Effects of nectar-robbing on plant reproduction and evolution

The relationship between plant and pollinator is considered as the mutualism because plant benefits from the pollinator's transport of male gametes and pollinator benefits from plant's reward.Nectar robbers are frequently described as cheaters in the plant-pollinator mutualism,because it is assumed that they obtain a reward (nectar) without providing a service (pollination).Nectar robbers are birds,insects,or other flower visitors that remove nectar from flowers through a hole pierced or bitten in the corolla.Nectar robbing represents a complex relationship between animals and plants.Whether plants benefit from the relationship is always a controversial issue in earlier studies.This paper is a review of the recent literatures on nectar robbing and attempts to acquire an expanded understanding of the ecological and evolutionary roles that robbers play.Understanding the effects of nectar robbers on the plants that they visited and other flower visitors is especially important when one considers the high rates of robbing that a plant population may experience and the high percentage of all flower visitors that nectar robbers make to some species.There are two standpoints in explaining why animals forage on flowers and steal nectar in an illegitimate behavior.One is that animals can only get food in illegitimate way because of the mismatch of the morphologies of animals'mouthparts and floral structure.The other point of view argues that nectar robbing is a relatively more efficient,thus more energy-saving way for animals to get nectar from flowers.This is probably associated with the difficulty of changing attitudes that have been held for a long time.In the case of positive effect,the bodies of nectar robbers frequently touch the sex organs of plants during their visiting to the flowers and causing pollination.The neutral effect,nectar robbers' behavior may destruct the corollas of flowers,but they neither touch the sex organs nor destroy the ovules.Their behavior does not affect the fruit sets or seed sets of the hosting plant.Besides the direct impacts on plants,nectar robbers may also have an indirect effect on the behavior of the legitimate pollinators.Under some circumstances,the change in pollinator behavior could result in improved reproductive fitness of plants through increased pollen flow and out-crossing.     

What shapes amino acid and sugar composition in Mediterranean floral nectars?

We studied the amino acid (AA) composition of the floral nectars of 73 plant species occurring in a phryganic (East Mediterranean garrigue) community and investigated whether AA and sugar composition is shaped by evolutionary (plant phylogeny), ecological (flowering time as a direct effect of summer drought) and coevolutionary (pollinator partnership) constraints. Our study utilised an extensive plant–pollinator matrix compiled in the same area where the plants had been sampled. Using HPLC we detected 22 AA compounds/groups of compounds, out of which 15 were commonly present in almost all nectars. Among all AAs, phenylalanine was the most abundant, especially in keystone (“cornucopian”) plant species visited by many insect species, such as the majority of the Lamiaceae. Amino acid quantities were transformed into percentages (% of each AA over the total AA content of a flower). Sugar composition was similarly expressed as % of each of the three sugars (glucose, fructose, sucrose) over the total content of these sugars; a number of other sugars, occurring in only a few plant species and in very low quantities were disregarded. The number of insect species of a particular family or guild was taken as a measure of the attraction of a nectar compound for such a family (guild). We found that taxonomical plant group had a weakly significant effect on nectar composition while neither life form nor flowering season had a discernable effect. Pollinators’ preference had the most important effect, with phenylalanine being the most consequent discriminatory compound for the response of the nectar consumers in phrygana, predominantly for long tongued bees, especially for Megachilidae. Gamma‐aminobutyric acid (GABA) had a similar, even stronger influence on bees (long tongued bees, Anthophoridae, Andrenidae) and flies (Syrphidae and other Diptera), whereas asparagine behaved as a general repellent together with tryptophane (rather as repellent). Considering total sugar and AA contents, as well as the volume of nectar, we found that total AA content was positively related to the number of species of long tongued bees and included families visiting the phryganic species; nectar volume was negatively related to flies (both hover flies and remaining Diptera), whereas total sugar content was not significant for any guild. We argue that due to the highly concentrated nectars in the dry Mediterranean communities that are characterised by outstanding melittophily, sugars play a less important role as phagostimulants compared to AAs in floral nectars. This is why phenylalanine, a phagostimulant tested earlier on honeybees, appears to be of high importance in phrygana, especially with long tongued bees and Megachilids as the main selective agents for phenylalanine‐rich nectars. The role of GABA, a strongly NaCl‐dependent AA, may be similar, probably because of the associated presence of NaCl.     

