The Restoration of Plant–Pollinator Interactions in Hay Meadows

Whether restoration programs successfully reinstate ecological interactions remains a contentious and largely untested issue. We investigated plant and pollinator interactions on two old and two restored hay meadows, with the aim of evaluating if quantitative patterns of insect visitation and pollen transport were comparable among old and restored meadows. In terms of structural diversity, few species of plants and insects were shared among the webs. In all four meadows, Diptera and Hymenoptera dominated the visitor community in terms of both species richness and abundance. Coleoptera, Hemiptera, and Lepidoptera comprised the remainder of the flower visitors. No significant difference was found between restored and old sites in plant or insect species richness or in plant and insect abundance. In terms of function, the meadows appeared more similar, although a slightly higher proportion of the potential links between plants and insects was realized for old meadows. No difference was found in the proportion of plant species visited, and visited plant species were generalized, with all having more than a single species of insect visitor. We also sampled approximately 400,000 pollen grains from the flower‐visiting insects. There were no differences between old and restored sites in the amount of pollen being transported or in the average number of pollen grains per insect. At both types of meadows, Hymenoptera carried most pollen, followed by Diptera. Again, generalization was the norm, with all plants having more than a single species of pollen carrier. No difference was observed in the connectance of pollen transport webs between old and restored sites. Overall, although the four meadows showed considerable structural variation, they showed similarity with regard to the functional processes we studied. Because structural variation is expected among localities, we conclude that the two restoration projects have been successful.     

Long-term observation of a pollination network: fluctuation in species and interactions, relative invariance of network structure and implications for estimates of specialization

We analysed the dynamics of a plant-pollinator interaction network of a scrub community surveyed over four consecutive years. Species composition within the annual networks showed high temporal variation. Temporal dynamics were also evident in the topology of the network, as interactions among plants and pollinators did not remain constant through time. This change involved both the number and the identity of interacting partners. Strikingly, few species and interactions were consistently present in all four annual plant-pollinator networks (53% of the plant species, 21% of the pollinator species and 4.9% of the interactions). The high turnover in species-to-species interactions was mainly the effect of species turnover (c. 70% in pairwise comparisons among years), and less the effect of species flexibility to interact with new partners (c. 30%). We conclude that specialization in plant-pollinator interactions might be highly overestimated when measured over short periods of time. This is because many plant or pollinator species appear as specialists in 1 year, but tend to be generalists or to interact with different partner species when observed in other years. The high temporal plasticity in species composition and interaction identity coupled with the low variation in network structure properties (e.g. degree centralization, connectance, nestedness, average distance and network diameter) imply (i) that tight and specialized coevolution might not be as important as previously suggested and (ii) that plant-pollinator interaction networks might be less prone to detrimental effects of disturbance than previously thought. We suggest that this may be due to the opportunistic nature of plant and animal species regarding the available partner resources they depend upon at any particular time.     

Variation in mutualisms: the spatiotemporal mosaic of a pollinator assemblage

Variation in time (annual and seasonal) and space (between- and within-population) is examined for the pollinator assemblage of Lavandula latifolia (Labiatae), an insect-pollinated, summer- flowering, evergreen shrub of Mediterranean woodlands in southeastern Spain. Lavandula latifolia is pollinated there by nearly 85 species of comparatively long-tongued Hymenoptera, Diptera and Lepidoptera. The diversity, composition and abundance of pollinators varied markedly between years. Lepidopteran taxa prevailed numerically in one year, hymenopterans in four years, and both groups had similar abundance in the remaining year. The vast majority of pollinators exhibited significant annual differences in average abundance. Only 35.7% of taxa were recorded in all of the six study years. The pollinator assemblage had marked seasonal dynamics. Important changes in species richness, abundance and composition took place over the flowering season of L. latifolia (nearly 3 months). Only 21.7% of insect taxa occurred throughout the flowering season. Lavandula latifolia populations at different distances from water courses differed broadly in the abundance and composition of pollinators. Hymenopterans dominated numerically at the water-distant sites, whereas lepidopterans prevailed in populations growing in the vicinity of streams. Only 40.7% of taxa were recorded at all of the four study populations. Within the same plant population, horizontal distances of the order of 25 m resulted in significant changes in pollinator composition. Variations at the four scales considered combine to produce, from the viewpoint of L. latifolia, a spatio-temporal mosaic of pollinators. As these differ broadly in frequency of pollen transfer, size of pollen loads deposited, and between-flower flight distance patterns, spatio-temporal variation will predictably result in inconsistent or contradictory selective pressures on the plant, thus hindering specialization in relation to particular pollinators.     

