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 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.

A sublethal imidacloprid concentration alters foraging and competition behaviour of ants.

Neonicotinoid pesticides, such as the widely used compound imidacloprid, are suspected to impair cognitive capacity, behaviour, and fitness of a number of non-target species. We tested whether sublethal imidacloprid concentrations alter the foraging and aggression behaviour of two European ant species. Even though the nestmate-recruitment of Lasius niger was not affected by pesticide exposure, these ants required more time to become active and the number of foraging workers was lower than in sub-colonies not exposed to imidacloprid. In interspecific confrontations, imidacloprid increased the aggressiveness of a usually subordinate species (Lasius flavus) enormously (3.7-fold increase in average number of aggressive encounters), whereas they did not affect a subdominant species (L. niger) that severely (1.2-fold increase in average number of aggressive encounters). The high frequency of aggressive encounters of L. flavus vs. non-exposed L. niger workers, reduced their survival probability significantly down to 60 %. The observed behavioural alterations of the two ant species have the potential to impair their viability and co-occurrence with behaviourally dominate species due to a decreased exploitative competition and a reduced chance to locate and use resources before competitors. As competition is considered key in structuring ant communities, changes in aggressiveness are likely to alter established dominance hierarchies and thereby the dynamic and structure of ant communities.

Non-random host tree infestation by the Neotropical liana Marcgravia longifolia.

The question whether or not tropical lianas infest host trees randomly or they exert host selection has implications for the structure and dynamics of tropical rainforests, particularly if colonization by lianas impacts host fitness. In this study, we present evidence that the Neotropical liana Marcgravia longifolia (Marcgraviaceae) infests host trees non-randomly. We identified host trees to species or genus level for 87 of the 100 M. longifolia individuals found in the study area of the Estación Biológica Quebrada Blanco (EBQB) in north-eastern Peruvian Amazonia. Data on host availability were taken from two 1-ha plots sampled at EBQB as part of a large-scale tree inventory in western Amazonia. Of the total of 88 tree genera with two or more individuals present in the inventory, 18 were represented amongst hosts. Host genera with a probability of colonization higher than expected by chance were Eschweilera (Lecythidaceae), Pouteria (Sapotaceae), Brosimum (Moraceae), and Hymenaea (Fabaceae). These findings suggest that M. longifolia exerts some level of host selectivity, but the mechanisms for this are completely unknown. Given the large number of animal species (41 bird species, three primate species) that are dispersing the seeds of M. longifolia and that have diverse ecological strategies, directed seed dispersal is unlikely to account for the observed patterns of host infestation.

Vertical stratification of seed-dispersing vertebrate communities and their interactions with plants in tropical forests.

Vertical stratification (VS) is a widespread phenomenon in plant and animal communities in forests and a key factor for structuring their species richness and biodiversity, particularly in tropical forests. The organisms composing forest communities adjust and shape the complex three-dimensional structure of their environment and inhabit a large variety of niches along the vertical gradient of the forest. Even though the degree of VS varies among different vertebrate groups, patterns of compositional stratification can be observed across taxa. Communities of birds, bats, primates, and non-flying small mammals are vertically stratified in terms of abundance, species richness, diversity, and community composition. Frugivorous members of these taxa play important roles as seed dispersers and forage on fruit resources that, in turn, vary in quantity and nutritional value along the vertical gradient. As a consequence, plant-seed disperser interaction networks differ among strata, which is manifested in differences in interaction frequencies and the degree of mutual specialization. In general, the canopy stratum is composed of strong links and generalized associations, while the lower strata comprise weaker links and more specialized interactions. Investigating the VS of communities can provide us with a better understanding of species habitat restrictions, resource use, spatial movement, and species interactions. Especially in the face of global change, this knowledge will be important as these characteristics can imply different responses of species and taxa at a fine spatial scale.