Effects of nectar-robbing on plant reproduction and evolution

The relationship between plant and pollinator is considered as the mutualism because plant benefits from the pollinator’s transport of male gametes and pollinator benefits from plant’s reward. Nectar robbers are frequently described as cheaters in the plant-pollinator mutualism, because it is assumed that they obtain a reward (nectar) without providing a service (pollination). Nectar robbers are birds, insects, or other flower visitors that remove nectar from flowers through a hole pierced or bitten in the corolla. Nectar robbing represents a complex relationship between animals and plants. Whether plants benefit from the relationship is always a controversial issue in earlier studies. This paper is a review of the recent literatures on nectar robbing and attempts to acquire an expanded understanding of the ecological and evolutionary roles that robbers play. Understanding the effects of nectar robbers on the plants that they visited and other flower visitors is especially important when one considers the high rates of robbing that a plant population may experience and the high percentage of all flower visitors that nectar robbers make to some species. There are two standpoints in explaining why animals forage on flowers and steal nectar in an illegitimate behavior. One is that animals can only get food in illegitimate way because of the mismatch of the morphologies of animals’ mouthparts and floral structure. The other point of view argues that nectar robbing is a relatively more efficient, thus more energy-saving way for animals to get nectar from flowers. This is probably associated with the difficulty of changing attitudes that have been held for a long time. In the case of positive effect, the bodies of nectar robbers frequently touch the sex organs of plants during their visiting to the flowers and causing pollination. The neutral effect, nectar robbers’ behavior may destruct the corollas of flowers, but they neither touch the sex organs nor destroy the ovules. Their behavior does not affect the fruit sets or seed sets of the hosting plant. Besides the direct impacts on plants, nectar robbers may also have an indirect effect on the behavior of the legitimate pollinators. Under some circumstances, the change in pollinator behavior could result in improved reproductive fitness of plants through increased pollen flow and out-crossing. __________ Translated from Acta phytoecologiaca Sinica, 2006, 30(4): 695–702 [译自: 植物生态学报]     

盗蜜行为在植物繁殖生态学中的意义

在动植物的相互关系中,盗蜜行为被认为是一种不同于普通传粉者的非正常访花行为。动物之所以要采取这种特殊的觅食策略,有假说认为是由访花者的口器和植物的花部形态不匹配造成的,也有认为是盗蜜行为提高了觅食效率从而使盗蜜者受益。在盗蜜现象中,盗蜜者和宿主植物之间的关系是复杂的。盗蜜对宿主植物的影响尤其是对其繁殖适合度的影响归纳起来有正面、负面以及中性3类。与此同时,盗蜜者的种类, 性别及其掠食行为差异不仅与生境因素密切相关,而且会对宿主植物的繁殖成功产生直接或间接的影响。另外,盗蜜者的存在无疑对其它正常传粉者的访花行为也产生一定的影响,从而间接地影响宿主植物的繁殖成功, 而植物在花部形态上也出现了对盗蜜现象的适应性进化。作者认为, 盗蜜是短嘴蜂对长管型花最有效的一种掠食策略, 它不仅增加了盗蜜者对资源的利用能力, 而且由于盗蜜对宿主植物繁殖成功的不同的影响使其具有调节盗蜜者和宿主之间种群动态的作用, 两者的彼此适应是一种协同进化的结果。     

东北地区早春蜜源植物传粉昆虫多样性研究

为了揭示蜜源植物与传粉昆虫的关系,科学的保护和利用传粉昆虫资源,本研究选取东北地区早春常见蜜源植物柳树Salix、延胡索Corydalis和银莲花Anemone为研究对象,对其传粉昆虫种类和访花行为进行调查,分析不同蜜源植物的传粉昆虫种类、优势类群、多样性和访花行为。结果表明：东北地区早春蜜源植物传粉昆虫共采集726头,隶属于5目14科53种,其中膜翅目4科35种,双翅目5科13种,鳞翅目3科3种,同翅目1科 1种,鞘翅目1科1种。柳树的优势传粉昆虫为蜂类,延胡索的优势传粉昆虫为熊蜂类,银莲花的优势传粉昆虫为蝇类。延胡索的传粉昆虫多样性、均匀度和丰富度均最高。蜜蜂对柳树单位时间内访花次数和单花停留时间最高,为94.33次和18.90 s。熊蜂对延胡索单位时间内访花次数和单花停留时间最高,为76.42次和15.37 s,熊蜂在采集延胡索花时有明显的盗蜜行为,该行为有利于其它传粉昆虫生存。东北地区保护生物多样性和传粉昆虫资源,应结合蜜源植物保护,可在早春时期补种柳树、延胡索和银莲花,并合理搭配,为传粉昆虫提供良好食物源和栖息地。     