Diversity, flower visitation frequency and generalism of pollinators in temperate rain forests of Chiloé Island, Chile

Species richness and taxonomic composition of pollinator assemblages are documented for 26 plant species from temperate rain forests of northern Chiloé Island, southern Chile (42°30'S). We investigated the patterns of generalism and specialization among plants and animal pollinators by comparing the flower visit frequency by different pollen vectors during the spring and summer months of three consecutive years (2000–2002). Species studied exhibited a range of floral morphologies (radial vs. zygomorphic, open vs. tubular) and rewards (nectar and/or pollen). Overall, we recorded 172 pollinator species, with an average of 6.6 species of pollen vectors/plant species. Pollinators visited an average of 15.2 plant species/pollen vector. Pollinator assemblages were dominated by Coleoptera (75 species), Diptera (56 species) and Hymenoptera (30 species), but passerine birds and hummingbirds were also important. The most specialized plants were vines, including the bee-pollinated genus Luzuriaga (Philesiaceae) and two endemic species of hummingbird-pollinated Gesneriaceae. Hymenoptera contributed 41.2% of all visits, with the bumblebee Bombus dalhbomii accounting for 22.5% of these. Plants with unspecialized flower morphology supported a higher species richness of pollinators, but visiting rates did not differ from specialized flowers.  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 147 , 399–416.     

Fine‐tuning the nested structure of pollination networks by adaptive interaction switching,biogeography and sampling effect in the Galápagos Islands

The structure of pollination networks, particularly its nestedness, contain important information on network assemblages. However, there is still limited understanding of the mechanisms underlying nested pollination network structures. Here, we investigate the role of adaptive interaction switching (AIS), island area, isolation, age and sampling effort in explaining the nestedness of pollination networks across ten Galápagos Islands. The AIS algorithm is inspired by Wallace's elimination of the unfit, where a species constantly replaces its least profitable mutualistic partner with a new partner selected at random. To explain network structures, we first use a dynamic model that includes functional response of pollination and AIS, with only species richness and binary connectance as input (hereafter the AIS model). Thereafter, other explanatory variables (isolation, area, age and sampling effort) were added to the model. In four out of ten islands, the pollination network was significantly nested, and predictions from the AIS model correlated with observed structures, explaining 69% variation in nestedness. Overall, in terms of independent contribution from hierarchical partitioning of variation in observed nestedness, the AIS model predictions contributed the most (37%), followed by sampling effort (28%) and island area (22%), with only trivial contributions from island isolation and age. Therefore, adaptive switching of biotic interactions seems to be key to ensure network function, with island biogeographic factors being only secondary. Although large islands could harbour more diverse assemblages and thus foster more nested structures, sufficient sampling proves to be essential for detecting non‐random network structures.     

Pollinator specialization increases with a decrease in a mass‐flowering plant in networks inferred from DNA metabarcoding

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Specialist pollinators deplete pollen in the spring ephemeral wildflower Claytonia virginica

