Co-flowering neighbors influence the diversity and identity of pollinator groups visiting plant species

The generalization–specialization continuum exhibited in pollination interactions currently receives much attention. It is well-known that the pollinator assemblage of particular species varies temporally and spatially, and therefore the ecological generalization on pollinators may be a contextual attribute. However, the factors causing such variation and its ecological and evolutionary consequences are still poorly understood. This variation can be caused by spatial or temporal variation in the pollinator community, but also by variation in the plant community. Here, we examined how the floral neighbourhood influenced the generalization on pollinators and the composition of pollinators of six plant species differing in generalization levels and main pollinators. The diversity, identity and density of floral species affected both the level of generalization on pollinators and the composition of visitors of particular plant species. Although the relationships to floral neighbourhood varied considerably among species, generalization level and visitation by uncommon pollinators generally increased with floral diversity and richness. The generalization level of the neighbourhood was negatively related to the generalization level of the focal species in two species. The number of flowers of the pollinator-sharing species and the number of flowers of the focal species had different effects on the composition of visits in different species; attributable to differences in facilitation/competition for pollinator attraction. We propose that an important ecological implication of our results is that variation in species interactions caused by the pollination context may result in increased community stability. The main evolutionary implication of our results is that selection on flower and pollinator traits may depend, to an unknown extent, on the composition of the co-flowering plant community.     

The role of local species abundance in the evolution of pollinator attraction in flowering plants

We present a population genetic model that incorporates aspects of pollinator efficiency and abundance to examine the effect of the local plant community on the evolution of floral trait specialization. Our model predicts that plant species evolve to be pollinator specialists on the most effective and common pollinators when their abundance is low relative to other plant species in the community (i.e., conspecific pollen is relatively rare) and evolve to be pollinator generalists when they are numerically dominant (i.e., conspecific pollen is abundant). Strong flower constancy also favors generalist floral traits. Furthermore, generalist species are predicted to differentiate when there is a concave trade-off in attracting pollinator species with different floral trait preferences. This result implies that populations that evolve toward a generalist strategy may be more prone to speciation. Ours is the first theoretical model to include local species abundance explicitly, despite the fact that it has been previously identified as an important factor in the evolution of plant specialization. Our results add a layer of ecological complexity to previous models of floral evolution and therefore have the potential to improve our power to predict circumstances under which specialized and generalized plant-pollinator interactions should evolve.     

Spatial variability in a plant–pollinator community across a continuous habitat: high heterogeneity in the face of apparent uniformity

Large‐scale spatial variability in plant–pollinator communities (e.g. along geographic gradients, across different landscapes) is relatively well understood. However, we know much less about how these communities vary at small scales within a uniform landscape. Plants are sessile and highly sensitive to microhabitat conditions, whereas pollinators are highly mobile and, for the most part, display generalist feeding habits. Therefore, we expect plants to show greater spatial variability than pollinators. We analysed the spatial heterogeneity of a community of flowering plants and their pollinators in 40 plots across a 40‐km² area within an uninterrupted Mediterranean scrubland. We recorded 3577 pollinator visits to 49 plant species. The pollinator community (170 species) was strongly dominated by honey bees (71.8% of the visits recorded). Flower and pollinator communities showed similar beta‐diversity, indicating that spatial variability was similar in the two groups. We used path analysis to establish the direct and indirect effects of flower community distribution and honey bee visitation rate (a measure of the use of floral resources by this species) on the spatial distribution of the pollinator community. Wild pollinator abundance was positively related to flower abundance. Wild pollinator visitation rate was negatively related to flower abundance, suggesting that floral resources were not limiting. Pollinator and flower richness were positively related. Pollinator species composition was weakly related to flower species composition, reflecting the generalist nature of flower–pollinator interactions and the opportunistic nature of pollinator flower choices. Honey bee visitation rate did not affect the distribution of the wild pollinator community. Overall, we show that, in spite of the apparent physiognomic uniformity, both flowers and pollinators display high levels of heterogeneity, resulting in a mosaic of idiosyncratic local communities. Our results provide a measure of the background of intrinsic heterogeneity within a uniform habitat, with potential consequences on low‐scale ecosystem function and microevolutionary patterns.     