Making sense of nectar scents: the effects of nectar secondary metabolites on floral visitors of Nicotiana attenuata

Flowers produce a plethora of secondary metabolites but only nectar sugars, floral pigments and headspace volatiles have been examined in the context of pollinator behavior. We identify secondary metabolites in the headspace and nectar of glasshouse- and field-grown Nicotiana attenuata plants, infer within-flower origins by analyzing six flower parts, and compare the attractiveness of 16 constituents in standardized choice tests with two guilds of natural pollinators (Manducasexta moths and Archilochus alexandri and Selasphorus rufus hummingbirds) and one nectar thief (Solenopsis xyloni ants) to determine whether nectar metabolites can 'filter' flower visitors: only two could. Moths responded more strongly than did hummingbirds to headspace presentation of nicotine and benzylacetone, the most abundant repellent and attractant compounds, respectively. For both pollinators, nectar repellents decreased nectaring time and nectar volume removed, but increased visitation number, particularly for hummingbirds. Fewer ants visited if the nectar contained repellents. To determine whether nicotine reduced nectar removal rates in nature, we planted transformed, nicotine-silenced plants into native populations in Utah over 2 years. Plants completely lacking nicotine in their nectar had 68-70% more nectar removed per night by the native community of floral visitors than did wild-type plants. We hypothesize that nectar repellents optimize the number of flower visitors per volume of nectar produced, allowing plants to keep their nectar volumes small.     

Floral plasticity: Herbivore‐species‐specific‐induced changes in flower traits with contrasting effects on pollinator visitation

Plant phenotypic plasticity in response to antagonists can affect other community members such as mutualists, conferring potential ecological costs associated with inducible plant defence. For flowering plants, induction of defences to deal with herbivores can lead to disruption of plant–pollinator interactions. Current knowledge on the full extent of herbivore‐induced changes in flower traits is limited, and we know little about specificity of induction of flower traits and specificity of effect on flower visitors. We exposed flowering Brassica nigra plants to six insect herbivore species and recorded changes in flower traits (flower abundance, morphology, colour, volatile emission, nectar quantity, and pollen quantity and size) and the behaviour of two pollinating insects. Our results show that herbivory can affect multiple flower traits and pollinator behaviour. Most plastic floral traits were flower morphology, colour, the composition of the volatile blend, and nectar production. Herbivore‐induced changes in flower traits resulted in positive, negative, or neutral effects on pollinator behaviour. Effects on flower traits and pollinator behaviour were herbivore species‐specific. Flowers show extensive plasticity in response to antagonist herbivores, with contrasting effects on mutualist pollinators. Antagonists can potentially act as agents of selection on flower traits and plant reproduction via plant‐mediated interactions with mutualists.     

Reproductive ecology of distylous Palicourea padifolia (Rubiaceae) in a tropical montane cloud forest. II. Attracting and rewarding mutualistic and antagonistic visitors

By definition, the floral morphs of distylous plants differ in floral architecture. Yet, because cross-pollination is necessary for reproductive success in both morphs, they should not differ in attributes that contribute to attracting and rewarding floral visitors. Floral and vegetative attributes that function in distylous polymorphism in hummingbird-pollinated Palicourea padifolia (Rubiaceae) and the responses of pollinators and insect herbivores to the resources offered by both morphs were investigated. The performance of each morph along multiple stages of the reproductive cycle, from inflorescence and nectar production to fruit production, was surveyed, and pollinator behavior and nectar standing crops were then observed. Costs associated with such attractiveness were also evaluated in terms of herbivore attack and of plant reproductive fitness (female function) as a function of leaf herbivory. The number of inflorescences, floral buds, open flowers, and ripe fruits offered by either floral morph were similar, but short-styled plants almost doubled the number of developing fruits of long-styled plants. Long-styled flowers produced higher nectar volumes and accumulated more nectar over time than short-styled flowers. Measures of nectar standing crop and data on pollinator behavior suggest that hummingbirds respond to this morph-specific scheduling of nectar production. Lastly, long-styled plants suffered a higher herbivore attack and lost more leaf area over time than those with short-styled flowers. Herbivory was negatively correlated with fruit number and fruit mass, and long-styled plants set significantly less fruit mass than short-styled plants. The results suggest that pollinators and herbivores may exert selective pressures on floral and vegetative traits that could also influence gender function.     

