Size constraints and flower abundance determine the number of interactions in a plant-flower visitor web

Species morphology better predicts plant-hummingbird interactions across elevations than nectar traits

Species traits greatly influence interactions between plants and pollinators where floral nectar is the primary energy source fostering this mutualism. However, very little is known about how nectar traits mediate interactions in pollination networks compared with morphological traits. Here, we evaluated the role of morphological and nectar traits in shaping plant-hummingbird interaction networks along an elevation gradient. For this, we assessed patterns in floral phenotypic traits and network properties of plant species across elevations in Costa Rica. We also analysed whether plant species with generalized flower traits are ecological generalists and how morphological trait matching versus nectar traits affect interactions. We found marked variation in floral phenotypic traits and flower abundance of hummingbird-visited plant species across 10 sites along the elevation gradient. We did not find evidence for a relationship between flower morphology and nectar traits or between morphological and ecological generalization of plant species. Plant-hummingbird interaction frequency increased when the lengths of hummingbird bill and flower corolla were similar, indicating morphological matching, whereas nectar traits were unrelated to interactions. While nectar may play a difficult-to-detect secondary role within plant-hummingbird networks, our results reinforce the idea that morphological matching is an important factor in structuring ecological communities.

Development and anatomy of petals with specialized nectar holder and pollen container in Fumarioideae (Papaveraceae)

MAIN CONCLUSION

Petal developmental characteristics in Fumarioideae were similar at early stages, and the specialized nectar holder/pollen container formed by the outer/inner petals. The micro-morphology of these two structures, however, shows diversity in seven species. Elaborate petals have been modified to form different types, including petal lobes, ridges, protuberances, and spurs, each with specialized functions. Nectar holder and pollen container presumably have a function in plant-pollinator interactions. In Fumarioideae, four elaborate petals of the disymmetric/zygomorphic flower present architecture forming the "nectar holder" and "pollen container" structure at the bottom and top separately. In the present study, the petals of seven species in Fumarioideae were investigated by scanning electron microscopy, light microscope, and transmission electron microscopes. The results show that petal development could divided into six stages: initiation, enlargement, adaxial/abaxial differentiation, elaborate specializations (sacs, spurs, and lobes formed), extension, and maturation, while the specialized "nectar holder" and "pollen container" structures mainly formed in stage 4. "Nectar holder" is developed from the shallow sac/spur differentiated at the base of the outer petal, eventually forming a multi-organized complex structure, together with staminal nectaries (1-2) with individual sizes. A semi-closed ellipsoidal "pollen container" is developed from the apical part of the 3-lobed inner petals fused by middle lobes and attain different sizes. The adaxial epidermis cells are specialized, with more distinct punctate/dense columnar protrusions or wavy cuticles presented on obviously thickening cell walls. In addition, a large and well-developed cavity appears between the inner and outer epidermis of the petals. As an exception, Hypecoum erectum middle lobes present stamen mimicry. Elaborate petal structure is crucial for comprehending the petal diversity in Fumarioideae and provides more evidence for further exploration of the reproductive study in Papaveraceae.

Body size of fungus-growing termites infers on the volume and density of their fungal cultivar

The body size of an animal plays a crucial role in determining its trophic level and position within the food web, as well as its interactions with other species. In the symbiosis between Termitomyces and fungus-growing termites, termites rely on nutrition of fungal nodules produced by Termitomyces. To understand whether the size of termites and fungal nodules are related to their partner specificity, we quantified the size of termite farmer caste, and the size and density of nodules in termite nests of four genera of fungus-growing termites, and identified their cultivated Termitomyces fungus species based on internal transcribed spacer regions and partial large subunit ribosomal RNA gene sequences. The results showed that the size and density of fungal nodules were different among Termitomyces clades and revealed a constant trade-off between size and density among clades. The nodule size of each clade has low variation and fits normal distribution, indicating that size is a stabilized trait. Moreover, we found larger termite genera cultivated Termitomyces with larger but less numerous nodules. Based on these results, we concluded that there is a size specificity between Termitomyces and fungus-growing termites, which may lead to diversification of Termitomyces as adaptations to different termite genera.

Plant Functional Dispersion, Vulnerability and Originality Increase Arthropod Functions from a Protected Mountain Mediterranean Area in Spring

Plant diversity often contributes to the shape of arthropod communities, which in turn supply important ecosystem services. However, the current biodiversity loss scenario, particularly worrying for arthropods, constitutes a threat for sustainability. From a trait-based ecology approach, our goal was to evaluate the bottom-up relationships to obtain a better understanding of the conservation of the arthropod function within the ecosystem. Specifically, we aim: (i) to describe the plant taxonomic and functional diversity in spring within relevant habitats of a natural protected area from the Mediterranean basin; and (ii) to evaluate the response of the arthropod functional community to plants. Plants and arthropods were sampled and identified, taxonomic and functional indices calculated, and the plant-arthropod relationships analyzed. Generally, oak forests and scrublands showed a higher plant functional diversity while the plant taxonomic richness was higher in grasslands and chestnut orchards. The abundance of arthropod functional groups increased with the plant taxonomic diversity, functional dispersion, vulnerability and originality, suggesting that single traits (e.g., flower shape or color) may be more relevant for the arthropod function. Results indicate the functional vulnerability of seminatural habitats, the relevance of grasslands and chestnut orchards for arthropod functions and pave the way for further studies about plant-arthropod interactions from a trait-based ecology approach.