Effects of different types of low‐intensity management on plant‐pollinator interactions in Estonian grasslands

In the face of global pollinator decline, extensively managed grasslands play an important role in supporting stable pollinator communities. However, different types of extensive management may promote particular plant species and thus particular functional traits. As the functional traits of flowering plant species (e.g., flower size and shape) in a habitat help determine the identity and frequency of pollinator visitors, they can also influence the structures of plant−pollinator interaction networks (i.e., pollination networks). The aim of this study was to examine how the type of low‐intensity traditional management influences plant and pollinator composition, the structure of plant−pollinator interactions, and their mediation by floral and insect functional traits. Specifically, we compared mown wooded meadows to grazed alvar pastures in western Estonia. We found that both management types fostered equal diversity of plants and pollinators, and overlapping, though still distinct, plant and pollinator compositions. Wooded meadow pollination networks had significantly higher connectance and specialization, while alvar pasture networks achieved higher interaction diversity at a standardized sampling of interactions. Pollinators with small body sizes and short proboscis lengths were more specialized in their preference for particular plant species and the specialization of individual pollinators was higher in alvar pastures than in wooded meadows. All in all, the two management types promoted diverse plant and pollinator communities, which enabled the development of equally even and nested pollination networks. The same generalist plant and pollinator species were important for the pollination networks of both wooded meadows and alvar pastures; however, they were complemented by management‐specific species, which accounted for differences in network structure. Therefore, the implementation of both management types in the same landscape helps to maintain high species and interaction diversity.     

The importance of interannual variation and bottom–up nitrogen enrichment for plant–pollinator networks

Striking changes in food web structure occur with alterations in resource supply. Like predator–prey interactions, many mutualisms are also consumer–resource interactions. However, no studies have explored how the structure of plant–pollinator networks may be affected by nutrient enrichment. For three years, we enriched plots of subalpine plant communities with nitrogen and observed subsequent effects on plant–pollinator network structure. Although nitrogen enrichment affects floral abundance and rates of pollinator visitation, we found no effects of nitrogen enrichment on the core group of generalist plants and pollinators or on plant–pollinator network structure parameters, such as network topology (the identity and frequency of interactions) and the degree of nestedness. However, individual plant and pollinator taxa were packed into the nested networks differently among nitrogen treatments. In particular, pollinators visited different numbers and types of plants in the nested networks, suggesting weak, widespread effects of nitrogen addition on individual taxa. Independent of nitrogen enrichment, there were large interannual differences in network structure and interactions, due to species turnover among years and flexibility in interacting with new partners. These data suggest that the community structure of small‐scale mutualistic networks may be relatively robust to short‐term bottom–up changes in the resource supply, but sensitive to variation in the opportunistic behavior and turnover of plant and pollinator species among years.     

Individual flowering phenology shapes plant–pollinator interactions across ecological scales affecting plant reproduction

The balance of pollination competition and facilitation among co-flowering plants and abiotic resource availability can modify plant species and individual reproduction. Floral resource succession and spatial heterogeneity modulate plant–pollinator interactions across ecological scales (individual plant, local assemblage, and interaction network of agroecological infrastructure across the farm). Intraspecific variation in flowering phenology can modulate the precise level of spatio-temporal heterogeneity in floral resources, pollen donor density, and pollinator interactions that a plant individual is exposed to, thereby affecting reproduction. We tested how abiotic resources and multi-scale plant–pollinator interactions affected individual plant seed set modulated by intraspecific variation in flowering phenology and spatio-temporal floral heterogeneity arising from agroecological infrastructure. We transplanted two focal insect-pollinated plant species (Cyanus segetum and Centaurea jacea, n = 288) into agroecological infrastructure (10 sown wildflower and six legume–grass strips) across a farm-scale experiment (125 ha). We applied an individual-based phenologically explicit approach to match precisely the flowering period of plant individuals to the concomitant level of spatio-temporal heterogeneity in plant–pollinator interactions, potential pollen donors, floral resources, and abiotic conditions (temperature, water, and nitrogen). Individual plant attractiveness, assemblage floral density, and conspecific pollen donor density (C. jacea) improved seed set. Network linkage density increased focal species seed set and modified the effect of local assemblage richness and abundance on C. segetum. Mutual dependence on pollinators in networks increased C. segetum seed set, while C. jacea seed set was greatest where both specialization on pollinators and mutual dependence was high. Abiotic conditions were of little or no importance to seed set. Intra- and interspecific plant–pollinator interactions respond to spatio-temporal heterogeneity arising from agroecological management affecting wild plant species reproduction. The interplay of pollinator interactions within and between ecological scales affecting seed set implies a co-occurrence of pollinator-mediated facilitative and competitive interactions among plant species and individuals.     

