophic interactions.

Methods: We investigated how the availability of multiple food resources for ants and the predation exerted by insectivorous birds on ants influence the performance of the tropical shrub Banisteriopsis malifolia. To address this, we conducted two field experiments. The first manipulated the individual and combined presence of mutualistic ant resources (EFNs, myrmecophilous caterpillars and treehoppers). The second experiment manipulated the presence or absence of both ants and birds to assess their interactive effects. Plant performance was evaluated by measuring leaf herbivore damage, the number of fruits and fruit weight.

Key results: Our findings show that B. malifolia plants offering a full complement of mutualistic food resources for ants, including EFNs and trophobiotic insects, experienced significantly lower leaf herbivory and higher fruit production in comparison to plants lacking these resources. Bird exclusion resulted in increased herbivore damage and reduced fruit output, with these negative effects being even more pronounced when both birds and ants were excluded.

Conclusions: Our findings highlight the complexity of multitrophic interactions in ant-plant mutualisms. They emphasize the importance of considering multiple trophic levels in plant defence strategies and underscore the cascading effects of predator interactions within natural ecosystems.