Novel nectar robbing negatively affects reproduction in Digitalis purpurea

With many plant–pollinator interactions undergoing change as species’ distributions shift, we require a better understanding of how the addition of new interacting partners can affect plant reproduction. One such group of floral visitors, nectar robbers, can deplete plants of nectar rewards without contributing to pollination. The addition of nectar robbing to the floral visitor assemblage could therefore have costs to the plant´s reproductive output. We focus on a recent plant colonist, Digitalis purpurea, a plant that in its native range is rarely robbed, but experiences intense nectar robbing in areas it has been introduced to. Here, we test the costs to reproduction following experimental nectar robbing. To identify any changes in the behavior of the principal pollinators in response to nectar robbing, we measured visitation rates, visit duration, proportion of flowers visited, and rate of rejection of inflorescences. To find the effects of robbing on fitness, we used proxies for female and male components of reproductive output, by measuring the seeds produced per fruit and the pollen export, respectively. Nectar robbing significantly reduced the rate of visitation and lengths of visits by bumblebees. Additionally, bumblebees visited a lower proportion of flowers on an inflorescence that had robbed flowers. We found that flowers in the robbed treatment produced significantly fewer seeds per fruit on average but did not export fewer pollen grains. Our finding that robbing leads to reduced seed production could be due to fewer and shorter visits to flowers leading to less effective pollination. We discuss the potential consequences of new pollinator environments, such as exposure to nectar robbing, for plant reproduction.     

Predicting the effects of nectar robbing on plant reproduction: implications of pollen limitation and plant mating system

The outcome of species interactions is often difficult to predict, depending on the organisms involved and the ecological context. Nectar robbers remove nectar from flowers, often without providing pollination service, and their effects on plant reproduction vary in strength and direction. In two case studies and a meta-analysis, we tested the importance of pollen limitation and plant mating system in predicting the impacts of nectar robbing on female plant reproduction. We predicted that nectar robbing would have the strongest effects on species requiring pollinators to set seed and pollen limited for seed production. Our predictions were partially supported. In the first study, natural nectar robbing was associated with lower seed production in Delphinium nuttallianum, a self-compatible but non-autogamously selfing, pollen-limited perennial, and experimental nectar robbing reduced seed set relative to unrobbed plants. The second study involved Linaria vulgaris, a self-incompatible perennial that is generally not pollen limited. Natural levels of nectar robbing generally had little effect on estimates of female reproduction in L. vulgaris, while experimental nectar robbing reduced seed set per fruit but not percentage of fruit set. A meta-analysis revealed that nectar robbing had strong negative effects on pollen-limited and self-incompatible plants, as predicted. Our results suggest that pollination biology and plant mating system must be considered to understand and predict the ecological outcome of both mutualistic and antagonistic plant-animal interactions.     

Plant–animal interactions in suburban environments: implications for floral evolution

Plant interactions with mutualists and antagonists vary remarkably across space, and have played key roles in the ecology and evolution of flowering plants. One dominant form of spatial variation is human modification of the landscape, including urbanization and suburbanization. Our goal was to assess how suburbanization affected plant–animal interactions in Gelsemium sempervirens in the southeastern United States, including interactions with mutualists (pollination) and antagonists (nectar robbing and florivory). Based on differences in plant–animal interactions measured in multiple replicate sites, we then developed predictions for how these differences would affect patterns of natural selection, and we explored the patterns using measurements of floral and defensive traits in the field and in a common garden. We found that Gelsemium growing in suburban sites experienced more robbing and florivory as well as more heterospecific but not conspecific pollen transfer. Floral traits, particularly corolla length and width, influenced the susceptibility of plants to particular interactors. Observational data of floral traits measured in the field and in a common garden provided some supporting but also some conflicting evidence for the hypothesis that floral traits evolved in response to differences in species interactions in suburban vs. wild sites. However, the degree to which plants can respond to any one interactor may be constrained by correlations among floral morphological traits. Taken together, consideration of the broader geographic context in which organisms interact, in both suburban and wild areas, is fundamental to our understanding of the forces that shape contemporary plant–animal interactions and selection pressures in native species.     