COMPETITION FOR HUMMINGBIRD POLLINATION SHAPES FLOWER COLOR VARIATION IN ANDEAN SOLANACEAE

One classic explanation for the remarkable diversity of flower colors across angiosperms involves evolutionary shifts among different types of pollinators with different color preferences. However, the pollinator shift model fails to account for the many examples of color variation within clades that share the same pollination system. An alternate explanation is the competition model, which suggests that color divergence evolves in response to interspecific competition for pollinators, as a means to decrease interspecific pollinator movements. This model predicts color overdispersion within communities relative to null assemblages. Here, we combine morphometric analyses, field surveys, and models of pollinator vision with a species‐level phylogeny to test the competition model in the primarily hummingbird‐pollinated clade Iochrominae (Solanaceae). Results show that flower color as perceived by pollinators is significantly overdispersed within sites. This pattern is not simply due to phylogenetic history: phylogenetic community structure does not deviate from random expectations, and flower color lacks phylogenetic signal. Moreover, taxa that occur in sympatry occupy a significantly larger volume of color space than those in allopatry, supporting the hypothesis that competition in sympatry drove the evolution of novel colors. We suggest that competition among close relatives may commonly underlie floral divergence, especially in species‐rich habitats where congeners frequently co‐occur.     

Between‐year changes in community composition shape species’ roles in an Arctic plant–pollinator network

Inter‐annual turnover in community composition can affect the richness and functioning of ecological communities. If incoming and outgoing species do not interact with the same partners, ecological functions such as pollination may be disrupted. Here, we explore the extent to which turnover affects species’ roles – as defined based on their participation in different motifs positions – in a series of temporally replicated plant–pollinator networks from high‐Arctic Zackenberg, Greenland. We observed substantial turnover in the plant and pollinator assemblages, combined with significant variation in species’ roles between networks. Variation in the roles of plants and pollinators tended to increase with the amount of community turnover, although a negative interaction between turnover in the plant and pollinator assemblages complicated this trend for the roles of pollinators. This suggests that increasing turnover in the future will result in changes to the roles of plants and likely those of pollinators. These changing roles may in turn affect the functioning or stability of this pollination network.     

Lifelong foraging and individual specialisation are influenced by temporal changes of resource availability

Resource availability largely determines the distribution and behaviour of organisms. In plant–pollinator communities, availability of floral resources may change so rapidly that pollinator individuals can benefit from switching between multiple resources, i.e. different flowering plant species. Insect pollinator individuals of a given generation often occur in different time windows during the reproductive season. This temporal variation in individual occurrences, together with the rapidly changing resource availability, may lead individuals of the same population to encounter and use different resources, resulting in an apparent individual specialisation. We hypothesized, that 1) individual pollinators change their resource use (flower visitation) during their lifetime according to the changing availability of floral resources, and that 2) temporal variation in individual occurrences of pollinators and in resource availability will partly explain individual specialisation. To test these hypotheses, we observed flower visitations of individually marked clouded Apollo butterflies Parnassius mnemosyne during one reproductive season. We found temporal changes in lifetime individual resource use that followed the changes in resource availability, indicating that butterflies can adjust foraging to varying resource availability. Individuals differed considerably in their resource use. This variation was partly explained by temporal variation in both floral resource availability and temporal occurrence of individual butterflies. We suggest the butterfly as a sequential specialist, i.e. short‐term specialist and long‐term generalist. This foraging plasticity can be essential for short‐living insect pollinators in rapidly changing environments. Although flowering dynamics do not fully explain the variability in foraging, our results highlight the importance of temporal dimension in resource use studies. Ultimately, the relative pace of environmental change compared to individual lifespan may be a key factor in resource use plasticity.     

Optimality modeling and fitness trade‐offs: when should plants become pollinator specialists?

The assumption that flowers readily evolve specializations for pollination by particular animals has been central to a standard view of pollinator-mediated adaptive divergence in angiosperms. Stebbins' Most Effective Pollinator Principle (MEPP) formalized this assumption in proposing that a plant should always evolve specializations to its most effective pollinator. I argue that the MEPP and its corollaries are unsupported by basic models of phenotypic selection which predict that a plant should evolve greater specialization to a particular pollinator when the marginal fitness gain exceeds the marginal fitness loss from becoming less adapted to all other pollinators. Differences in pollinator effectiveness are neither necessary nor sufficient to predict specialization. Differences in effectiveness certainly can foster floral specialization to the most effective pollinator in some cases, but when adaptation to a relatively ineffective pollinator requires little loss in the fitness contribution of a more effective pollinator, plants may exhibit striking specializations for the less effective pollinator. Recognizing that the effectiveness of pollinators is not tightly coupled to their importance in selecting for phenotypic novelty may resolve the mismatch between floral features that appear to represent clear evolutionary responses to specific pollinators and patterns of flower visitation that often seem generalized. To shed light on agents of selection and the adaptive value of floral traits I argue that we must go beyond measures of pollinator effectiveness to investigate pollinator-mediated fitness trade-offs over a range of floral phenotypes.     