Different influences of phylogenetically conserved and independent floral traits on plant functional specialization and pollination network structure

Plant specialization and pollination network structure play important roles in community assembly. Floral traits can mediate plant-pollinator interactions and thus have important impacts on nestedness and modularity of pollination network. When such traits are phylogenetically conserved, therefore, phylogeny and traits should predict network structure to similar degrees. Moreover, conserved network structures were also found attributed to pollination syndrome or pollination system. However, we still know little about the relation between pollination syndrome and pollination network, especially under a phylogenetic framework. Herein, we established a phylogenetic framework including five floral traits (flower density, floral size, floral shape, floral symmetry, and floral color) and five species-level metrics (species strength, weighted closeness, specialization d', nestedness contribution, and modularity contribution) to test how floral traits could directly or indirectly influence species' specialization and network structure in central China. Phylogenetic signals were found in all floral traits except flower density. Structural equation model and phylogenetic structural equation model results showed that both floral size and floral density affected plant specialization and its contribution to network modularity indirectly. However, compared with phylogenetic independent flower density, phylogenetic conserved floral size had much more complexed influences, having a direct influence both on species' specialization and on modularity contribution. In this nested and modular network, abundant species with larger flowers tend to be more central and had larger values of z. Floral shape, symmetry, and color could act as co-flowering filters in pollination sharing and help to shape network modularity. Our results emphasize that phylogenetically conserved traits partially represent pollination syndrome and are important drivers for modular structure of local pollination network. This study may improve the understanding how the evolutionary history and ecological process drive local network structure and dynamics.

The evolution, ecology, and conservation of hummingbirds and their interactions with flowering plants

The ecological co-dependency between plants and hummingbirds is a classic example of a mutualistic interaction: hummingbirds rely on floral nectar to fuel their rapid metabolisms, and more than 7000 plant species rely on hummingbirds for pollination. However, threats to hummingbirds are mounting, with 10% of 366 species considered globally threatened and 60% in decline. Despite the important ecological implications of these population declines, no recent review has examined plant-hummingbird interactions in the wider context of their evolution, ecology, and conservation. To provide this overview, we (i) assess the extent to which plants and hummingbirds have coevolved over millions of years, (ii) examine the mechanisms underlying plant-hummingbird interaction frequencies and hummingbird specialization, (iii) explore the factors driving the decline of hummingbird populations, and (iv) map out directions for future research and conservation. We find that, despite close associations between plants and hummingbirds, acquiring evidence for coevolution (versus one-sided adaptation) is difficult because data on fitness outcomes for both partners are required. Thus, linking plant-hummingbird interactions to plant reproduction is not only a major avenue for future coevolutionary work, but also for studies of interaction networks, which rarely incorporate pollinator effectiveness. Nevertheless, over the past decade, a growing body of literature on plant-hummingbird networks suggests that hummingbirds form relationships with plants primarily based on overlapping phenologies and trait-matching between bill length and flower length. On the other hand, species-level specialization appears to depend primarily on local community context, such as hummingbird abundance and nectar availability. Finally, although hummingbirds are commonly viewed as resilient opportunists that thrive in brushy habitats, we find that range size and forest dependency are key predictors of hummingbird extinction risk. A critical direction for future research is to examine how potential stressors - such as habitat loss and fragmentation, climate change, and introduction of non-native plants - may interact to affect hummingbirds and the plants they pollinate.

A Systematic Review on Opuntia (Cactaceae; Opuntioideae) Flower-Visiting Insects in the World with Emphasis on Mexico: Implications for Biodiversity Conservation

Opuntia spp. are cacti with high ecological, economic and conservation interest in semiarid environments, particularly in Mexico. We conducted a systematic search of the existing peer-reviewed literature about the state of knowledge of pollination ecology on these plants. We documented the most studied Opuntia species worldwide with an emphasis on Mexico. We found that only 15% of Opuntia species described have been investigated so far, and studies were mainly focused on comprehension of the biology of a single species. Despite the economic and cultural importance of Opuntia, there is a significant lack of knowledge about the flower-visiting insects and their taxonomic identity. We provide a checklist of the insect species associated with Opuntia spp. Through a circular network, we visualize the complex Opuntia flower-visiting insect relationship, and we detected a set of key species constituting the generalist core of the networks constructed. Since pollination is crucial for crop production, a better understanding of ecological interactions would inform management measures to strengthen biodiversity and agriculture sustainability as well as productivity in arid and marginal lands. Further research on pollination ecology is needed to improve the conservation status of the insects associated with Opuntia species.

Morphological trait-matching in plant-Hymenoptera and plant-Diptera mutualisms across an elevational gradient

Morphological trait-matching and species abundance are thought to be the main factors affecting the frequency and strength of mutualistic interactions. However, the relative importance of trait-matching and species abundance in shaping species interactions across environmental gradients remains poorly understood, especially for plant-insect mutualisms involving generalist species. Here, we characterised variation in species and trait composition and the relative importance of trait-matching and species abundance in shaping plant-Hymenoptera and plant-Diptera mutualisms in four meadows across an elevational gradient (2,725-3,910 m) in Yulong Snow Mountain, Southwest China. We also evaluated the effects of morphological traits of flower visitors and plant composition on their foraging specialisation (d' and normalised degree). There was a high degree of dissimilarity in the composition of Hymenoptera and Diptera visitors and their visited plants between communities. This variation was mainly driven by the spatial replacement of species. Both for plant-Hymenoptera and plant-Diptera networks, trait-matching between nectar tube depth and proboscis length was a stronger predictor of the interactions between temporally co-occurring plants and flower visitors than species abundance. Fourth-corner analyses revealed statistically significant trait-matching between nectar tube depth and proboscis length in plant-Hymenoptera networks at all sites, suggesting that Hymenoptera consistently foraged on plant species with nectar tube depths matching their proboscis lengths. By contrast, significant trait-matching in plant-Diptera networks was only observed at the two lower elevation sites. The species-level specialisation d' of flower visitors increased significantly as the proboscis length and the difference in nectar tube depth between the plant community and the plants visited by flower visitors increased. Our results highlight that the importance of trait-matching in shaping pairwise interactions and niche partitioning depends on the specific features (e.g. species composition and trait availability) of the plant-pollinator system. For specialised plant-Hymenoptera systems, trait-matching is an important determinant of species interactions, whereas for generalist plant-Diptera systems, trait-matching is relatively unimportant.

