Insect colour preference compared to flower colours in the Australian Alps

An apparent predominance of plant taxa with pale flowers in the alpine floras of Australia and New Zealand may be due to the prevalence of insects, such as flies, that prefer pale colours and the absence of other types of potential pollinators that are attracted to bright colours such as social bees and birds. In this study, the diversity of flower colours, and the preference of insects for different colours were examined for the largest contiguous alpine area in Australia, around Mt Kosciuszko. Out of an alpine flora of 204 taxa, 127 species were found to have large showy flowers. The most common flower colour among these taxa was white (53.5%), then yellow (21.3%), followed by pink (6.3%), and cream (6.3%). Only a handful of taxa had red, blue, brown, green, orange or purple flowers. When the colour preference of insects was tested using five different coloured traps (white, yellow, orange, red and purple), the most successful traps were white then yellow, with these two colours accounting for 66% of all individual insects collected. Diptera were the most common insects caught (576 insects greater than 4 mm in length, 31 morphotaxa) showing an apparent preference for white and yellow coloured traps over others. Therefore, the results add some support to the proposition that the 'white' flora of the Australian Alps may be associated with the colour preference of flies, which have previously been found to be the most common type of pollinators in the Kosciuszko alpine zone.     

The relative importance of solitary bees and syrphid flies as pollinators of two outcrossing plant species in the New Zealand alpine

Pollinators vary in their relative contribution to the conspecific pollen deposited onto receptive stigmas, because of variation in both visitation rate and effectiveness of pollen transfer. Syrphid flies and short‐tongued solitary bees are common flower visitors in alpine New Zealand, yet their relative importance as pollinators is unknown. We measured pollinator performance of the New Zealand alpine endemics Hylaeus matamoko (Hymenoptera: Colletidae) and Allograpta spp. (Diptera: Syrphidae) on two New Zealand alpine herbs, Ourisia glandulosa (Plantaginaceae) and Wahlenbergia albomarginata (Campanulaceae). Ourisia glandulosa received visits by solitary bees and syrphid flies at equal frequencies, whereas W. albomarginata was mostly visited by H. matamoko. Based on single‐visit pollen deposition to virgin stigmas, H. matamoko was a much more effective pollinator than Allograpta spp., delivering 10 times as much pollen per visit to O. glandulosa stigmas and 3 times as much to W. albomarginata stigmas. By multiplying visitation frequency by single‐visit pollen deposition, we estimated that H. matamoko performed 90% and 95% of the pollination of O. glandulosa and W. albomarginata, respectively. Although H. matamoko bees are short‐tongued and small in size, they are critically important to plant reproductive success in the New Zealand alpine. These bees contributed most of the pollination, even to a species that received just as many visits by flies, underscoring the need to consider per‐visit effectiveness as well as visitation rate in assessing the importance of different pollinators.     

Where have all the blue flowers gone: pollinator responses and selection on flower colour in New Zealand Wahlenbergia albomarginata

Although pollinators are thought to select on flower colour, few studies have experimentally decoupled effects of colour from correlated traits on pollinator visitation and pollen transfer. We combined selection analysis and phenotypic manipulations to measure the effect of petal colour on visitation and pollen export at two spatial scales in Wahlenbergia albomarginata. This species is representative of many New Zealand alpine herbs that have secondarily evolved white or pale flowers. The major pollinators, solitary bees, exerted phenotypic selection on flower size but not colour, quantified by bee vision. When presented with manipulated flowers, bees visited flowers painted blue to resemble a congener over white flowers in large, but not small, experimental arrays. Pollen export was higher for blue flowers in large arrays. Pollinator preference does not explain the pale colouration of W. albomarginata, as commonly hypothesized. Absence of bright blue could be driven instead by indirect selection of correlated characters.     

Flower colour change in Banksia ilicifolia: A signal for pollinators

Both birds and insects visit yellow flower heads of Banksia ilicifolia rather than those in the pink or red phases. Birds carry most pollen. Substantial nectar and pollen rewards are present only in the yellow phase. The timing of flower colour change also corresponds to a decline in viability of presented pollen and stigma receptivity. Colour change is age-dependent rather than pollinator-induced. Bird visits to yellow or red heads are essentially determined by the availability of nectar in each rather than differences in their visibility. Fruit set is negligible in the absence of pollinators but still < 1% in their presence. Banksia ilicifolia has the smallest heads and is the most localized of five co-occurring and partly co-flowering Banksia species. It is hypothesized that the restriction of flower colour change to B. ilicifolia increases the competitiveness of this species: bird visitors are directed to flower heads with abundant nectar, viable pollen and receptive stigmas, foraging and pollination efficiency thereby being enhanced without a marked reduction in long-distance attractiveness of the tree to potential pollinators.     