Nectar robbers deter legitimate pollinators by mutilating flowers

Nectar robbing – harvesting nectar illegitimately – can have a variety of outcomes for plant sexual reproduction and for the pollinator community. Nectar robbers can damage flowers while robbing nectar, which could affect the behavior of subsequent flower visitors and, consequently, plant reproduction. However, only nectar manipulation by nectar robbers has so far received attention. We found a short-tongued bee, Hoplonomia sp. (Halictidae), mutilating the conspicuous lower petal of the zygomorphic flowers of Leucas aspera (Lamiaceae) while robbing nectar. We hypothesized that the mutilation of the conspicuous lower petal deters legitimate pollinators on L. aspera flowers, which, in turn, might affect plant reproduction. We first assessed the proportion of naturally-robbed flowers in plant populations for three years to confirm that it was not a purely local phenomenon due to a few individual bees. We then studied diversity, community and visitation characteristics of pollinators, nectar dynamics and fruit set in unrobbed and robbed open flowers in naturally-robbed populations. The proportion of robbed flowers varied significantly across sites and years. Robbing did not affect nectar dynamics in flowers, but it did alter flower morphology, so much so that it reduced pollinator visitation and altered the pollinator community on robbed flowers. However, the maternal function of plant reproduction was not affected by nectar robbing. This study for the first time shows that a nectar robber can have an ecologically significant impact on floral morphology.     

Realized tolerance to nectar robbing: compensation to floral enemies in Ipomopsis aggregata

Background and Aims

Although the ecological and evolutionary consequences of foliar herbivory are well understood, how plants cope with floral damage is less well explored. Here the concept of tolerance, typically studied within the context of plant defence to foliar herbivores and pathogens, is extended to floral damage. Variation in tolerance to floral damage is examined, together with some of the mechanisms involved.

Methods

The study was conducted on Ipomopsis aggregata, which experiences floral damage and nectar removal by nectar-robbing bees. High levels of robbing can reduce seeds sired and produced by up to 50 %, an indirect effect mediated through pollinator avoidance of robbed plants. Using an experimental common garden with groups of I. aggregata, realized tolerance to robbing was measured. Realized tolerance included both genetic and environmental components of tolerance. It was hypothesized that both resource acquisition and storage traits, and traits involved in pollination would mitigate the negative effects of robbers.

Key Results

Groups of I. aggregata varied in their ability to tolerate nectar robbing. Realized tolerance was observed only through a component of male plant reproduction (pollen donation) and not through components of female plant reproduction. Some groups fully compensated for robbing while others under- or overcompensated. Evidence was found only for a pollination-related trait, flower production, associated with realized tolerance. Plants that produced more flowers and that had a higher inducibility of flower production following robbing were more able to compensate through male function.

Conclusions

Variation in realized tolerance to nectar robbing was found in I. aggregata, but only through an estimate of male reproduction, and traits associated with pollination may confer realized tolerance to robbing. By linking concepts and techniques from studies of plant–pollinator and plant–herbivore interactions, this work provides insight into the role of floral traits in pollinator attraction as well as plant defence.Key words: Compensation, herbivory, indirect effects, Ipomopsis aggregata, male reproductive success, nectar robbing, pollen donation, pollination, resistance, tolerance     

Consumption of a nectar alkaloid reduces pathogen load in bumble bees

Diet has a significant effect on pathogen infections in animals and the consumption of secondary metabolites can either enhance or mitigate infection intensity. Secondary metabolites, which are commonly associated with herbivore defense, are also frequently found in floral nectar. One hypothesized function of this so-called toxic nectar is that it has antimicrobial properties, which may benefit insect pollinators by reducing the intensity of pathogen infections. We tested whether gelsemine, a nectar alkaloid of the bee-pollinated plant Gelsemium sempervirens, could reduce pathogen loads in bumble bees infected with the gut protozoan Crithidia bombi. In our first laboratory experiment, artificially infected bees consumed a daily diet of gelsemine post-infection to simulate continuous ingestion of alkaloid-rich nectar. In the second experiment, bees were inoculated with C. bombi cells that were pre-exposed to gelsemine, simulating the direct effects of nectar alkaloids on pathogen cells that are transmitted at flowers. Gelsemine significantly reduced the fecal intensity of C. bombi 7 days after infection when it was consumed continuously by infected bees, whereas direct exposure of the pathogen to gelsemine showed a non-significant trend toward reduced infection. Lighter pathogen loads may relieve bees from the behavioral impairments associated with the infection, thereby improving their foraging efficiency. If the collection of nectar secondary metabolites by pollinators is done as a means of self-medication, pollinators may selectively maintain secondary metabolites in the nectar of plants in natural populations.     