Buzz-Pollination in a Tropical Montane Cloud Forest: Compositional Similarity and Plant-Pollinator Interactions

Buzz-pollinated plants are an essential source of pollen for a significant portion of local bee communities. Buzz pollination research has focused on studying the properties of bee buzzes and their implications on pollen release, morphological specialization of flowers, and the reproductive ecology of buzz-pollinated plants. In contrast, diversity patterns and ecological interactions between bees and buzz-pollinated plants have been studied less. This study analyzed the buzzing bee community of twelve tropical buzz-pollinated co-occurring plant species in a tropical montane cloud forest during the flowering periods of two consecutive years, focusing on diversity, compositional similarity, structure, and specialization (H₂´) of the network. Twenty-one bee species belonging to Apidae, Colletidae, and Halictidae were recorded, fifteen species in 2014, and eighteen in 2015. Floral display and visited flowers doubled from first to second year, although the flowering period was 2 months longer in the first year. Bee compositional similarity between plants tended to be low; however, this was due rather to a high nestedness than species replacement. Temporal bee compositional similarity was also low but variable, and different plant species showed the highest similarity between years. The number of bee visits depended significantly on the number of flowers and years. Interactions between bees and plants showed a tendency to generalization. Compared to other buzz-pollinated networks, specialization (H₂´) was similar, but diversity was low and the network small. In endangered ecosystems like the Mexican cloud forest, however, buzzing bees support biodiversity and provide an essential ecological service by pollinating dominant understory flora.

Using Matching Traits to Study the Impacts of Land-Use Intensification on Plant-Pollinator Interactions in European Grasslands: A Review

Simple Summary

Permanent grasslands are main habitats for many plant species and pollinators. Their destruction as well as their intensification has a major impact on plant and pollinator biodiversity, which has a cascading effect on pollination. However, we lack an understanding of these effects, thereby limiting our ability to predict them. In this review, we synthesised the literature on the mechanisms behind this cascade to provide new insights into the relationship between land-use intensification and pollination. By matching functional traits that mediate the relationship between the two trophic levels, we identified major knowledge gaps about how land-use intensification affects plant–pollinator interactions and how it favours plants with generalised floral traits, which are likely harmful to pollination.

Abstract

Permanent grasslands are suitable habitats for many plant and animal species, among which are pollinating insects that provide a wide range of ecosystem services. A global crisis in pollination ecosystem service has been highlighted in recent decades, partly the result of land-use intensification. At the grassland scale, however, the underlying mechanisms of land-use intensification that affect plant–pollinator interactions and pollination remain understudied. In this review, we first synthesise the literature to provide new insights into the relationships between land-use intensification and pollination by using matching community and interaction traits. We then identify knowledge gaps and summarise how land-use intensification of grassland influences floral traits that may in turn be associated with modifications to pollinator matching traits. Last, we summarise how these modifications may affect pollination function on permanent grasslands. Overall, land-use intensification may lead to a shift in flower colour, a decrease in mean nectar tube depth and a decrease in reward production and pollen quality at the community level. This, in turn, may generate a decrease in pollinator mouthparts length and body size, that may favour pollinators that require a low amount of floral reward. We found no study citing the effect of land-use intensification on volatile organic compounds emitted by flowers despite the importance of these molecules in pollinator community composition. Overall, our review highlighted major knowledge gaps about the effects of land-use intensification on plant–pollinator interactions, and suggests that land-use intensification could favour plants with generalised floral traits that adversely affect pollination.

Phenological traits foster persistence of mutualistic networks by promoting facilitation

Morphological and phenological traits are key determinants of the structure of mutualistic networks. Both traits create forbidden links, but phenological traits can also decouple interaction in time. While such difference likely affects the indirect effects among species and consequently network persistence, it remains overlooked. Here, using a dynamic model, we show that networks structured by phenology favour facilitation over competition within guilds of pollinators and plants, thereby increasing network persistence, while the contrary holds for networks structured by morphology. We further show that such buffering of competition by phenological traits mostly beneficiate to specialists, the most vulnerable species otherwise, which propagate the most positive effects within guilds and promote nestedness. Our results indicate that beyond trophic mismatch, phenological shifts such as those induced by climate change are likely to affect indirect effects within mutualistic assemblages, with consequences for biodiversity.

Edible Fruit Plant Species in the Amazon Forest Rely Mostly on Bees and Beetles as Pollinators

Edible fruit plants of tropical forests are important for the subsistence of traditional communities. Understanding the most important pollinators related to fruit and seed production of these plants is a necessary step to protect their pollination service and assure the food security of these communities. However, there are many important knowledge gaps related to floral biology and pollination in megadiverse tropical rainforests, such as the Amazon Forest, due mainly to the high number of plant species. Our study aims to indicate the main pollinators of edible plants (mainly fruits) of the Amazon forest. For this, we adopted a threefold strategy: we built a list of edible plant species, determined the pollination syndrome of each species, and performed a review on the scientific literature searching for their pollinator/visitors. The list of plant species was determined from two specialized publications on Amazon fruit plants, totaling 188 species. The pollination syndrome was determined for 161 species. The syndromes most frequently found among the analyzed species were melittophily (bee pollination), which was found in 101 of the analyzed plant species (54%) and cantharophily (beetle pollination; 26 species; 14%). We also found 238 pollinator/visitor taxa quoted for 52 (28%) plant species in previous publications, with 124 taxa belonging to Apidae family (bees; 52%), mainly from Meliponini tribe (58 taxa; 47%). Knowledge about pollinators is an important step to help on preserving their ecosystem services and maintaining the productivity of fruit trees in the Amazon.