Beyond the pollination syndrome: nectar ecology and the role of diurnal and nocturnal pollinators in the reproductive success of Inga sessilis (Fabaceae)

Inga species present brush‐type flower morphology allowing them to be visited by distinct groups of pollinators. Nectar features in relation to the main pollinators have seldom been studied in this genus. To test the hypothesis of floral adaptation to both diurnal and nocturnal pollinators, we studied the pollination ecology of Inga sessilis, with emphasis on the nectar secretion patterns, effects of sequential removals on nectar production, sugar composition and the role of diurnal and nocturnal pollinators in its reproductive success. Inga sessilis is self‐incompatible and pollinated by hummingbirds, hawkmoths and bats. Fruit set under natural conditions is very low despite the fact that most stigmas receive polyads with sufficient pollen to fertilise all ovules in a flower. Nectar secretion starts in the bud stage and flowers continually secreting nectar for a period of 8 h. Flowers actively reabsorbed the nectar a few hours before senescence. Sugar production increased after nectar removal, especially when flowers were drained during the night. Nectar sugar composition changed over flower life span, from sucrose‐dominant (just after flower opening, when hummingbirds were the main visitors) to hexose‐rich (throughout the night, when bats and hawkmoths were the main visitors). Diurnal pollinators contributed less than nocturnal ones to fruit production, but the former were more constant and reliable visitors through time. Our results indicate I. sessilis has floral adaptations, beyond the morphology, that encompass both diurnal and nocturnal pollinator requirements, suggesting a complementary and mixed pollination system.     

Floral colour change in Pedicularis monbeigiana (Orobanchaceae)

We examined the effects of the retention of colour-changed flowers on long- and short-distance attractiveness of bumblebees and the likelihood of successive flower visits by bumblebees in Pedicularis monbeigiana. The lower lip changed colour with age from white to purple. Hand geitonogamous pollination significantly reduced seed production. No pollen limitation occurred in this species. Purple-phase flowers contributed minimally to pollinator attractiveness at long distance. The combination of less reproductive flowers with a lower amount of reward and floral colour change enabled plants to direct pollinators to reproductive, highly rewarding white flowers at close range. A high percentage of purple-phase flowers in an inflorescence was associated with a marked reduction in the frequency of successive flower visits to individual plants. We suggest floral colour change in P. monbeigiana may serve as a mechanism for enhancing inter-individual pollen transfer and reducing intra-individual pollen transfer.     

Nectar secretion dynamic links pollinator behavior to consequences for plant reproductive success in the ornithophilous mistletoe Psittacanthus robustus

The mistletoe Psittacanthus robustus was studied as a model to link flower phenology and nectar secretion strategy to pollinator behaviour and the reproductive consequences for the plant. The bright‐coloured flowers presented diurnal anthesis, opened asynchronously throughout the rainy season and produced copious dilute nectar as the main reward for pollinators. Most nectar was secreted just after flower opening, with little sugar replenishment after experimental removals. During the second day of anthesis in bagged flowers, the flowers quickly reabsorbed the offered nectar. Low values of nectar standing crop recorded in open flowers can be linked with high visitation rates by bird pollinators. Eight hummingbirds and two passerines were observed as potential pollinators. The most frequent flower visitors were the hummingbirds Eupetomena macroura and Colibri serrirostris, which actively defended flowering mistletoes. The spatial separation between anthers, stigma and nectar chamber promotes pollen deposition on flapping wings of hovering hummingbirds that usually probe many flowers per visit. Seed set did not differ between hand‐, self‐ and cross‐pollinated flowers, but these treatments set significantly more seeds than flowers naturally exposed to flower visitors. We suggest that the limitation observed in the reproductive success of this plant is not related to pollinator scarcity, but probably to the extreme frequency of visitation by territorial hummingbirds. We conclude that the costs and benefits of plant reproduction depend on the interaction strength between flowers and pollinators, and the assessment of nectar secretion dynamics, pollinator behaviour and plant breeding system allows clarification of the complexity of such associations.     

Intra‐floral resource partitioning between endemic and invasive flower visitors: consequences for pollinator effectiveness

1. Sympatric flower visitor species often partition nectar and pollen and thus affect each other's foraging pattern. Consequently, their pollination service may also be influenced by the presence of other flower visiting species. Ants are solely interested in nectar and frequent flower visitors of some plant species but usually provide no pollination service. Obligate flower visitors such as bees depend on both nectar and pollen and are often more effective pollinators. 2. In Hawaii, we studied the complex interactions between flowers of the endemic tree Metrosideros polymorpha (Myrtaceae) and both, endemic and introduced flower‐visiting insects. The former main‐pollinators of M. polymorpha were birds, which, however, became rare. We evaluated the pollinator effectiveness of endemic and invasive bees and whether it is affected by the type of resource collected and the presence of ants on flowers. 3. Ants were dominant nectar‐consumers that mostly depleted the nectar of visited inflorescences. Accordingly, the visitation frequency, duration, and consequently the pollinator effectiveness of nectar‐foraging honeybees (Apis mellifera) strongly decreased on ant‐visited flowers, whereas pollen‐collecting bees remained largely unaffected by ants. Overall, endemic bees (Hylaeus spp.) were ineffective pollinators. 4. The average net effect of ants on pollination of M. polymorpha was neutral, corresponding to a similar fruit set of ant‐visited and ant‐free inflorescences. 5. Our results suggest that invasive social hymenopterans that often have negative impacts on the Hawaiian flora and fauna may occasionally provide neutral (ants) or even beneficial net effects (honeybees), especially in the absence of native birds.     