盗蜜对角蒿传粉者行为和生殖成功的影响

一些研究显示盗蜜对自交植物的结实和结籽没有显著影响。然而, 对于既有传粉者为其传粉实现异交又能通过自交实现生殖保障的兼性自交植物来说, 盗蜜对其生殖的影响还知之甚少。由于兼性自交植物可以自交, 盗蜜对其总体结实可能不会有显著影响, 但可能会通过影响传粉者行为而影响传粉者介导的结实。为了验证这一假说, 本研究以兼性自交的一年生角蒿(Invarvillea sinensis var. sinensis)为研究材料, 通过野外调查和控制实验, 探讨了盗蜜对传粉者介导的结实(传粉者行为)和总体结实率的影响。结果表明: 角蒿的盗蜜者和主要传粉者相同, 均为密林熊蜂(Bombus patagiatus)。熊蜂盗蜜频率平均为20.24% (范围为0-51.43%)。盗蜜对角蒿总体结实率、每果结籽数和每果种子重量没有显著影响。然而, 被盗蜜花的柱头闭合比率显著高于未被盗蜜花, 说明盗蜜影响传粉者的访花行为和传粉者介导的结实率。另外, 被盗蜜花的高度显著高于未被盗蜜花, 说明盗蜜者倾向于从较大较高的花上盗蜜。这些结果为全面认识盗蜜对植物生殖的影响提供了新的信息。     

Additive effects of herbivory,nectar robbing and seed predation on male and female fitness estimates of the host plant <Emphasis Type="Italic">Ipomopsis aggregata</Emphasis>

Many antagonistic species attack plants and consume specific plant parts. Understanding how these antagonists affect plant fitness individually and in combination is an important research focus in ecology and evolution. We examined the individual and combined effects of herbivory, nectar robbing, and pre-dispersal seed predation on male and female estimates of fitness in the host plant Ipomopsis aggregata. By examining the effects of antagonists on plant traits, we were able to tease apart the direct consumptive effects of antagonists versus the indirect effects mediated through changes in traits important to pollination. In a three-way factorial field experiment, we manipulated herbivory, nectar robbing, and seed predation. Herbivory and seed predation reduced some male and female fitness estimates, whereas plants tolerated the effects of robbing. The effects of herbivory, robbing, and seed predation were primarily additive, and we found little evidence for non-additive effects of multiple antagonists on plant reproduction. Herbivory affected plant reproduction through both direct consumptive effects and indirectly through changes in traits important to pollination (i.e., nectar and phenological traits). Conversely, seed predators primarily had direct consumptive effects on plants. Our results suggest that the effects of multiple antagonists on estimates of plant fitness can be additive, and investigating which traits respond to damage can provide insight into how antagonists shape plant performance.     

Pollinators and nectar robbers cause directional selection for large spur circle in Impatiens oxyanthera (Balsaminaceae)

Nectar spurs have an important role in floral evolution and plant–pollinator coadaptation. The flowers of some species possess spurs curving into a circle. However, it is unclear whether spur circle diameter is under direct selection pressure from different sources, such as pollinators and nectar robbers. In this study, we quantified selection on some floral traits, such as spur circle diameter in Impatiens oxyanthera (Balsaminaceae) using phenotypic selection analysis and compared the relative importance of pollinators and nectar robbers as selective agents using mediation analysis. The study showed that pollinators caused significant selection on corolla length, spur curvature and spur circle diameter while nectar robbers only imposed strong selection on spur circle diameter. Pollinators favored flowers with large corolla, curly spurs and large spur circle while nectar robbers preferred flowers with small spur circle. More pollinator visits resulted in higher female reproductive success, while robbery reduced female fitness. Conflicting selection on spur traits from pollinators and nectar robbers was not found. Mediation analysis showed that selection on floral traits through nectar robbing was stronger than selection through pollination. The results suggested that pollinators and nectar robbers jointly mediated the directional selection for large spur circle, and nectar robbers caused stronger selection than pollinators on floral traits.     