Evolution of autonomous selfing accompanies increased specialization in the pollination system of Schizanthus (Solanaceae)

The co-occurrence of elaborate flowers visited by specific groups of pollinators and capacity for autonomous selfing in the same plant species has puzzled evolutionary biologists since the time of Charles Darwin. To examine whether autonomous selfing and floral specialization evolved in association, we quantified the autofertility level (AFI) in nine Schizanthus species characterized by a wide range of pollination specialization, revealing AFI values of 0.02 to complete selfing. An independent contrasts analysis conducted on AFIs and number of functional pollinator groups showed that autonomous selfing evolved from an ancestral outcrossing system as plants became increasingly specialized (r = -0.82). To assess whether autonomous selfing together with specialization acts as a reproductive assurance mechanism, we estimated spatial and interannual variation in fruit set due to pollinator failure in two closely related high Andean Schizanthus species differing in their specialization levels. Variation in pollinator failure rate was more pronounced and autonomous selfing increased fruit production over biotically assisted pollination in the more specialized species. Our study suggests that specialized pollination deems species more vulnerable to pollinator fluctuation thus promoting the evolution of delayed autonomous selfing.     

Comparing the conservatism of ecological interactions in plant–pollinator and plant–herbivore networks

Conservatism in species interaction, meaning that related species tend to interact with similar partners, is an important feature of ecological interactions. Studies at community scale highlight variations in conservatism strength depending on the characteristics of the ecological interaction studied. However, the heterogeneity of datasets and methods used prevent to compare results between mutualistic and antagonistic networks. Here we perform such a comparison by taking plant–insect communities as a study case, with data on plant–herbivore and plant–pollinator networks. Our analysis reveals that plants acting as resources for herbivores exhibit the strongest conservatism in species interaction among the four interacting groups. Conservatism levels are similar for insect pollinators, insect herbivores and plants as interacting partners of pollinators, although insect pollinators tend to have a slightly higher conservatism than the two others. Our results thus clearly support the current view that within antagonistic networks, conservatism is stronger for species as resources than for species as consumer. Although the pattern tends to be opposite for plant–pollinator networks, our results suggest that asymmetry in conservatism is much less pronounced between the pollinators and the plant they interact with. We discuss these differences in conservatism strength in relation with the processes structuring plant–insect communities.     

Resource heterogeneity, diet shifts and intra-cohort competition: effects on size divergence in YOY fish

The pollination syndrome hypothesis has provided a major conceptual framework for how plants and pollinators interact. However, the assumption of specialization in pollination systems and the reliability of floral traits in predicting the main pollinators have been questioned recently. In addition, the relationship between ecological and evolutionary specialization in pollination interactions is still poorly understood. We used data of 62 plant species from three communities across southern Norway to test: (1) the relationships between floral traits and the identity of pollinators, (2) the association between floral traits (evolutionary specialization) and ecological generalization, and (3) the consistency of both relationships across communities. Floral traits significantly affected the identity of pollinators in the three communities in a way consistent with the predictions derived from the pollination syndrome concept. However, hover flies and butterflies visited flowers with different shapes in different communities, which we mainly attribute to among-community variation in pollinator assemblages. Interestingly, ecological generalization depended more on the community-context (i.e. the plant and pollinator assemblages in the communities) than on specific floral traits. While open yellow and white flowers were the most generalist in two communities, they were the most specialist in the alpine community. Our results warn against the use of single measures of ecological generalization to question the pollination syndrome concept, and highlight the importance of community comparisons to assess the pollination syndromes, and to understand the relationships between ecological and evolutionary specialization in plant-pollinator interactions.     