Pollinators that collect pollen – and specifically, pollen‐specialist bees – are often considered to be the best pollinators of a (host) plant. Although pollen collectors and pollen specialists often benefit host plants, especially in the pollen that they deliver (their pollination “effectiveness”), they can also exact substantial costs because they are motivated to collect as much pollen as possible, reducing the proportion of pollen removed that is subsequently delivered to stigmas (their pollination “efficiency”). From the plant perspective, pollen grains that do not pollinate conspecific stigmas are “wasted”, and potentially costly. We measured costs and benefits of nectar‐collecting, pollen‐collecting, and pollen‐specialist pollinator visitation to the spring ephemeral Claytonia virginica. Visits by the pollen‐specialist bee Andrena erigeniae depleted pollen quickly and thoroughly. Although all pollinators delivered roughly the same number of grains, the pollen specialist contributed most to C. virginica pollen delivery because of high visitation rates. However, the pollen specialist also removed a large number of grains; this removal may be especially costly because it resulted in the depletion of pollen grains in C. virginica populations. While C. virginica appears to rely on pollen transfer by the pollen specialist in these populations, nectar‐collecting visitors could provide the same benefit at a lower cost if their visitation rates increased. Pollen depletion affects a pollinator's value to plants, but is frequently overlooked. If they lower the effectiveness of future floral visitors, visits by A. erigeniae females to C. virginica may be more detrimental than beneficial compared to other pollinators and may, in some circumstances, reduce plant fitness rather than increase it. Therefore, A. erigeniae and C. virginica may vary in their degree of mutualism depending on the ecological context.     

The topological differences between visitation and pollen transport networks: a comparison in species rich communities of the Himalaya–Hengduan Mountains

Pollination networks are usually constructed and assessed by direct field observations which commonly assume that all flower visitors are true pollinators. However, this assumption is often invalid and the use of data based on mere visitors to flowers may lead to a misunderstanding of intrinsic pollination networks. Here, using a large dataset by both sampling floral visitors and analyzing their pollen loads, we constructed 32 networks pairs (visitation versus pollen transport) across one flowering season at four elevation sites in the Himalaya–Hengduan Mountains region. Pollen analysis was conducted to determine which flower visitors acted as potential pollinators (pollen vectors) or as cheaters (those not carrying pollen of the visited plants). We tested whether there were topological differences between visitation and pollen transport networks and whether different taxonomic groups of insect visitors differed in their ability to carry pollen of the visited plants. Our results indicated that there was a significantly higher degree of specialization at both the network and species levels in the pollen transport networks in contrast to the visitation networks. Modularity was lower but nestedness was higher in the visitation networks compared to the pollen transport networks. All the cheaters were identified as peripheral species and most of them contributed positively to the nested structure. This may explain in part the differences in modularity and nestedness between the two network types. Bees carried the highest proportion of pollen of the visited plants. This was followed by Coleoptera, other Hymenoptera and Diptera. Lepidoptera carried the lowest proportion of pollen of the visited plants. Our study shows that the construction of pollen transport networks could provide a more in‐depth understanding of plant–pollinator interactions. Moreover, it suggests that detecting and removing cheater interactions when studying the topology of other mutualistic networks might be also important.     

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.     

Primary pollinator exclusion has divergent consequences for pollen dispersal and mating in different populations of a bird‐pollinated tree

Pollination by nectarivorous birds is predicted to result in different patterns of pollen dispersal and plant mating compared to pollination by insects. We tested the prediction that paternal genetic diversity, outcrossing rate and realized pollen dispersal will be reduced when the primary pollinator group is excluded from bird‐pollinated plants. Pollinator exclusion experiments in conjunction with paternity analysis of progeny were applied to Eucalyptus caesia Benth. (Myrtaceae), a predominantly honeyeater‐pollinated tree that is visited by native insects and the introduced Apis mellifera (Apidae). Microsatellite genotyping at 14 loci of all adult E. caesia at two populations (n = 580 and 315), followed by paternity analysis of 705 progeny, revealed contrasting results between populations. Honeyeater exclusion did not significantly impact pollen dispersal or plant mating at Mount Caroline. In contrast, at the Chiddarcooping site, the exclusion of honeyeaters led to lower outcrossing rates, a threefold reduction in the average number of sires per fruit, a decrease in intermediate‐distance mating and an increase in near‐neighbour mating. The results from Chiddarcooping suggest that bird pollination may increase paternal genetic diversity, potentially leading to higher fitness of progeny and favouring the evolution of this strategy. However, further experimentation involving additional trees and study sites is required to test this hypothesis. Alternatively, insects may be effective pollinators in some populations of bird‐adapted plants, but ineffective in others.     