Nectar robbing impacts pollinator behavior but not plant reproduction

Trait‐mediated indirect effects (TMIEs) refer to interactions in which the effect of one species on another is mediated by the behavior of a third species. A mechanistic approach that identifies the direction and impact of TMIEs can shed light on why different net outcomes are observed in the same general phenomena across systems. Nectar robbing has variable net effects through TMIEs on animal‐pollinated plants across systems, but the mechanistic steps underlying this range of outcomes are often unclear. To address this knowledge gap, we assessed linkages between nectar robbing, pollinator behavior and plant reproductive success in the Andean tree, Oreocallis grandiflora. We found that robbing in this system led to lower nectar volumes, higher nectar sucrose concentration, and higher nectar viscosity, which together negatively impact nectar quality. This drop in nectar quality was associated with decreased visitation rates by hummingbirds, which might be expected to impact plant reproduction negatively by pollen limitation. However, it was also associated with increased diversity (Shannon's) and evenness in the pollinator community due to reduced visitation by a territorial hummingbird, which might be expected to impact reproduction positively via enhanced genetic diversity of pollen as non‐territorial pollinators forage over greater areas. We measured seed set and mass to distinguish the relative intensity of these two possible outcomes, but found no detectable effect. We tentatively conclude that these two consequences of TMIEs may have balanced each other out to yield a neutral net effect of nectar robbing on plant reproduction, though other explanations are also possible. This study highlights ways in which ecologically important TMIEs may act in opposing directions to mask important ecological forces, and underscores the continued need for detailed study of the mechanisms through which TMIEs operate.     

Leaf herbivory and nutrients increase nectar alkaloids

Correlations between traits may constrain ecological and evolutionary responses to multispecies interactions. Many plants produce defensive compounds in nectar and leaves that could influence interactions with pollinators and herbivores, but the relationship between nectar and leaf defences is entirely unexplored. Correlations between leaf and nectar traits may be mediated by resources and prior damage. We determined the effect of nutrients and leaf herbivory by Manduca sexta on Nicotiana tabacum nectar and leaf alkaloids, floral traits and moth oviposition. We found a positive phenotypic correlation between nectar and leaf alkaloids. Herbivory induced alkaloids in nectar but not in leaves, while nutrients increased alkaloids in both tissues. Moths laid the most eggs on damaged, fertilized plants, suggesting a preference for high alkaloids. Induced nectar alkaloids via leaf herbivory indicate that species interactions involving leaf and floral tissues are linked and should not be treated as independent phenomena in plant ecology or evolution.     

Factors affecting pollinator movement and plant fitness in a specialized pollination system

The rate of pollen exchange within and among flowers may depend on pollinator attraction traits such as floral display size and flowering plant density. Variations in these traits may influence pollinator movements, pollen receipt, and seed number. To assess how floral display size and flowering plant density affect parameters of pollinator visitation rate, pollen receipt per flower, seed number per fruit and the between-plant pollinator movements, we studied the self-incompatible plant, Nierembergia linariifolia. Per-flower pollinator visitation rate and bout length increased linearly with increasing floral display size. Pollen receipt per flower increased linearly with increasing flowering plant density. For seed number per fruit, a polynomial model describing an increased seed number per fruit at low density and a decreased seed number per fruit at high density provided a significant fit. Per-flower pollinator visitation rate was not associated with pollen receipt per flower and seed number per fruit. Bees visited plants located near to the center of the population more frequently than plants located at the periphery. Increases in both floral display size and flowering plant density led to an increased chance of a plant being chosen as the center of the pollinator foraging area. These results suggest that even though large floral displays and high flowering plant density are traits that attract more pollinators, they may also reduce potential mate diversity by restricting pollen movement to conspecific mates that are closely located.     

