Vertically Stratified Interactions Of Nectarivores And Nectar-Inhabiting Bacteria In A Liana Flowering Across Forest Strata

Premise: Vertical stratification is a key feature of tropical forest and structures plant–frugivore interactions. However, it is unclear whether equally strong patterns of vertical stratification exist for plant–nectarivore interactions and, if so, which factors drive these patterns. Further, nectar-inhabiting bacteria acting as “hidden players” in plant–nectarivore interactions might be vertically stratified, either in response to differences among strata in microenvironmental conditions or to the nectarivore community serving as vectors. Methods: To test this, we observed visitation of a diverse nectarivore community in the liana Marcgravia longifolia in a Peruvian rainforest, as well as diversity and community composition of nectar-inhabiting bacteria. Unlike most other plants, Marcgravia longifolia produces inflorescences across forest strata. This enabled us to study effects of vertical stratification on plant–nectarivore interactions without confounding effects of plant species and stratum. Results: We found a significantly higher number of visits of nectarivorous bats and hummingbirds in the midstorey than in the understorey and canopy, being strongly correlated to flower availability and nectar quantity and quality. Trochiline hummingbirds foraged across all strata, whereas hermits remained in the lower strata. The Shannon diversity of nectar-inhabiting bacteria communities was highest in the midstorey. Conclusions: Our findings suggest that vertical niche differentiation in plant–nectarivore interactions seems to be partly driven by resource abundance, but other factors such as species-specific preferences of hummingbirds likely caused by competition play an important role. We conclude that vertical stratification is an important driver of a species' interaction niche highlighting its role for promoting biodiversity and ecosystem functioning.