花蜜微生物及其生态功能研究进展

花蜜是虫媒植物提供给传粉者最有效的报酬,对花蜜特征介导的植物-传粉者相互关系的研究已成为当今传粉生物学研究中最活跃的领域之一。开花植物分泌的原始花蜜是无菌的,不过一些微生物可经由空气传播至花蜜或(和)通过与传粉者的喙接触而聚集于花蜜中,并利用花蜜中的营养物质进行快速繁殖。花蜜的高渗透压环境导致花蜜中微生物(酵母菌,细菌)的物种多样性相对较低。此外,某些生物(传粉者组成,微生物间的竞争)与非生物因素(渗透压,糖组成,次生代谢物质,抗菌化合物,可利用氮源,温度,pH)也可影响花蜜中微生物群落的形成。花蜜中微生物的代谢活动能够改变花蜜物理(温度,粘度)与化学(pH,H_2O_2含量,糖组成和浓度,氨基酸组分和浓度,以及气味)特性,进而影响传粉者的访花行为与植物的繁殖适合度。因而,对花蜜中微生物及其生态功能的研究近年来颇受传粉生物学家的关注。在总结已发表研究成果的基础上,提出今后的研究有必要结合分子生物学与化学分析技术,以进一步揭示影响花蜜中微生物群落的潜在因素的作用机制,同时对花蜜微生物改变花蜜的物理、化学特性及植物-传粉者之间相互作用的可能原因进行更详尽的阐释,特别是对花蜜微生物在生态系统中所发挥的生态功能进行进一步的研究与认识。     

Effects of nectar robbing on pollinator behavior and plant reproductive success of Pitcairnia angustifolia (Bromeliaceae)

Nectar robbers may have direct and indirect effects on plant reproductive success but the presence of nectar robbing is not proof of negative fitness effects. We combined census data and field experiments to disentangle the complex effects of nectar robbing on nectar production rates, pollinator behavior, pollen export, and female reproductive success of Pitcairnia angustifolia. Under natural conditions flowers were visited by four different animal species including a robber‐like pollinator and a secondary robber. Natural levels of nectar robbing ranged from 40 to 100%. Natural variation in nectar robbing was not associated with fruit set in any year whereas seed set was weakly positively associated for 1 year only. Artificial nectar robbing did not increase nectar production or concentration, did not affect the behavior of long‐billed hummingbirds, and when faced with artificially robbed flowers, these visitors behaved as secondary nectar robbers. The number of stigmas within a patch that received pollen dye analogs and the average distance traveled by these analogs were not significantly different between robbing treatments (robbed flowers versus unrobbed flowers), but the maximum distance traveled by these pollen analogs was higher when nectar robbing was not prevented. Overall, the proportion of robbed flowers on an inflorescence had a neutral effect to a weak positive effect on the reproduction of individual plants (i.e. positive association between nectar robbing and fruit set in 2002) even when it clearly changed the behavior of its most efficient pollinator potentially increasing the frequency of nectar robbing within a plant.     

The influence of nectar production on hawkmoth behavior,self pollination,and seed production in Mirabilis multiflora (Nyctaginaceae)

Nectar is the most common floral reward that plants produce to attract pollinators. To determine the effect of nectar production on hawkmoth behavior, pollen movement, and reproductive success in Mirabilis multiflora, I manipulated nectar volumes and observed the subsequent foraging behavior of the hawkmoth Hyles lineata and the resulting pollen movement patterns. Individual hawkmoths visited significantly more flowers on plants with more nectar. The increase in flower visits significantly increased pollen deposition on stigmas and pollen removal from anthers when nectar volume was raised to twice the highest level found in nature. As hawkmoths visited flowers consecutively on a plant, the proportion of self pollen deposited on stigmas increased significantly and rapidly. Based on simulated hawkmoth visits, seed set was significantly reduced for flowers later in a visit sequence. A simple model combining these results predicts that the form of selection on nectar production varies depending on pollinator abundance. Using a multiple regression analysis a nearly significant (P < 0.08) effect of stabilizing selection was detected during a single season as predicted by the model for the prevailing hawkmoth abundance. Although increased nectar production may indirectly affect plant fitness by reducing resources available for other plant functions, the direct effect of high nectar production on pollinator behavior and self pollination may generally limit floral nectar production.     