Contrasting effects of invasive plants in plant–pollinator networks

The structural organization of mutualism networks, typified by interspecific positive interactions, is important to maintain community diversity. However, there is little information available about the effect of introduced species on the structure of such networks. We compared uninvaded and invaded ecological communities, to examine how two species of invasive plants with large and showy flowers (Carpobrotus affine acinaciformis and Opuntia stricta) affect the structure of Mediterranean plant–pollinator networks. To attribute differences in pollination to the direct presence of the invasive species, areas were surveyed that contained similar native plant species cover, diversity and floral composition, with or without the invaders. Both invasive plant species received significantly more pollinator visits than any native species and invaders interacted strongly with pollinators. Overall, the pollinator community richness was similar in invaded and uninvaded plots, and only a few generalist pollinators visited invasive species exclusively. Invasive plants acted as pollination super generalists. The two species studied were visited by 43% and 31% of the total insect taxa in the community, respectively, suggesting they play a central role in the plant–pollinator networks. Carpobrotus and Opuntia had contrasting effects on pollinator visitation rates to native plants: Carpobrotus facilitated the visit of pollinators to native species, whereas Opuntia competed for pollinators with native species, increasing the nestedness of the plant–pollinator network. These results indicate that the introduction of a new species to a community can have important consequences for the structure of the plant–pollinator network.     

Pollination networks and functional specialization: a test using Lesser Antillean plant–hummingbird assemblages

Network analysis has in recent years improved our understanding of pollination systems. However, there is very little information about how functionally specialized plants and pollinators interact directly and indirectly in pollination networks. We have developed a parameter, Functional specialization index, to quantify functional specialization in pollination networks. Using this parameter, we examined whether different sized hummingbirds visit a distinct set of flowers in five hummingbird-pollinated plant assemblages from the Lesser Antilles, obtaining a simple relationship between hummingbird body size, network parameter and ecological function. In the Lesser Antilles, functionally specialized hummingbird pollination is distinct for plant species pollinated by the largest hummingbird species, whereas the pollination niche gradually integrates with the insect pollinator community as hummingbird body size decreases. The network approach applied in this study can be used to validate functional specialization and community-level interdependence between plants and pollinators, and it is therefore useful for evaluating and predicting plant resilience to pollinator loss, presently a global concern.     

Urbanization increases floral specialization of pollinators

Understanding how urbanization alters functional interactions among pollinators and plants is critically important given increasing anthropogenic land use and declines in pollinator populations. Pollinators often exhibit short‐term specialization and visit plants of the same species during one foraging trip. This facilitates plant receipt of conspecific pollen—pollen on a pollinator that is the same species as the plant on which the pollinator was foraging. Conspecific pollen receipt facilitates plant reproductive success and is thus important to plant and pollinator persistence. We investigated how urbanization affects short‐term specialization of insect pollinators by examining pollen loads on insects’ bodies and identifying the number and species of pollen grains on insects caught in urban habitat fragments and natural areas. We assessed possible drivers of differences between urban and natural areas, including frequency dependence in foraging, species richness and diversity of the plant and pollinator communities, floral abundance, and the presence of invasive plant species. Pollinators were more specialized in urban fragments than in natural areas, despite no differences in the species richness of plant communities across site types. These differences were likely driven by higher specialization of common pollinators, which were more abundant in urban sites. In addition, pollinators preferred to forage on invasive plants at urban sites and native plants at natural sites. Our findings reveal indirect effects of urbanization on pollinator fidelity to individual plant species and have implications for the maintenance of plant species diversity in small habitat fragments. Higher preference of pollinators for invasive plants at urban sites suggests that native species may receive fewer visits by pollinators. Therefore, native plant species diversity may decline in urban sites without continued augmentation of urban flora or removal of invasive species.     

Temporal stability of pollinator preference in an alpine plant community and its implications for the evolution of floral traits

A traditional view of diverse floral traits is that they reflect differences in foraging preferences of pollinators. The role of pollinators in the evolution of floral traits has been questioned recently by broad community surveys, especially studies concerning variation in pollinator assemblages and visitation frequency, which suggest a diminished role of pollinators in floral evolution. Here, we investigate the relationships between six categories of floral traits of 29 species and 10 pollinator functional groups in an alpine meadow in the Hengduan Mountains of China, over three consecutive years. Simpson’s diversity index was used to estimate the level of pollinator generalization of each plant species by considering both pollinator groups and their relative visitation frequencies. Multivariate analyses indicated that eight of the ten pollinator groups showed constant preferences for at least two floral traits, leading to a relatively stable level of ecological generalization for most floral traits (two out of three categories), despite the fact that the level of generalization of the entire community varied across years. Shape preferences of butterflies, honeybees and beeflies varied such that open flowers exhibited a lower level of ecological generalization in 2007 than closed flowers, in contrast with the other 2 years. These results suggest that temporally stabilized preferences of diverse pollinators may contribute to the evolution of specialized versus generalized floral traits; however, their role may be moderated by variation in community structure, including both the composition and abundance of plants and pollinators.     