Drought-induced reduction in flower size and abundance correlates with reduced flower visits by bumble bees

Reduced water availability can cause physiological stress in plants that affects floral development leading to changes in floral morphology and traits that mediate interactions with pollinators. As pollinators can detect small changes in trait expressions, drought-induced changes in floral traits could affect pollinator visitations. However, the linkage between changes in floral traits and pollinator visitations under drought conditions is not well explored. We, therefore, tested whether drought-induced changes in floral morphology and abundance of flowers are linked to changes in pollinator visitations. We conducted flight cage experiments with a radio frequency identification system for automated visitation recordings with bumble bees (Bombus terrestris) and common charlock (Sinapis arvensis) as the model system. In total, we recorded interactions for 31 foraging bumble bees and 6569 flower visitations. We found that decreasing soil moisture content correlated with decreasing size of all measured morphological traits except stamen length and nectar tube width. The reductions in floral size, petal width and length, nectar tube depth and number of flowers resulted in decreasing visitation rates by bumble bees. These decreasing visitations under lower soil moisture availability might be explained by lower numbers of flowers and thus a reduced attractiveness and/or by increased difficulties experienced by bumble bees in handling smaller flowers. Whether these effects act additively or synergistically on pollinator behaviour and whether this leads to changes in pollen transfer and to different selectionp ressures require further investigation.

Bumblebee Behavior on Flowers, but Not Initial Attraction, Is Altered by Short-Term Drought Stress

Climate change is leading to increasing drought and higher temperatures, both of which reduce soil water levels and consequently water availability for plants. This reduction often induces physiological stress in plants, which in turn can affect floral development and production inducing phenotypic alterations in flowers. Because flower visitors notice and respond to small differences in floral phenotypes, changes in trait expression can alter trait-mediated flower visitor behavior. Temperature is also known to affect floral scent emission and foraging behavior and, therefore, might modulate trait-mediated flower visitor behavior. However, the link between changes in flower visitor behavior and floral traits in the context of increasing drought and temperature is still not fully understood. In a wind-tunnel experiment, we tested the behavior of 66 Bombus terrestris individuals in response to watered and drought-stressed Sinapis arvensis plants and determined whether these responses were modulated by air temperature. Further, we explored whether floral traits and drought treatment were correlated with bumblebee behavior. The initial attractiveness of drought and watered plants did not differ, as the time to first visit was similar. However, bumblebees visited watered plants more often, their visitation rate to flowers was higher on watered plants, and bumblebees stayed for longer, indicating that watered plants were more attractive for foraging. Bumblebee behavior differed between floral trait expressions, mostly independently of treatment, with larger inflorescences and flowers leading to a decrease in the time until the first flower visit and an increase in the number of visits and the flower visitation rate. Temperature modulated bumblebee activity, which was highest at 25°C; the interaction of drought/water treatment and temperature led to higher visitation rate on watered plants at 20°C, possibly as a result of higher nectar production. Thus, bumblebee behavior is influenced by the watered status of plants, and bumblebees can recognize differences in intraspecific phenotypes involving morphological traits and scent emission, despite overall morphological traits and scent emission not being clearly separated between treatments. Our results indicate that plants are able to buffer floral trait expressions against short-term drought events, potentially to maintain pollinator attraction.

The role of individual variation in flowering and pollination in the reproductive success of a crepuscular buzz-pollinated plant

BACKGROUND AND AIMS

Plant individuals within a population differ in their phenology and interactions with pollinators. However, it is still unknown how individual differences affect the reproductive success of plants that have functionally specialized pollination systems. Here, we evaluated whether plant individual specialization in phenology (temporal specialization) and in pollination (pollinator specialization) affect the reproductive success of the crepuscular-bee-pollinated plant Trembleya laniflora (Melastomataceae).

METHODS

We quantified flowering activity (amplitude, duration and overlap), plant-pollinator interactions (number of flowers visited by pollinators) and reproductive success (fruit set) of T. laniflora individuals from three distinct locations in rupestrian grasslands of southeastern Brazil. We estimated the degree of individual temporal specialization in flowering phenology and of individual specialization in plant-pollinator interactions, and tested their relationship with plant reproductive success.

KEY RESULTS

Trembleya laniflora presented overlapping flowering, a temporal generalization and specialized pollinator interactions. Flowering overlap among individuals and populations was higher than expected by chance but did not affect the individual interactions with pollinators and nor their reproductive success. In contrast, higher individual generalization in the interactions with pollinators was related to higher individual reproductive success.

CONCLUSIONS

Our findings suggest that individual generalization in plant-pollinator interaction reduces the potential costs of specialization at the species level, ensuring reproductive success. Altogether, our results highlight the complexity of specialization/generalization of plant-pollinator interactions at distinct levels of organization, from individuals to populations, to species.