Variation in nectar volume and composition of Impatiens capensis at the individual,plant, and population levels

Although the volume and chemical composition of nectars are known to vary among plant species and to affect pollinator response to plants, relatively little is known of the variation in volume, and sugar and amino acid composition within species. We collected nectar from Impatiens capensis in a nested design: three flowers from each of three plants from each of three populations. This design enabled us to quantify variation within individual plants, among plants within populations, and among populations. Using high performance liquid chromatography, we analyzed the sugar and amino composition of the 27 flowers. Analysis of variance showed that none of the parameters (volume, concentrations of three sugars and 24 amino compounds) varied within individuals. Variation in nectar volume was not significant among plants but was nearly significant among populations. Of the three sugars detected (sucrose, glucose, and fructose), the only significant variation was that of sucrose among populations. Concentrations of 12 amino compounds varied significantly at the plant level while 7 amino compounds varied among populations. The results indicate that: (1) pooling of nectar samples from flowers of individual plants can be an acceptable methodology for those seeking to understand within species variation; (2) amino compounds appear to vary more than either volumes or sugar concentrations; (3) future studies should assess how much of the observed variation is due to genetic versus environmental differences; (4) additional studies should examine the geographic variation in nectar parameters and pollinators of I. capensis in order to assess the role different pollinators play in shaping nectar composition.     

Relationships among nectar‐dwelling yeasts,flowers and ants: patterns and incidence on nectar traits

Nectar‐dwelling yeasts are emerging as widely distributed organisms playing a potentially significant and barely unexplored ecological role in plant pollinator mutualisms. Previous efforts at understanding nectar–pollinator–yeast interactions have focused on bee‐pollinated plants, while the importance of nectarivorous ants as vectors for yeast dispersal remains unexplored so far. Here we assess the abundance and composition of the nectar fungal microbiota of the ant‐pollinated plant Cytinus hypocistis, study whether yeast transmission is coupled with ant visitation, and discern whether ant‐ transported yeasts promote changes in nectar characteristics. Our results show that a high percentage of flowers (77%) and plants (94%) contained yeasts, with yeast cell density in nectar reaching up to 6.2 × 10⁴ cells mm^?3, being the highest densities associated with the presence of the nectar‐specialist yeast Metschnikowia reukaufii. The establishment of fungal microbiota in nectar required flower visitation by ants, with 70% of yeast species transported by them being also detected in nectar. Ant‐vectored yeasts diminished the nutritional quality of nectar, with flowers exposed to pollinators and yeasts containing significantly lower nectar sugar concentration than virgin flowers (13.4% and 22.8%, respectively). Nectar of flowers that harbored M. reukaufii showed the lowest quality, with nectar concentration declining significantly with increasing yeast density. Additionally, yeasts modified patterns of interpopulation variation in nectar traits, homo genizing differences between populations in some nectar attributes. We show for the first time that the outcome of the tripartite pollinator–flower–yeast interaction is highly dependent on the identity and inherent properties of the participants, even to the extent of influencing the species composition of this ternary system, and can be mediated by ecological characteristics of plant populations. Through their influence on plant functional traits, yeasts have the potential to alter nectar consumption, pollinator foraging behavior and ultimately plant reproduction.     

Does spatial aggregation of total nectar production lead to genetic structure?

Spatial aggregation of plants of high nectar production, receiving an enhanced pollinator service is known to occur in Echium vulgare. Moreover, an emanating effect of nectar production on pollinator visits may occur, i.e. many pollinator visits may be observed around high nectar patches. Consequently, gene flow within patches of plants of high nectar production and their close neighbours may result in genetic structure. In this study, we investigated whether aggregation of total nectar production (nectar production per flower×number of flowers) and its emanating effect resulted in genetic structure in a natural E. vulgare population. We compared the spatial structure of total nectar production, pollinator visits and microsatellite markers using spatial autocorrelation analysis. Increased geitonogamy, caused by longer boutlengths in plants of high nectar production may generate genetic structure. We estimated selfing rates of plants of the highest and lowest total nectar production. Spatial aggregation of total nectar production occurred on a relatively small scale up to 2.83 m. A significant emanating correlation between total nectar production and pollinator visits was observed on a relatively large scale up to a 4.24 m. Thus, around patches of high nectar production numbers of pollinator visits were relatively high, while few visits were observed around patches of low nectar production. Weak genetic structure was present on a small scale up to 2.20 m. This corresponded with the scale of aggregation of total nectar production. High gene flow around the patches of high nectar production seems to weaken genetic structure. This is supported by the relatively low selfing rates. The average selfing rate of the plants of highest nectar production was 8.8% and that of the plants of lowest nectar production 5.0%. Low gene flow within and around low nectar patches sustain a weak genetic structure or, conversely, may have caused it in the first instance. Results indicate the importance of spatial structure of nectar production for pollinator movement.     