传粉昆虫下降背景下的授粉生态弹性: 内涵、机制和展望

全球传粉昆虫多样性正在下降, 如何保障农林生态系统传粉功能是当前研究的热点。理论上说, 传粉功能不仅与生态系统的传粉昆虫多样性相关, 还与生态系统的调节能力有关。近年来, 学者们逐渐认识到授粉生态弹性对传粉功能的影响。本文在回顾已有研究的基础之上, 总结传粉昆虫授粉生态弹性的内涵, 厘清授粉生态弹性与工程弹性、稳定性和抗性的异同。目前, 学者对授粉生态弹性形成机制开展广泛探讨, 提出功能冗余假说、密度补偿假说、响应多样性假说、连接周转假说和跨尺度弹性假说, 但这5个假说间的关系仍不清楚, 存在一词多义、词意混淆等现象。我们依次阐述功能冗余假说、密度补偿假说、响应多样性假说、连接周转假说和跨尺度弹性假说, 介绍不同假说中授粉生态弹性形成过程、研究热点和发展动态。通过解析授粉生态弹性的形成机制可知, 5个假说在内涵上存在紧密联系, 它们从不同空间尺度和研究对象下解释传粉昆虫授粉生态弹性的形成机制。未来授粉生态弹性研究将整合传粉昆虫群落动态和传粉功能动态的量化方法, 通过实验验证5个假说的合理性, 并揭示不同假说间的联系, 由此阐明授粉生态弹性的发生条件、形成阈值和动态规律。随着研究的深入, 授粉生态弹性理论有望用于指导农林生态系统传粉功能的经营管理。     

The role of the endemic and critically endangered Colorful Puffleg Eriocnemis mirabilis in plant‐hummingbird networks of the Colombian Andes

Ecological network approaches may contribute to conservation practices by quantifying within‐community importance of species. In mutualistic plant‐pollinator systems, such networks reflect potential pollination of the plants and a considerable portion of the energy consumption by the pollinators, two key components for each party. Here, we used two different sampling approaches to describe mutualistic plant‐hummingbird networks from a cloud forest in the Colombian Western Andes, home to the Colorful Puffleg Eriocnemis mirabilis, an endemic and critically endangered hummingbird. We contrast networks between two localities (a protected area inside a National park vs. its buffer zone) and across sampling methods (floral visitation vs. pollen loads) to assess how the network structure and the importance of each hummingbird species within the networks may change. Visitation networks were characterized as having higher sampling completeness, yet pollen load network recorded more pollen types than plant species recorded by visitation. Irrespective of the sampling methods, the Colorful Puffleg was one of the most important hummingbird species in the network within the protected area inside the National park, but not in the buffer zone. Moreover, most species‐level network indices were related to hummingbirds’ abundance. This suggests that conservation initiatives aimed at the endangered Colorful Puffleg may both help on the survival of this endangered hummingbird, as well as on maintaining its key role in the mutualistic interaction network inside the National Park. Our study illustrates how conservation practitioners could assess the local importance of endangered species using interaction network approaches.     

Testing the potential for conflicting selection on floral chemical traits by pollinators and herbivores: predictions and case study

1.  There are myriad ways in which pollinators and herbivores can interact via the evolutionary and behavioural responses of their host plants.
2.  Given that both herbivores and pollinators consume and are dependent upon plant-derived nutrients and secondary metabolites, and utilize plant signals, plant chemistry should be one of the major factors mediating these interactions.
3.  Here we build upon a conceptual framework for understanding plant-mediated interactions of pollinators and herbivores. We focus on plant chemistry, in particular plant volatiles and aim to unify hypotheses for plant defence and pollination. We make predictions for the evolutionary outcomes of these interactions by hypothesizing that conflicting selection pressures from herbivores and pollinators arise from the constraints imposed by plant chemistry.
4.  We further hypothesize that plants could avoid conflicts between pollinator attraction and herbivore defence through tissue-specific regulation of pollinator reward chemistry, as well as herbivore-induced changes in flower chemistry and morphology.
5.  Finally, we test aspects of our predictions in a case study using a wild tomato species, Solanum peruvianum , to illustrate the diversity of tissue-specific and herbivore-induced differences in plant chemistry that could influence herbivore and pollinator behaviour, and plant fitness.     