Urban fragmentation leads to lower floral diversity, with knock-on impacts on bee biodiversity

Bees and flowering plants are two closely interacting groups of organisms. Habitat loss and fragmentation associated with urbanisation are major threats to both partners. Yet how and why bee and floral richness and diversity co-vary within the urban landscape remain unclear. Here, we sampled bees and flowering plants in urban green spaces to investigate how bee and flowering plant species richness, their phylogenetic diversity and pollination-relevant functional trait diversity influence each other in response to urban fragmentation. As expected, bee abundance and richness were positively related to flowering plant richness, with bee body size (but not bee richness and diversity) increasing with nectar-holder depth of flowering plants. Causal modelling indicated that bottom-up effects dictated patterns of bee-flower relationships, with urban fragmentation diminishing flowering plants richness and thereby indirectly reducing bee species richness and abundance. The close relationship between bees and flowering plants highlights the risks of their parallel declines in response to land-use change within the urban landscape.

Floral Complexity Traits as Predictors of Plant-Bee Interactions in a Mediterranean Pollination Web

Despite intensive research, predicting pairwise species associations in pollination networks remains a challenge. The morphological fit between flowers and pollinators acts as a filter that allows only some species within the network to interact. Previous studies emphasized the depth of floral tubes as a key shape trait that explains the composition of their animal visitors. Yet, additional shape-related parameters, related to the handling difficulty of flowers, may be important as well. We analyzed a dataset of 2288 visits by six bee genera to 53 flowering species in a Mediterranean plant community. We characterized the plant species by five discrete shape parameters, which potentially affect their accessibility to insects: floral shape class, tube depth, symmetry, corolla segmentation and type of reproductive unit. We then trained a random forest machine-learning model to predict visitor identities, based on the shape traits. The model’s predictor variables also included the Julian date on which each bee visit was observed and the year of observation, as proxies for within- and between-season variation in flower and bee abundance. The model attained a classification accuracy of 0.86 (AUC = 0.96). Using only shape parameters as predictors reduced its classification accuracy to 0.76 (AUC = 0.86), while using only the date and year variables resulted in a prediction accuracy of 0.69 (AUC = 0.80). Among the shape-related variables considered, flower shape class was the most important predictor of visitor identity in a logistic regression model. Our study demonstrates the power of machine-learning algorithms for understanding pollination interactions in a species-rich plant community, based on multiple features of flower morphology.

Floral traits are associated with the quality but not quantity of heterospecific stigmatic pollen loads

Background

In flowering communities, plant species commonly share pollinators and therefore plant individuals receive heterospecific pollen (HP). However, the patterns of HP transfers can deviate from patterns of plant-pollinator visitations. Although flower-visitor interactions are known to be mediated by floral traits, e.g. floral size or nectar tube depth, the explanatory power of these traits for HP transfer patterns remains elusive. Here, we have explored pollen transfer patterns at three sites in Southern Germany on three dates (early, mid and late summer). At the plant level, we tested whether flower abundance and floral traits are correlated with HP reception and donation. At the community level, we determined whether flower and bee diversity are correlated with network modularity and whether floral traits explain the module affiliation of plant species. We collected the stigmas of flowering plant species, analysed HP and conspecific pollen (CP) loads and measured floral traits, flower and bee diversity.

Results

Our results show that the degree and intensity of HP reception or donation at the plant level do not correlate with floral traits, whereas at the community level, the module affiliation of who is sharing pollen with whom is well-explained by floral traits. Additionally, variation in network modularity between communities is better explained by plant diversity and abundance than by bee diversity and abundance.

Conclusions

Overall, our results indicate that floral traits that are known to mediate flower-visitor interactions can improve our understanding of qualitative HP transfer but only provide limited information about the quantity of HP transfer, which more probably depends on other floral traits, flower-visitor identity or community properties.

Size and isolation of naturally isolated habitats do not affect plant-bee interactions: A case study of ferruginous outcrops within the eastern Amazon forest

Pollination may be severely affected by the decreasing size and increasing isolation of habitat patches. However, most studies that have considered the effects of these two variables on plant-pollinator interactions have been carried out in areas that have undergone anthropogenic fragmentation, and little is known about their effects in natural habitats. The Carajás National Forest and Campos Ferruginosos National Park are two protected areas in the eastern Amazon where one can find isolated ferruginous outcrops characterized by iron-rich soil and herbaceous-shrub vegetation surrounded by Amazon forest. These patches of canga provide an opportunity to analyze plant-pollinator interactions in naturally fragmented areas. Our objective was to test whether the size and isolation of naturally isolated outcrops located in Carajás affect plant-pollinator interactions by using pollination syndromes and interaction networks. We determined the pollination syndromes of 771 plant species that occurred in eleven canga patches and performed field work to analyze plant-pollinator networks in nine canga patches. The structure of the plant-pollinator networks was not affected by the size or isolation of the canga patches. Generalist species were present in all canga areas, indicating that they are important in maintaining the plant communities in isolated canga patches. The lack of significance related to the distance between canga patches suggests that the forest does not prevent pollinator movement between canga patches.

Disentangling direct and indirect effects of habitat fragmentation on wild plants' pollinator visits and seed production

Habitat fragmentation threatens plant and pollinator communities, as well as their interactions. However, the effects of landscape fragmentation on the pollination of wild plant species are not well understood yet, partly because there are many correlated features in fragmented landscapes (e.g., decreased patch size, increased isolation, and patch complexity) whose influences are difficult to disentangle. Using a structural equation modeling approach, we assessed the direct and indirect effects of landscape fragmentation (patch size, isolation and complexity, percentage of surrounding land in forest) on the abundance, functional-group richness, and evenness of pollinators of 24 habitat fragments within an agricultural landscape in Southern Norway. In addition, we studied how these variables affected visitation rates (visits per flower) and seed production (seed set, seed mass) in the four most abundant plant species in the area. Flower abundance was higher in larger and complex patches and decreased with the percentage of forest in the surroundings, while flower richness increased with patch complexity. We found a direct negative relationship between patch complexity and the overall number of pollinator visits that the habitat fragments received. Apart from this direct landscape effect, pollinator visits were mostly affected by the floral communities, with overall flower abundance and richness increasing both total number of pollinator visits and pollinator-group richness, and flower richness having an additional negative influence on pollinator-group evenness. Interestingly, we did not find any direct link between visitation rates and reproductive success for any of the study plant species. Instead, several landscape variables directly affected species seed production, although the effects of landscape on seed production were highly species specific. Patch complexity had a negative effect on seed production in two of the four focal species, while other components of the landscape had species-specific effects. Increasing fragmentation of agricultural landscapes affects pollination interactions at the community level and the reproduction of wild plants. However, understanding the effects of fragmentation on seed production requires going beyond estimating visitation rates, since landscape effects on plant reproduction are not always related to overall interaction frequencies.