The generalist Inga subnuda subsp. luschnathiana (Fabaceae): negative effect of floral visitors on reproductive success?

Inga species are characterised by generalist or mixed pollination system. However, this feature does not enhance reproductive rates in species with very low fruit set under natural conditions. Some ecological and genetic factors are associated with this feature, and to test the effect of massive visits on pollination success in Inga subnuda subsp. luschnathiana, we studied the efficacy of polyads deposited on stigmas of flowers isolated from visitors and polyads exposed to visitors. The proportion of polyads fixed in stigmas decreased after exposure to visitors (24 h) in comparison to stigmas isolated from visitors (hummingbirds, bees, wasps, hawkmoths and bats), and fruit set was very low. Furthermore, nectar production, sugar composition and other floral biology traits were evaluated. Increased nectar production, sugar availability and sucrose dominance during the night indicates adaptation to nocturnal visitors and supports their role as main pollinators; although the brush‐flower morphology, time of anthesis, nectar dynamics and chemical composition also allow daytime visitors. Thus the species is an important resource for a diverse group of floral visitors. We conclude that excess visits (diurnal and nocturnal) are responsible for the decrease in fixed polyads in stigmas of I. subnuda subsp. luschnathiana flowers, thus contributing, with others factors, to its low fruit set. Therefore, the generalist pollination system does not result in reproductive advantages because the low fruit set in natural conditions could be the result of a negative effect of visitors/pollinators.     

MORPHOLOGICAL STUDIES OF THE NYMPHAEACEAE. XI. THE FLORAL BIOLOGY OF NELUMBO PENTAPETALA

The floral biology of Nelumbo pentapetala (Walter) Fernald, the American lotus, native to Texas, was investigated. Anthesis occurs over three consecutive days with flowers opening each morning and closing around noon. First-day flowers are protogynous with the perianth parts partially expanded so that pollen-covered insects which are attracted by floral color and the intense “fruity” odor (diffused with the aid of increased floral temperature) are directed on to the flattened receptacle (= carpellary receptacle) from which the receptive stigmas protrude, thus accomplishing pollination. During the second morning anther dehiscence begins and insects which visit and forage within the flower become covered with pollen and typically crawl over the still receptive stigmas achieving “facilitated” self-pollination (indirect autogamy). By mid-morning of the second day the stigmas dry and become non-receptive to pollen. During the third day of anthesis perianth and staminal parts quickly abscise and over the period of a few weeks the receptacle and enclosed fruits mature. In most populations studied, Hymenoptera (e.g., Lusioglossum spp., and Apis mellifera) were the most abundant and effective pollinators. In some populations, however, Coleoptera (e.g., Chauliognathus) were also numerous and effective pollinators. It is suggested that the overall floral structure (e.g., large numbers of stamens, masses of pollen, staminal appendages) are adaptations which facilitate the pollination of Nelumbo by beetles.     

Visual signalling of nectar‐offering flowers and specific morphological traits favour robust bee pollinators in the mass‐flowering tree Handroanthus impetiginosus (Bignoniaceae)

Mass flowering is a widespread blooming strategy among Neotropical trees that has been frequently suggested to increase geitonogamous pollination. We investigated the pollination ecology of the mass‐flowering tree Handroanthus impetiginosus, addressing its breeding system, the role in pollination of different visitors, the impact of nectar robbers on fruit set and the function of colour changes in nectar guides. This xenogamous species is mainly pollinated by Centris and Euglossa bees (Apidae) seeking nectar, which are known to fly long distances. The flowers favour these bees by having: (1) a closed entrance in newly opened flowers which provides access only to strong bees capable of deforming the flower tube; and (2) a nectar chamber that is accessible only to long‐tongued bees. Only first‐day flowers with yellow nectar guides produce nectar. Pollinators prefer these flowers over second‐ and third‐day flowers with orange and red nectar guides, respectively. Nectar robbers damage two‐thirds of the flowers and this robbing activity decreases fruit set by half. We attribute the low fruit set of H. impetiginosus to the intense nectar robbing and hypothesize that visual signalling of nectar presence in newly opened (receptive) flowers reduces geitonogamy by minimizing bee visits to unrewarding (non‐receptive) flowers. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 396–407.     

