Vertically stratified interactions of nectarivores and nectar-inhabiting bacteria in a liana flowering across forest strata.

ABSTRACT

PREMISE Vertical stratification is a key feature of tropical forests and 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: We observed visitations by a diverse nectarivore community to the liana Marcgravia longifolia in a Peruvian rainforest and characterized diversity and community composition of nectar-inhabiting bacteria. Unlike most other plants, M. longifolia produces inflorescences across forest strata, enabling us to study effects of vertical stratification on plant-nectarivore interactions without confounding effects of plant species and stratum. RESULTS: A significantly higher number of visits were by nectarivorous bats and hummingbirds in the midstory than in the understory and canopy, and the visits were 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 index for nectar-inhabiting bacterial communities was highest in the midstory. 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.