Cognition-mediated coevolution – context-dependent evaluations and sensitivity of pollinators to variability in nectar rewards

 In this paper we emphasize the role of pollinator perception and decision-making processes in mediating floral nectar distribution strategies. Since pollinator choice behavior is guided by how the pollinator perceives and evaluates floral rewards, we hypothesize that plants have evolved strategies that maximize their perceived profitability, through pollinator cognition-mediated coevolution. We focus on two classes of cognitive phenomena, context-dependent evaluations and risk-sensitivity. These phenomena are of interest to psychologists and biologists. Our paper is an attempt to show the value of cross-disciplinary exchange of theories and ideas. A review of the ecology literature suggests that pollinators evaluate variability in nectar volume in proportion to the mean, and thus choice behavior is guided by the coefficient of variation (standard deviation/mean) of the distribution. This functional relationship is predicted by Weber's law, which describes a wide range of psychophysical phenomena. Simulations show that this phenomenon also affects how pollinators perceive skewed nectar distributions. Cognition-mediated coevolution theory should be a fruitful approach to understanding the evolution of pollinator-plant interactions. Received November 12, 2002; accepted January 15, 2003 Published online: June 2, 2003     

ADAPTIVE SIGNIFICANCE OF IPOMOPSIS AGGREGATA NECTAR PRODUCTION: OBSERVATION AND EXPERIMENT IN THE FIELD

Production of floral nectar is generally thought to be an adaptation that increases plant fitness by altering pollinator behavior, and therefore pollination success. To test this hypothesis, I investigated the effects of floral nectar production rate on pollination success of the hermaphroditic plant Ipomopsis aggregata (Polemoniaceae). Success through male function (estimated by the export of fluorescent dyes) was significantly greater for plants with naturally high nectar production rates than for nearby plants with low nectar production rates, whereas success through female function (receipt of fluorescent dye) was unrelated to nectar production rate. Experimental addition of artificial nectar also produced a significant increase in male function success and no increase in several estimates of female function success. Observations confirmed that hummingbirds probed a larger proportion of flowers on plants that received supplemental nectar, as they do in response to natural variation in nectar production. The concordance of results across these observational and experimental studies indicates that nectar production acts primarily to increase pollination success through male function for this species.     

Innate Adjustment of Visitation Behavior to Rewarding and Reward‐Minimized Petunia axillaris (Solanacea) Plants by Hawkmoth Manduca sexta (Sphingidae)

Plant–pollinator interactions offer an excellent system to study the stability of mutualistic interactions. While nectar production requires resources and a reduction could in principle benefit plant fitness, only few angiosperms lack nectar, and thus cheat from a pollinator's perspective. Cheating behavior may be scarce because of pollinator foraging behaviors that select for nectariferous plants. Shorter inspection duration, interaction with fewer flowers, or even complete avoidance of plants with low/no nectar may reduce the fitness of cheating plants. The effectiveness of pollinator strategies may depend on how they are implemented. Innate strategies would invariably decrease the fitness of a cheating plant, while learned responses allow cheaters to exploit naïve pollinators. Here, we studied the foraging strategies of the hawkmoth Manduca sexta during interactions with nectariferous and reward‐minimized Petunia axillaris. We found that neither naïve nor experienced hawkmoths discriminated a priori between rewarding and nectar‐less plants. However, naïve hawkmoths displayed reduced probing time per flower and number of flowers visited on reward‐minimized plants during the first trial, without showing further improved discrimination with experience. In conclusion, the foraging decision rules of hawkmoths that may reduce the fitness of reward‐minimized plants appear to be innate, with little scope for additional learning.     