Nectar robbers influence the trait–fitness relationship of Primula secundiflora

• The trait–fitness relationship influences the strength and direction of floral evolution. To fully understand and predict the evolutionary trajectories of floral traits, it is critical to disentangle the direct and indirect effects of floral traits on plant fitness in natural populations.
• We experimentally quantified phenotypic selection on floral traits through female fitness and estimated the casual effects of nectar robbing with different nectar robbing intensities on trait–fitness relationships in both the L‐ (long‐style and short‐anther phenotype) and S‐morph (short‐style and long‐anther phenotype) flowers among Primula secundiflora populations.
• A larger number of flowers and wider corolla tubes had both direct and indirect positive effects on female fitness in the P. secundiflora populations. The indirect effects of these two traits on female fitness were mediated by nectar robbers. The indirect effect of the number of flowers on female fitness increased with increasing nectar robbing intensity. In most populations, the direct and/or indirect effects of floral traits on female fitness were stronger in the S‐morph flowers than in the L‐morph flowers. In addition, nectar robbers had a direct positive effect on female fitness, but this effect varied between the L‐ and S‐morph flowers.
• These results show the potential role of nectar robbers in influencing the trait–fitness relationships in this primrose species.

     

Are nectar-robbers mutualists or antagonists?

As exploiters of plant-pollinator mutualisms, nectar-robbers remove rewards (nectar) without providing pollination services. Though one might expect nectar-robbing to be costly to plants, it may instead benefit plants by indirectly increasing pollen dispersal. I investigated the direct effects of nectar-robbing bees (Xylocopa californica) on floral rewards and behaviors of pollinators visiting desert willow (Chilopsis linearis) and indirect effects of robbing on the reproductive success of the plant. Nectar-robbers reduced nectar; while unrobbed and robbed flowers were equally likely to contain nectar, nectar volumes were smaller in robbed flowers with nectar. Apis mellifera (honeybees), ineffective pollinators in terms of pollen deposition, avoided robbed flowers. In contrast, Bombus sonorus (bumblebees), effective pollinators, did not avoid robbed flowers. While bumblebees tended to spend less time in robbed flowers, the time that they spent in flowers was not correlated with pollen deposition. Using powder mimicking pollen, I found that on some days, powder was dispersed farther or to more flowers from robbed flowers, indicating that robbing may sometimes benefit plants by increasing male reproductive success. Powder movement suggested that the effect of robbing on male reproductive success ranged from costly to beneficial. The outcome for flowers that were marked early each morning was a function of prevalence of robbing and abundances of effective pollinators, but not a function of spatial variability among trees in prevalence of robbing or the abundance of ineffective honeybees. Unlike powder dispersal, female reproductive success, measured by fruit set and the number of pollen tubes growing in styles, was not affected by robbing. Thus, robbers did not reduce plants female reproductive success either directly by damaging flowers or indirectly by reducing pollen deposition by pollinators. Overall, this study indicates that nectar-robbers were not often costly to plants, and sometimes even benefited plants.     

Reproductive biology and effect of nectar robbing on fruit production in Macleania bullata (Ericaceae)

Hummingbird-pollinated flowers are frequently subjected to nectar robbing. In this paper, I examine the impact of nectar robbing on plant reproductive success on a hummingbird-pollinated species. After studying the basic aspects of the floral morphology and reproduction of Macleania bullata (Ericaceae) in a tropical montane wet forest in southwest Colombia, I examined the percent of flowers robbed and the effect of nectar robbery on fruit set. The flowers of this species are typical for plants pollinated by long-bill hummingbirds. They are protandrous and open for four days. Fruit production requires a pollinator visit; fruit set following pollinator exclusion was zero. Fruit set following xenogamous pollen transfer (36.8%) differed significantly from that of population controls (11.9%) and of autogamous pollen transfer (6.3%). Nectar volume, sugar concentration and sugar production were measured at daily intervals from bud opening until the fading of flowers. Daily nectar production (both volume and amount of sugar) varied considerably with flower age. Sugar production peaked on the second day, coinciding with the male phase. The frequency of nectar robbing in the studied population was very high (75% of examined flowers) and was positively correlated with reduced fruit set. I discuss the probability of a relation between reduced fruit set on robbed flowers and an energetic investment. Robbing by non-pollinating visitors can suppose the plant to re-synthesize more nectar. The high incidence of nectar robbing impugns the advantage of specialization.     

