Patterns and drivers of vertical stratification in nectarivore and frugivore communities and their interactions with plants in Neotropical rainforests

Tropical forests harbour more than 50% of our world’s biodiversity. Crucially, this biological diversity is under serious pressure due to anthropogenic factors such as forest degradation, land-use, and climate changes. These factors alter tropical forest structure and threaten the persistence of native populations and communities. In order to successfully manage and conserve tropical forests and their affiliated species and ecosystem functions, we need a better understanding of the mechanisms that drive their structure and unparalleled diversity. In this context, the vertical stratification of tropical forest ecosystems as a key factor promoting tropical forest diversity came into focus. Plant and animal organisms inhabit a variety of niches along the vertical forest gradient. Communities of birds, bats, primates, and small non-flying mammals are vertically stratified in terms of species abundance, richness, and community composition. As many animal species from these taxa are important nectarivores and frugivores in tropical forests, the vertical structure of their communities likely induces differences among strata in the patterning of associated ecological processes such as pollination and seed dispersal. These mutualistic interactions are considered the backbone of ecological communities as they are ultimately linked to the organization, functioning, and integrity of ecosystems. However, our understanding of how interactions among plants and animals are structured across the vertical gradient is very limited. To date, the few studies examining patterns of vertical stratification in plant–animal interactions are scarce and limited to plant–frugivore interactions. Further, we know little about the drivers creating the potentially vertically stratified structure of interaction networks. Generally, resource availability and animals’ functional traits were considered as the most important drivers of network structure. Recently, the role of microbes as “hidden players” influencing mutualistic interactions also came into focus. Plants harbour a wide diversity of microbes on their internal and external tissues, rendering their interactions with animals a tripartite interaction among the plants, its microbes, and the animals. Yet, the roles animals’ functional traits and microbes as “hidden players” might play in structuring differences among strata in plant–animal interactions are poorly understood. With this thesis, I aim to narrow these gaps by addressing the following questions: i) how pronounced are the patterns of vertical stratification in seed-dispersing vertebrate communities and their interactions with plants in tropical forests? ii) can patterns of vertical stratification in plant–animal interactions reported elsewhere be generalized for a specific resource that is available across forest strata? More specifically, iii) do nectarivores and frugivores preferentially forage in a distinct vertical niche even if the same resource is available along the entire vertical gradient and iv) if so, which drivers are causing the vertically stratified interaction structure? Chapter II is a literature review that summarizes current knowledge about the vertical stratification of vertebrate seed disperser communities and their resources in tropical forests, with particular emphasis on plant–seed disperser interactions and species networks. In Chapter III and IV, I used Marcgravia longifolia as a model species to study whether vertebrate nectarivore and frugivore communities feeding in a single liana species that flowers and fruits across forest strata show patterns of vertical stratification and which driving factors cause these patterns. I applied a network approach to assess differences in community composition and interaction frequency of nectarivores and frugivores among strata. Then, I examined whether animals’ functional traits, differences in environmental characteristics, or resource quantity and quality among strata, played an important role in determining the vertical foraging niche. Considering the role of microbes as important “hidden players” influencing plant–animal interactions, in Chapter III, I further investigated if the bacteria communities inhabiting the nectar of M. longifolia show patterns of vertical stratification in terms of diversity and community composition. My work shows that i) vertical stratification is a key feature of tropical forests that shapes differences among strata in the patterning of plant–animal interactions. This was manifested in differing interaction frequencies and mutual specialization among strata (Chapter II). Further, I found ii) strong patterns of vertical stratification of plant–animal interactions at a single plant species, which implies that their vertical structure is not primarily driven by differences in plant community composition among strata (Chapter III and IV). I also demonstrated that iii) both studied mutualistic interactions differed among strata in terms of interaction frequency and community composition. Nectarivores and frugivores preferentially foraged in a distinct vertical niche, even though the same resource was available across strata (Chapter III and IV). However, the drivers acting behind the vertically stratified structure of plant–frugivore and plant–nectarivore interactions differed. I showed that iv) the strong vertical stratification of plant–frugivore interactions was primarily driven by inherent preferences of animals for certain strata. Birds’ functional traits, such as their dietary dependence on fruit and their morphology, played an important role in determining their vertical foraging niche (Chapter III). In contrast, plant–nectarivore interactions were mainly driven by variation in resource abundance among strata as well as competitive interactions among species of different hummingbird subfamilies. Further, the diversity of nectar-inhabiting bacteria differed among strata. Considering their role as “hidden players” influencing the interactions among plants and nectarivores, they might be a critical driver in structuring vertically stratified plant–nectarivore interactions (Chapter IV). Understanding the structure and drivers of vertically stratified nectarivore and frugivore communities and their interaction with plants is crucial to predict how vertically stratified tropical forests and their affiliated species and ecosystem functions will perform under global change