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Recent environmental changes due to land-use and climate change threaten biodiversity and the ecosystem services it provides. Understanding the true scope of these changes is complicated by the lack of historical baselines for many of the interactions underpinning ecosystem services, such as pollination, or disservices, such as disease spreading. To assess changes in such services, it is vital to find ways of comparing past and current interactions between species. Here, we focus on interactions between honey bees – one of the world’s most important agricultural pollinators, the plants they visit, and the microbes they encounter in the environment. DNA in honey offers insights into the contemporary interactions of honey bees. Old honey samples could serve to describe honey bees’ interactions in previous decades, providing a baseline against which to assess changes in interactions over time. By identifying the taxonomic origin of plant, bacterial and fungal DNA in fifty-year-old honey samples, we show that plant DNA can reveal which plants honey bees visited in the past. Likewise, microbe DNA records the microbes, including pollinator and plant pathogens, honey bees encountered and possibly spread. However, some differences in the DNA recovered between old and new honey suggest that differences in DNA degradation of different microbes could bias naive comparisons between samples. Like other types of ancient samples, old honey may be most useful for identifying interactions that historically occurred and should not be taken as proof that an interaction did not occur. Keeping these limits of the data in mind, time series of honey may offer unique information about how honey bees’ associations with flowers and microbes have changed during decades of environmental change.

Background: Contrasting hypotheses suggest that the number of biotic interactions per species could either increase towards the equator due to the increasing richness of potential interaction partners (Neutral theory), or decrease in the tropics due to increased biotic competition (Latitudinal Biotic Interaction Hypothesis). Empirical testing of these hypotheses remains limited due to practical limitations, differences in methodology, and species turnover across latitudes. Here, we focus on a single species with a worldwide distribution, the honey bee (Apis mellifera L.), to assess how the number of different types of interactions vary across latitudes. Foraging honey bees interact with many organisms in their local environment, including plants they actively select to visit and microbes that they largely encounter passively (i.e., unintentionally and more or less randomly). Tissue pieces and spores of these organisms are carried to the hive by foraging honey bees and end up preserved within honey, providing a rich record of the species honey bees encounter in nature.

Results: Using honey samples from around the globe, we show that while honey bees visit more plant taxa at higher latitudes, they encounter more bacteria in the tropics.

Conclusions: These different components of honey bees’ biotic niche support the latitudinal biotic interaction hypothesis for actively-chosen interactions, but are more consistent with neutral theory (assuming greater bacterial richness in the tropics) for unintentional interactions.

Global environmental change may lead to changes in community structure and in species interactions, ultimately changing ecosystem functioning. Focusing on spatial variation in fungus–plant interactions across the rapidly changing Arctic, we quantified variation in the identity of interaction partners. We then related interaction turnover to variation in the bioclimatic environment by combining network analyses with general dissimilarity modelling. Overall, we found species associations to be highly plastic, with major rewiring among interaction partners across variable environmental conditions. Of this turnover, a major part was attributed to specific environmental properties which are likely to change with progressing climate change. Our findings suggest that the current structure of plant-root associated interactions may be severely altered by rapidly advancing global warming. Nonetheless, flexibility in partner choice may contribute to the resilience of the system.

Microbiota, the communities of microbes on and in organisms or organic matter, are essential for the functioning of ecosystems. How microbes are shared and transmitted delineates the formation of a microbiota. As pollinators forage, they offer a route to transfer microbes among the flowering plants, themselves, and their nests. To assess how the two components of the microbiota, bacteria and fungi, in pollination communities are shared and transferred, we focused on the honey bee Apis mellifera and collected honey bee, honey (representing the hive microbiota), and flower samples three times during the summer in Finland. We identified the bacteria and fungi by DNA metabarcoding. To determine the impact of honey bees’ flower choices on the honey bee and hive microbiota, we identified also plant DNA in honey. The bacterial communities of honey bees, honey, and flowers all differ greatly from each other, while the fungal communities of honey bees and honey are very similar, yet different from flowers. The time of the summer and the sampling area influence all these microbiota. For flowers, the plant identity impacts both bacterial and fungal communities’ composition the most. For the dispersal pathways of bacteria to honey bees, they are acquired directly from the honey and indirectly from flowers through the honey, while fungi are directly transmitted to honey bees from flowers. Overall, the distinctiveness of the microbiota of honey bees, honey, and the surrounding flowers suggests the sharing of microbes among them occurs but plays a minor role for the established microbiota.

Most animal pollination results from plant–insect interactions, but how we perceive these interactions may differ with the sampling method adopted. The two most common methods are observations of visits by pollinators to plants and observations of pollen loads carried by insects. Each method could favour the detection of different species and interactions, and pollen load observations typically reveal more interactions per individual insect than visit observations. Moreover, while observations concern plant and insect individuals, networks are frequently analysed at the level of species. Although networks constructed using visitation and pollen-load data have occasionally been compared in relatively specialised, bee-dominated systems, it is not known how sampling methodology will affect our perception of how species (and individuals within species) interact in a more generalist system. Here we use a Diptera-dominated high-Arctic plant–insect community to explore how sampling approach shapes several measures of species' interactions (focusing on specialisation), and what we can learn about how the interactions of individuals relate to those of species. We found that species degrees, interaction strengths, and species motif roles were significantly correlated across the two method-specific versions of the network. However, absolute differences in degrees and motif roles were greater than could be explained by the greater number of interactions per individual provided by the pollen-load data. Thus, despite the correlations between species roles in networks built using visitation and pollen-load data, we infer that these two perspectives yield fundamentally different summaries of the ways species fit into their communities. Further, individuals' roles generally predicted the species' overall role, but high variability among individuals means that species' roles cannot be used to predict those of particular individuals. These findings emphasize the importance of adopting a dual perspective on bipartite networks, as based on the different information inherent in insect visits and pollen loads.