Post‐ingestive effects of nectar alkaloids depend on dominance status of bumblebees

Abstract 1. Secondary metabolites have acute or chronic post‐ingestive effects on animals, ranging from death to growth inhibition to reduced nutrient assimilation. 2. Although characterised as toxic, the nectar of Gelsemium sempervirens is not lethal to pollinators, even when the concentration of the nectar alkaloid gelsemine is very high. However, little is known about the sublethal costs of nectar alkaloids. 3. Using a microcolony assay and paired worker bumblebees, the present study measured the effects of artificial nectar containing gelsemine on oocyte development. Oocytes are a sensitive indicator of protein utilisation and general metabolic processes. We also calculated carbohydrate concentrations in the haemolymph to examine energetic costs of gelsemine consumption. 4. High concentrations of gelsemine significantly reduced mean oocyte width in subordinate bees, while dominant bees showed only a trend towards oocyte inhibition. Gelsemine consumption did not reduce carbohydrate concentrations in haemolymph. 5. The cost of ingesting gelsemine may be due to direct toxicity of alkaloids or may be an expense associated with detoxifying gelsemine. Detoxification of alkaloids can require reallocation of resources away from essential metabolic functions like reproduction. The risks associated with nectar alkaloid consumption are tied to both the social and nutritional status of the bee.     

Nectar robbing improves male reproductive success of the endangered Aconitum napellus ssp. lusitanicum

Nectar robbery is usually thought to impact negatively on the reproductive success of plants, but also neutral or even positive effects have been reported. Very few studies have investigated the effects of nectar robbing on the behaviour of legitimate pollinators so far. Such behavioural changes may lead to the reduction of geitonogamy or to increased pollen movement. We simulated nectar robbing in experimental sites as well as in natural populations of Aconitum napellus ssp. lusitanicum, a rare plant pollinated by long-tongued bumblebees. In an experimental setup, we removed the nectaries of 40 % of the flowers, which is similar to rates of robbing observed in wild populations. Patches of plants with experimentally robbed flowers were compared with control patches containing plants with untreated flowers. We observed pollinator behaviour, mimicked male reproductive success (pollen dispersal) using fluorescent dye, and measured female reproductive success (seed set). The main legitimate visitors were bumblebees while honeybees were often observed robbing nectar. They did so by “base working”, i.e. sliding between tepals. Bumblebees tended to visit fewer flowers per plant and spent less time per single flower when these had been experimentally robbed. This change in behaviour consequently increased the proportion of flowers visited by bumblebees in patches with robbed flowers. Fluorescent dye mimicking pollen flow was dispersed larger distances after pollinators had visited patches with robbed flowers compared to control patches. Average seed set per plant was not affected by nectar robbing. Our results demonstrated that A. napellus does not suffer from nectar robbery but may rather benefit via improved pollen dispersal and thus, male reproductive success. Knowledge on such combined effects of behavioural changes of pollinators due to nectar robbery is important to understand the evolutionary significance of exploiters of such mutualistic relationships between plants and their pollinators.     