Diversity and Distribution of Floral Resources Influence the Restoration of Plant–Pollinator Networks on a Reclaimed Strip Mine

Plant–pollinator mutualisms are one of the several functional relationships that must be reinstated to ensure the long‐term success of habitat restoration projects. These mutualisms are unlikely to reinstate themselves until all of the resource requirements of pollinators have been met. By meeting these requirements, projects can improve their long‐term success. We hypothesized that pollinator assemblage and structure and stability of plant–pollinator networks depend both on aspects of the surrounding landscape and of the restoration effort itself. We predicted that pollinator species diversity and network stability would be negatively associated with distance from remnant habitat, but that local floral diversity might rescue pollinator diversity and network stability in locations distant from the remnant. We created plots of native prairie on a reclaimed strip mine in central Ohio, U.S.A. that ranged in floral diversity and isolation from the remnant habitat. We found that the pollinator diversity declined with distance from the remnant habitat. Furthermore, reduced pollinator diversity in low floral diversity plots far from the remnant habitat was associated with loss of network stability. High floral diversity, however, compensated for losses in pollinator diversity in plots far from the remnant habitat through the attraction of generalist pollinators. Generalist pollinators increased network connectance and plant‐niche overlap. As a result, network robustness of high floral diversity plots was independent of isolation. We conclude that the aspects of the restoration effort itself, such as floral community composition, can be successfully tailored to incorporate the restoration of pollinators and improve success given a particular landscape context.     

Pollinator individual‐based networks reveal the specialized plant–pollinator mutualism in two biodiverse communities

Generalization of pollination systems is widely accepted by ecologists in the studies of plant–pollinator interaction networks at the community level, but the degree of generalization of pollination networks remains largely unknown at the individual pollinator level. Using potential legitimate pollinators that were constantly visiting flowers in two alpine meadow communities, we analyzed the differences in the pollination network structure between the pollinator individual level and species level. The results showed that compared to the pollinator species‐based networks, the linkage density, interaction diversity, interaction evenness, the average plant linkage level, and interaction diversity increased, but connectance, degree of nestedness, the average of pollinator linkage level, and interaction diversity decreased in the pollinator individual‐based networks, indicating that pollinator individuals had a narrower food niche than their counterpart species. Pollination networks at the pollinator individual level were more specialized at the network level (H′₂) and the plant species node level (d′) than at the pollinator species‐level networks, reducing the chance of underestimating levels of specialization in pollination systems. The results emphasize that research into pollinator individual‐based pollination networks will improve our understanding of the pollination networks at the pollinator species level and the coevolution of flowering plants and pollinators.     

Pollinator diversity and specialization in relation to flower diversity

In the face of global decline in biodiversity, the relationship between diversity and species interactions deserves particular attention. If pollinators are strongly dependent on floral diversity due to mutual specialization, declines in plant diversity, e.g. caused by land use intensification, may be associated with linked extinctions of pollinators. However, the general extent of pollinator specialization is still poorly known. To explore the dependence of local bee and hoverfly communities on flower diversity, we recorded flower supply and flower‐visiting insects on 27 meadows with varying flower diversity in southern Germany and analyzed (a) whether the diversity of flower visitors is correlated with flower diversity, (b) whether the degree of dietary specialization of flower visitors changes with flower diversity and (c) whether flower preferences of individual flower visitor species are constant or variable between different communities. Flower–visitor interaction webs were compiled during a single day on each meadow. This approach prevents relating pollinator species to flowers they never encounter because of non‐overlapping phenology or spatial segregation. (a) Flower diversity and flower visitor diversity were positively correlated. (b) Flower visitor assemblies were significantly specialized at a relatively high level, contrasting to the opinion that plant–pollinator webs are highly generalized, and providing a possible explanation for the positive diversity correlation. However, the level of specialization did not change significantly across the gradient of flower diversity, suggesting that pollinators are partitioned to a similar extent in each meadow. (c) In the analysis of ten common flower visitor species previously categorized as generalists, strong evidence was found for both, consistent preferences and preferences that differ between sites. These results indicate a flexibility in flower preferences and a dynamic resource partitioning among pollinators. Generally, our findings highlight the complexity of plant–pollinator interactions and confirm the importance of flower diversity for bee and hoverfly communities.