Trait matching and phenological overlap increase the spatio-temporal stability and functionality of plant-pollinator interactions

Morphology and phenology influence plant-pollinator network structure, but whether they generate more stable pairwise interactions with higher pollination success remains unknown. Here we evaluate the importance of morphological trait matching, phenological overlap and specialisation for the spatio-temporal stability (measured as variability) of plant-pollinator interactions and for pollination success, while controlling for species' abundance. To this end, we combined a 6-year plant-pollinator interaction dataset, with information on species traits, phenologies, specialisation, abundance and pollination success, into structural equation models. Interactions among abundant plants and pollinators with well-matched traits and phenologies formed the stable and functional backbone of the pollination network, whereas poorly matched interactions were variable in time and had lower pollination success. We conclude that phenological overlap could be more useful for predicting changes in species interactions than species abundances, and that non-random extinction of species with well-matched traits could decrease the stability of interactions within communities and reduce their functioning.

Relative contribution of ecological and biological attributes in the fine-grain structure of ant-plant networks

Background

Ecological communities of interacting species analyzed as complex networks have shown that species dependence on their counterparts is more complex than expected at random. As for other potentially mutualistic interactions, ant-plant networks mediated by extrafloral nectar show a nested (asymmetric) structure with a core of generalist species dominating the interaction pattern. Proposed factors structuring ecological networks include encounter probability (e.g., species abundances and habitat heterogeneity), behavior, phylogeny, and body size. While the importance of underlying factors that influence the structure of ant-plant networks have been separately explored, the simultaneous contribution of several biological and ecological attributes inherent to the species, guild or habitat level has not been addressed.

Methods

For a tropical seasonal site we recorded (in 48 censuses) the frequency of pairwise ant-plant interactions mediated by extrafloral nectaries (EFN) on different habitats and studied the resultant network structure. We addressed for the first time the role of mechanistic versus neutral determinants at the ‘fine-grain’ structure (pairwise interactions) of ant-plant networks. We explore the simultaneous contribution of several attributes of plant and ant species (i.e., EFN abundance and distribution, ant head length, behavioral dominance and invasive status), and habitat attributes (i.e., vegetation structure) in prevailing interactions as well as in overall network topology (community).

Results

Our studied network was highly-nested and non-modular, with core species having high species strengths (higher strength values for ants than plants) and low specialization. Plants had higher dependences on ants than vice versa. We found that habitat heterogeneity in vegetation structure (open vs. shaded habitats) was the main factor explaining network and fine-grain structure, with no evidence of neutral (abundance) effects.

Discussion

Core ant species are relevant to most plants species at the network showing adaptations to nectar consumption and deterrent behavior. Thus larger ants interact with more plant species which, together with higher dependence of plants on ants, suggests potential biotic defense at a community scale. In our study site, heterogeneity in the ant-plant interactions among habitats is so prevalent that it emerges at community-level structural properties. High frequency of morphologically diverse and temporarily-active EFNs in all habitats suggests the relevance and seasonality of plant biotic defense provided by ants. The robust survey of ecological interactions and their biological/ecological correlates that we addressed provides insight of the interplay between adaptive-value traits and neutral effects in ecological networks.

Interconnectedness of the Grinnellian and Eltonian Niche in Regional and Local Plant-Pollinator Communities

Understanding the causes and consequences of coexistence and thus biodiversity is one of the most fundamental endeavors of ecology, which has been addressed by studying species' requirements and impacts - conceptualized as their Grinnellian and Eltonian niches. However, different niche types have been mostly studied in isolation and thus potential covariation between them remains unknown. Here we quantified the realized Grinnellian niche (environmental requirements), the fundamental (morphological phenotype) and realized Eltonian niche (role in networks) of plant and pollinator taxa at a local and regional scale to investigate the interconnectedness of these niche types. We found a strong and scale-independent co-variation of niche types suggesting that taxa specialized in environmental factors are also specialized in their position in trait spaces and their role in bipartite networks. The integration of niche types thus will help to detect the true causes for species distributions, interaction networks, as well as the taxonomic and functional diversity of communities.

Wild pollinator activity negatively related to honey bee colony densities in urban context

As pollinator decline is increasingly reported in natural and agricultural environments, cities are perceived as shelters for pollinators because of low pesticide exposure and high floral diversity throughout the year. This has led to the development of environmental policies supporting pollinators in urban areas. However, policies are often restricted to the promotion of honey bee colony installations, which resulted in a strong increase in apiary numbers in cities. Recently, competition for floral resources between wild pollinators and honey bees has been highlighted in semi-natural contexts, but whether urban beekeeping could impact wild pollinators remains unknown. Here, we show that in the city of Paris (France), wild pollinator visitation rates are negatively correlated to honey bee colony densities present in the surrounding landscape (500m -slope = -0.614; p = 0.001 -and 1000m -slope = -0.489; p = 0.005). Regarding the morphological groups of wild pollinators, large solitary bee and beetle visitation rates were negatively affected by honey bee colony densities within a 500m buffer (slope = -0.425, p = 0.007 and slope = - 0.671, p = 0.002, respectively) and bumblebee visitation rates were negatively affected by honey bee colony density within a 1000m buffer (slope = - 0.451, p = 0.012). Further, lower interaction evenness in plant-pollinator networks was observed with high honey bee colony density within a 1000m buffer (slope = -0.487, p = 0.008). Finally, honey bees tended to focus their foraging activity on managed rather than wild plant species (student t-test, p = 0.001) whereas wild pollinators equally visited managed and wild species. We advocate responsible practices mitigating the introduction of high density of honey bee colonies in urban environments. Further studies are however needed to deepen our knowledge about the potential negative interactions between wild and domesticated pollinators.