The nectar alkaloid, gelsemine, does not affect offspring performance of a native solitary bee, Osmia lignaria (Megachilidae)

Abstract.  1. The ecology and evolution of foliar-feeding insects are thought to be closely tied to plant secondary compounds. Although secondary compounds are also abundant in floral nectar, their role in mediating pollinator preference and performance remains relatively unexplored.
2. This study tested the effects of an alkaloid, gelsemine, found in the nectar of Carolina jessamine ( Gelsemium sempervirens L., Loganiaceae), on the performance of a native solitary bee ( Osmia lignaria lignaria Say, Megachilidae). Nectar gelsemine reduces visits from pollinators, including O. lignaria lignaria , and gelsemine is toxic to vertebrates and possibly non-native honey bees ( Apis mellifera L., Apidae). To test the hypothesis that the deterrent effects of nectar gelsemine reflect negative consequences for pollinator performance, O. lignaria lignaria offspring provisions were supplemented with nectar containing different gelsemine concentrations. Effects on larval development time, prepupa cocoon mass, adult emergence, and adult mass were measured.
3. Nectar gelsemine had no effect on any measure of offspring performance. Thus, although gelsemine deters foraging by adult bees, this behaviour did not optimize offspring performance under the experimental conditions of this study. In contrast, sugar added to nectar treatments increased offspring mass.
4. While adult pollinators may avoid nectar with secondary compounds, this could hinder offspring performance by reducing sugar in provisions if nectar is limiting in the environment. Preference-performance trade-offs, which are studied extensively with foliar herbivores, have seldom been tested for pollinating plant consumers. Future studies of nectar secondary compounds and insect pollinator preference and performance may help to integrate studies of foliage-consuming insect herbivores with nectar-consuming insect pollinators.     

Effects of flowering plant density on pollinator visitation, pollen receipt, and seed production in Delphinium barbeyi (Ranunculaceae)

Variation in flowering plant density can have conflicting effects on pollination and seed production. Dense flower patches may attract more pollinators, but flowers in those patches may also compete for pollinator visits and abiotic resources. We examined how natural and experimental conspecific flowering plant density affected pollen receipt and seed production in a protandrous, bumble bee-pollinated wildflower, Delphinium barbeyi (Ranunculaceae). We also compared floral sex ratios, pollinator visitation rates, and pollen limitation of seed set from early to late in the season to determine whether these factors mirrored seasonal changes in pollen receipt and seed production. Pollen receipt increased with natural flowering plant density, while seed production increased across lower densities and decreased across higher flower densities. Experimental manipulation of flowering plant density did not affect pollinator visitation rate, pollen receipt, or seed production. Although pollinator visitation rate increased 10-fold from early to late in the season, pollen receipt and seed set decreased over the season. Seed set was never pollen-limited. Thus, despite widespread effects of flowering plant density on plant reproduction in other species, the effects of conspecific flowering plant density on D. barbeyi pollination and seed production are minor.     

Nectar robbing: a common phenomenon mainly determined by accessibility constraints,nectar volume and density of energy rewards

Nectar robbers use a hole made in the perianth to extract nectar. Since robbers may modify plant fitness, they play an important role by driving evolution on floral traits, shaping population structure and influencing community dynamics. Although nectar robbing is widespread in angiosperms, the causes and ecological implications of this behaviour on large ecological scales are still unexplored. Our aim is to study the frequency of nectar robbing in plants of temperate and tropical regions and examine its association with plant traits. We characterised the levels of nectar robbing in 88 species of Mediterranean, Alpine, Antillean and Andean plant communities and identified the most important nectar robbers. We analysed associations between the levels of robbing and floral morphology, production and density of energy rewards, mechanisms of protection against nectar robbers, plant life form and geographic origin. Nectar robbing was present at all sampling sites. Within communities two patterns of robbing levels related to the diversity and specialization of robbers were detected. In most communities one plant species presented very high levels of robbing while other species had intermediate to low robbing levels. There, nectar robbers are opportunists, robbing highly rewarding plants. In the Andean community the high specialization of several co‐existing flowerpiercers produced an even pattern of robbing levels in the plant community. Plants with long flowers, abundant nectar and a high energy density are more likely to be robbed by both insects and birds. A high aggregation of the flowers within the plants and the presence of long calyxes and bracts are associated to low robbing rates by insects and to a lesser extent by birds. Besides the morphological constraints that operate on a single flower basis, nectar robbing is a phenomenon dependent upon the density of energy rewards reflecting the presence of mechanisms on higher ecological scales.