Evaluating the effects of pollinator‐mediated interactions using pollen transfer networks: evidence of widespread facilitation in south Andean plant communities

Information about the relative importance of competitive or facilitative pollinator‐mediated interactions in a multi‐species context is limited. We studied interspecific pollen transfer (IPT) networks to evaluate quantity and quality effects of pollinator sharing among plant species on three high‐Andean communities at 1600, 1800 and 2000 m a.s.l. To estimate the sign of the effects (positive, neutral or negative), the relation between conspecific and heterospecific pollen deposited on stigmas was analysed with GLMMs. Network analyses showed that communities were characterised by the presence of pollen hub‐donors and receptors. We inferred that facilitative and neutral pollinator‐mediated interactions among plants prevailed over competition. Thus, the benefits from pollinator sharing seem to outweigh the costs (i.e. heterospecific deposition and conspecific pollen loss). The largest proportion of facilitated species was found at the highest elevation community, suggesting that under unfavourable conditions for the pollination service and at lower plant densities facilitation can be more common.     

传粉网络的研究进展:网络的结构和动态

植物与传粉者之间相互作用,构成了复杂的传粉网络。近年来,社会网络分析技术的发展使得复杂生态网络的研究成为可能。从群落水平上研究植物与传粉者之间的互惠关系,为理解群落的结构和动态以及花部特征的演化提供了全新的视角。传粉网络的嵌套结构说明自然界的传粉服务存在冗余,而且是相对泛化的物种主导了传粉。在多年或者多季度的传粉网络中,虽然有很高的物种替换率,但是其网络结构仍然保持相对稳定,说明传粉网络对干扰有很强的抗性。尽管有关网络结构和动态的研究逐渐增多,但传粉网络维持的机制仍不清楚。网络结构可以部分由花部特征与传粉者的匹配来解释,也受到系统发生的制约,影响因素还包括群落构建的时间和物种多样性,以及物种在群落中的位置。开展大尺度群落动态的研究,为探索不同时间尺度、不同物种多样性水平上的传粉网络的生态学意义提供了条件。但已有的研究仍存在不足,比如基于访问观察的网络无法准确衡量传粉者的访问效率和植物间的花粉流动,以及结果受到调查精度区域研究不平衡的制约等。目前的研究只深入到传粉者携带花粉构成成分的水平,传粉者访问植物的网络不能代表植物的整个传粉过程。因此,研究应当更多地深入到物种之间关系对有性生殖的切实影响上。     

Flower‐visitor networks only partially predict the function of pollen transport by bees

Mutualistic networks display distinct structural and organizational features such as nestedness, power‐law degree distribution and asymmetric dependencies. Attention is now focused on how these structural properties influence network function. Most plant‐pollinator networks are constructed using records of animals contacting flowers, which is based on the assumption that all visitors to flowers are pollinators; however, animals may visit flowers as nectar robbers, florivores, or to prey upon other visitors. To differentiate potential pollinator interactions from other interaction types, we examined individual bees that had visited flowers to detect if they carried pollen. Using these data, we constructed visitation and pollen‐transport networks for a spinifex‐dominated arid zone grassland. To determine how the structure of the visitation network reflects pollen transport, we compared the two networks using a null model approach to account for differences in network size. Differences in number of species, nestedness and connectance observed between the visitation and pollen‐transport networks were within expected ranges generated under the null model. The pollen‐transport network was more specialized, had lower interaction evenness, and fewer links compared to the visitation network. Almost half the number of species of the visitation network participated in the pollen‐transport network, and one‐third of unique visitation interactions resulted in pollen transport, highlighting that visitation does not always result in pollination. Floral visitor data indicate potential pollen transporters, but inferring pollination function from visitation networks needs to be performed cautiously as pollen transport resulted from both common and rare interactions, and depended on visitor identity. Although visitation and pollen‐transport networks are structurally similar, the function of all species cannot be predicted from the visitation network alone. Considering pollen transport in visitation networks is a simple first step towards determining pollinators from non‐pollinators. This is fundamental for understanding how network structure relates to network function.