How does urbanization affect the reproductive characteristics and ecological affinities of street plant communities?

Anthropogenic activities in urban ecosystems induce a myriad of environmental changes compared with adjacent rural areas. These environmental changes can be seen as series of abiotic and biotic selection filters affecting the distribution of plant species. What are the attributes of plant species that compose urban communities, compared with rural communities, as related to their ecological affinities (e.g., to temperature, humidity), and reproductive traits (e.g., entomophily, autogamy, floral morphology)? Using a floristic dataset from a citizen science project recording plant species growing spontaneously in the streets, we analyzed the distribution of species according to their ecological requirements and reproductive traits along an urbanization gradient in the Parisian region. We developed an original floral and pollinator typology composed of five floral and four pollinator morphotypes. The proportion of impervious areas, used as a proxy of urbanization, was measured at different spatial scales, to reveal at which spatial scales urbanization is selecting plant traits. We found significant differences in plant communities along the urbanization gradient. As expected with the warmer and drier conditions of urban areas, species with higher affinities to higher temperature, light and nutrient soil content, and lower atmospheric moisture were over-represented in urban plant communities. Interestingly, all of the significant changes in plant abiotical affinities were the most pronounced at the largest scale of analysis (1,000 m buffer radius), probably because the specific urban conditions are more pronounced when they occur on a large surface. The proportion of autogamous, self-compatible, and nonentomophilous species was significantly higher in urban plant communities, strongly suggesting a lower abundance or efficiency of the pollinating fauna in urban environments. Last, among insect-pollinated species, those with relatively long and narrow tubular corollas were disadvantaged in urban areas, possibly resulting from a reduction in pollinator abundance particularly affecting specialized plant-pollinator interactions.

Pollinators and visitors of the generalized food-deceptive orchid Dactylorhiza majalis in North-Eastern Poland

Pollinator foraging behavior plays a key role in breeding and therefore affects the evolution of the orchid reproductive strategy. Food-deceptive orchids usually implement a generalized plant pollination strategy and a relatively diverse group of pollinators visit them. Dactylorhiza majalis is a food-deceptive, early-flowering orchid that relies on insect-mediated pollination. This study’s objectives were to identify D. majalis’ pollinators and flower visitors and their foraging behaviors on D. majalis inflorescences. We also assessed the bending movement time to determine the relationship between bending time and the duration of pollinators’ visits. To assess pollination efficiency, we measured the spur length of D. majalis flowers, which is expected to affect the mechanical fit to pollinators/“potential” pollinators. The arthropod fauna were investigated to examine the availability of “potential” pollinators in populations. We identified Apis mellifera as this orchid’s main pollinator and confirmed that few of the flower visitors belonged to Diptera (12 individuals, 9 taxa), Hymenoptera (3 individuals, 3 taxa), or Coleoptera (2 individuals, 2 taxa) in our dataset, which was collected over a 2-year period and includes 360 h of video. The arthropods were collected by a sweep net in D. majalis populations and there were fewer Hymenoptera (2.9–23.2%) and Coleoptera (4.4–23.8%) visitors but more Diptera (23.3–58.6%) visitors. We found that A. mellifera foraged in different ways on D. majalis inflorescences, thereby resulting in cross-pollination and/or geitonogamy; however, the bending time data supported the hypothesis about promoting cross-pollination while decreasing self-pollination, but these data do not exclude the possibility of geitonogamy.

Foraging efficiency and size matching in a plant-pollinator community: the importance of sugar content and tongue length

A long-standing question in ecology is how species interactions are structured within communities. Although evolutionary theory predicts close size matching between floral nectar tube depth and pollinator proboscis length of interacting species, such size matching has seldom been shown and explained in multispecies assemblages. Here, we investigated the degree of size matching among Asteraceae and their pollinators and its relationship with foraging efficiency. The majority of pollinators, especially Hymenoptera, choose plant species on which they had high foraging efficiencies. When proboscides were shorter than nectar tubes, foraging efficiency rapidly decreased because of increased handling time. When proboscides were longer than nectar tubes, a decreased nectar reward rather than an increased handling time made shallow flowers more inefficient to visit. Altogether, this led to close size matching. Overall, our results show the importance of nectar reward and handling time as drivers of plant-pollinator network structure.