Bird pollination syndrome is the plant's adaptation to ornithophily,but nectarivorous birds are not so selective

Many tropical plants are pollinated by birds and several bird phylogenetical lineages have specialised to a nectar diet. The long-assumed, intimate ecological and evolutionary relationship between ornithophilous plants and phenotypically specialised nectarivorous birds has nevertheless been questioned in recent decades, where such plant–pollinator interactions have been shown to be highly generalised. In our study, we analysed two extensive interaction datasets: bird–flower and insect–flower interactions, both collected on Mt Cameroon, west-central Africa. We tested if: 1) insects and birds interact with distinct groups of plants; 2) plants with a typical set of ornithophilous floral traits (i.e. bird pollination syndrome) interact mainly with birds; 3) birds favour plants with bird pollination syndrome and; 4) if and how the individual floral traits and plant level nectar production predict bird visitation. Bird-visited plants were typically also visited by insects, while approximately half of the plants were visited by insects only. We confirmed the validity of the bird pollination syndrome hypothesis, as plants with bird-pollination syndrome traits were visited by birds at a higher rate and mostly hosted a lower frequency of visiting insects. However, these ornithophilous plants were not more attractive than the other plants for nectar-feeding birds. Nectar production per plant individual was a better predictor of bird visitation than any other floral trait traditionally related to the bird pollination syndrome. Our study thus demonstrated the highly asymmetrical relationship between ornithophilous plants and nectarivorous birds.     

Spatial distribution of nectar production in a natural Echium vulgare population: Implications for pollinator behaviour

The distribution of trait values in many populations is not homogenous but creates a mosaic of patches. This may lead to differences in selection on the patch level compared to selection on the population level. As an example we investigated the spatial distribution of nectar production and its effects on pollinator behaviour in a natural population of Echium vulgare. Nectar production per flower, number of flowers and total nectar production showed a hierarchy and spatial aggregation as expressed by Gini coefficients and significant Moran's I values. Plants in patches of high nectar production received significantly more pollinator visits and had a significant emanating effect on pollinator visits of neighbouring plants. The same was true for plants in patches with high number of flowers. To disentangle these effects a path analysis was applied, which suggested that the direct effect of nectar production per flower although present, seems to be small compared to the effect of the number of flowers. Nectar production per flower affected pollinator visits mainly indirectly by way of total nectar production, which includes the effect of number of flowers. Assuming a minor pollinator-mediated selection for number of flowers, pollinator-mediated selection for total nectar production equals that for nectar production per flower. If so, the observed spatial structure of nectar production and its emanating effect on pollinator behaviour is of importance for natural selection. Plants of low nectar production occurring close to patches of plants with high nectar production benefited from the enhanced pollinator service of their neighbours while saving costs of increased nectar production. Consequently, plants with low nectar production may have a selective advantage at patch level while plants with high nectar production may have a selective advantage at population level. Results presented stress the importance of small-scale patterns for ecological relationships and evolutionary change.     

Phenotypic selection on nectar amino acid composition in the Lepidoptera pollinated orchid species Gymnadenia conopsea

Plant–pollinator interactions are potential drivers of evolution in floral traits. Because nectar chemical composition is known to mediate both plant–pollinator interactions and plant reproductive success, it can be expected that chemical composition of nectar is subjected to strong pollinator‐mediated selective forces. However, the extent of natural selection on different nectar components has not been studied so far. Using the Lepidoptera pollinated fragrant orchid Gymnadenia conopsea as a model species, we used high‐performance anion‐exchange chromatography (HPAEC) to characterize the sugar and amino acid composition of floral nectar in three calcareous grassland populations of G. conopsea. We then measured phenotypic selection on nectar composition and on other plant and floral traits through applying both linear regression and structural equation modelling. We demonstrate phenotypic selection on plant height, inflorescence height and on specific nectar amino acids, whereas spur length, total sugar and amino acid concentration were not direct targets of selection. Chemical nectar composition is thus indeed under selective pressure but nectar amino acids are much more important to fitness of G. conopsea, as compared to nectar sugars. Furthermore, as we found no evidence of selection on the total amino acid concentration, it is unlikely that amino acids increase pollinator attraction because they are a pollinator nitrogen source. To further unravel the evolutionary ecology of floral nectar, behavioural experiments with pollinators exposed to different nectar components and studies experimentally identifying the selective agents are recommended.