Flower visitation by hoverflies (Diptera: Syrphidae) in a temperate plant-pollinator network

Hoverflies (Diptera: Syrphidae) are among the most important pollinators, although they attract less attention than bees. They are usually thought to be rather opportunistic flower visitors, although previous studied demonstrated that they show colour preferences and their nectar feeding is affected by morphological constraints related to flower morphology. Despite the growing appreciation of hoverflies and other non-bee insects as pollinators, there is a lack of community-wide studies of flower visitation by syrphids. The aim of this paper is to provide a detailed analysis of flower visitation patterns in a species rich community of syrphids in a Central European grassland and to evaluate how species traits shape the structure of the plant-hoverfly flower visitation network. We found that different species varied in the level of specialisation, and while some species visited a similar spectre of flowers, others partitioned resources more strongly. There was a consistent difference in both specialisation and flower preferences between three syrphid subfamilies. Eristalinae and Pipizinae were more specialised than Syrphinae. Trait-based analyses showed that relative flower visitation (i) increased with plant height, but most strongly in Eristalinae; (ii) increased with inflorescence size in small species from all three subfamilies, but was independent of inflorescence size in large species of Eristalinae and Syrphinae; and (iii) depended on flower colour, but in a subfamily-specific way. Eristalinae showed the strongest flower colour preferences for white flowers, Pipizinae visited mostly white and yellow flowers, while Syrphinae were less affected by flower colour. Exploration of the structure of the plant-hoverfly flower visitation network showed that the network was both modular and nested. We also found that there were almost no differences in specialisation and relative visitation frequency between males and females. Overall, we showed that flower visitation in syrphids was affected by phylogenetic relatedness, body size of syrphids and several plant traits.

Zooming into plant-flower visitor networks: an individual trait-based approach

Understanding how ecological communities are structured is a major goal in ecology. Ecological networks representing interaction patterns among species have become a powerful tool to capture the mechanisms underlying plant-animal assemblages. However, these networks largely do not account for inter-individual variability and thus may be limiting our development of a clear mechanistic understanding of community structure. In this study, we develop a new individual-trait based approach to examine the importance of individual plant and pollinator functional size traits (pollinator thorax width and plant nectar holder depth) in mutualistic networks. We performed hierarchical cluster analyses to group interacting individuals into classes, according to their similarity in functional size. We then compared the structure of bee-flower networks where nodes represented either species identity or trait sets. The individual trait-based network was almost twice as nested as its species-based equivalent and it had a more symmetric linkage pattern resulting from of a high degree of size-matching. In conclusion, we show that by constructing individual trait-based networks we can reveal important patterns otherwise difficult to observe in species-based networks and thus improve our understanding of community structure. We therefore recommend using both trait-based and species-based approaches together to develop a clearer understanding of the properties of ecological networks.

Unmasking the architecture of ant-diaspore networks in the Brazilian Savanna

Ant–diaspore interactions are directly related to fruit consumption, seed predation and dispersal, being determinant for the plant fitness. However, although abundant and diversified, these ecological interactions have been neglected in network studies. Understanding the structure of these networks is the first step in preserving these ecological functions. However, describing the network structure is not enough; we need to understand what mechanisms are behind the network patterns. In this study, for the first time, we describe the structure of the ant–diaspore network, considering only the interactions that can benefit plants, separating it into fruit consumption and diaspore removal networks in the Brazilian Savanna. We postulated that ant–diaspore interactions tend to be more specialized in the diaspore removal network compared to the fruit consumption network. Furthermore, we tested whether morphological features, such as size of mandibles of ants and diaspores, could modulate these ecological networks. Overall, we recorded 24 ant and 29 plant species interacting. We found that fruit consumption and diaspore removal networks exhibited similar patterns of interactions (i.e., non-modular), although only the diaspore removal network was nested. The diaspore removal network did not show a more specialized pattern than the fruit consumption network, since both networks consisted of opportunistic interactions. We found that ant mandible and diaspore size does not explain the structure of ecological networks, but in diaspore removal networks the relationship between these morphological traits may explain the pattern of interactions. Thus, we showed that mandible size of ants may have implications on seedling recruitment, suggesting that mandible size can predict possible effects on plant fitness within in diaspore removal networks. Overall, ant–diaspore networks maintain important ecological functions, such as fruit consumption and seed dispersal, which often implies an increase in reproductive success of the plants.

Plant and animal functional diversity drive mutualistic network assembly across an elevational gradient

Species’ functional traits set the blueprint for pair-wise interactions in ecological networks. Yet, it is unknown to what extent the functional diversity of plant and animal communities controls network assembly along environmental gradients in real-world ecosystems. Here we address this question with a unique dataset of mutualistic bird–fruit, bird–flower and insect–flower interaction networks and associated functional traits of 200 plant and 282 animal species sampled along broad climate and land-use gradients on Mt. Kilimanjaro. We show that plant functional diversity is mainly limited by precipitation, while animal functional diversity is primarily limited by temperature. Furthermore, shifts in plant and animal functional diversity along the elevational gradient control the niche breadth and partitioning of the respective other trophic level. These findings reveal that climatic constraints on the functional diversity of either plants or animals determine the relative importance of bottom-up and top-down control in plant–animal interaction networks.

Differential responses of plant and animal functional diversity to climatic variation could affect trait matching in mutualistic interactions. Here, Albrecht et al. show that network structure varies across an elevational gradient owing to bottom-up and top-down effects of functional diversity.

Phenotypic trait matching predicts the topology of an insular plant-bird pollination network

Conceptualizing species interactions as networks has broadened our understanding of ecological communities. However, the factors shaping interaction patterns among species and, therefore, network structure remain unclear. One potentially important factor is the matching of phenotypic traits. Here, we tested for trait matching in a bird-flower visitation network from New Zealand. We first quantified the overall network structure and tested whether flower size could account for differences in the visitation rates of flowering plants. We then explored the relationship between the flower size and bill size. The results showed that the interaction network is nested. Plant species with large flowers received more visits from birds than plant species with small flowers. Moreover, plant species with large flowers were visited more frequently by birds with large bills, while species with smaller flowers were visited more frequently by birds with small bills. Overall, the interaction patterns between birds and flowering plants could be predicted by their morphology, suggesting that phenotypic trait matching is an important predictor of network structure.