Dark purple nectar as a foraging signal in a bird-pollinated Himalayan plant

? Some plants secrete coloured nectar to attract pollinators, but little is known about the chemical origins of nectar colouration and its ecological function. Leucosceptrum canum stands out as the only plant with coloured nectar recorded in the Himalayas. Here, we focused on the compound associated with the dark colour of the nectar, as well as its secretion dynamics during the flowering season and its relationship to pollinators. ? Fresh nectar was analysed by semi-preparative reversed-phase high-performance liquid chromatography (HPLC), LC-MS and HRESIMS (high resolution electronspray ionization mass spectroscopy) to determine which compound causes the nectar colouration. Behavioural experiments were conducted with birds and honeybees to elucidate the effect of the nectar colour and volume on pollinators. ? We identified a purple anthocyanidin, 5-hydroxyflavylium, as a natural nectar product for the first time. Two short-billed birds were found to pollinate this plant, which employs two nectar-based mechanisms to direct bird pollinators to reproductively active flowers, controlling nectar palatability and presenting a foraging signal for birds by altering nectar volume and colour in a developmental stage-specific manner. ? 5-Hydroxyflavylium was found to be the cause of the nectar colouration, the function of which is to act as a foraging signal to increase pollination efficiency through nectar visibility and palatability.     

Do pollinators influence the assembly of flower colours within plant communities?

The co-occurrence of plant species within a community is influenced by local deterministic or neutral processes as well as historical regional processes. Floral trait distributions of co-flowering species that share pollinators may reflect the impact of pollinator preference and constancy on their assembly within local communities. While pollinator sharing may lead to increased visitation rates for species with similar flowers, the receipt of foreign pollen via interspecific pollinator movements can decrease seed set. We investigated the pattern of community flower colour assembly as perceived by native honeybee pollinators within 24 local assemblages of co-flowering Oxalis species within the Greater Cape Floristic Region, South Africa. To explore the influence of pollinators on trait assembly, we assessed the impact of colour similarity on pollinator choices and the cost of heterospecific pollen receipt. We show that flower colour is significantly clustered within Oxalis communities and that this is not due to historical constraint, as flower colour is evolutionarily labile within Oxalis and communities are randomly structured with respect to phylogeny. Pollinator observations reveal that the likelihood of pollinators switching between co-flowering species is low and increases with flower colour similarity. Interspecific hand pollination significantly reduced seed set in the four Oxalis species we investigated, and all were dependant on pollinators for reproduction. Together these results imply that flower colour similarity carries a potential fitness cost. However, pollinators were highly flower constant, and remained so despite the extreme similarity of flower colour as perceived by honeybees. This suggests that other floral traits facilitate discrimination between similarly coloured species, thereby likely resulting in a low incidence of interspecific pollen transfer (IPT). If colour similarity promotes pollinator attraction at the community level, the observed clustering of flower colour within communities might result from indirect facilitative interactions.     

Reproductive biology of Butea monosperma (Fabaceae)

The reproductive biology encompassing phenology, floral biology, pollination and breeding systems, of Butea monosperma, a beautiful tree of the Indian subcontinent, was investigated in a protected dry, deciduous forest located in New Delhi. Phenological studies indicated that although the species shows a regular flowering season, all trees do not flower every year. Flowers are typically papilionaceous; the stigma is wet papillate and the style is hollow. The flowers show characteristics of bird pollination being large and bright orange-red in colour with copious amounts of nectar, and exhibiting diurnal anthesis. Although the flowers are frequented by as many as seven species of birds belonging to six families, only one species, the purple sunbird (Nectarinia asiatica), is the effective pollinator. The flowers are also pollinated by the three-striped squirrel (Funambulus tristiatus). Unlike other flower visitors, these two pollinators forage the nectar from the open side of the keel (legitimate path) during which pollen grains are deposited on their body parts. After the first visit of a sunbird or a squirrel, virgin flowers showed pollen load on the stigma and developed into fruits. B. monosperma shows a weak form of self-incompatibility. Fruit set following manual self-pollination (5.25 %) was comparable with open-pollination (approx. 5 %) but was significantly lower than manual cross-pollination (22.51 %). This indicates that there is a high degree of geitonogamous pollination in this species, which may lead to a weakening of self-incompatibility as a means of reproductive assurance. The results are analysed in the light of prevailing discussions on specialized vs. generalized pollination systems.     

Sexual differences in reproductive characters and pollinator attractiveness in gynodioecious Glechoma longituba (Lamiaceae)

We quantified the differences in floral characters and attractiveness to flower visitors under natural conditions between the sexual types in the gynodioecious plant Glechoma longituba. We also manipulated flowers by altering corolla size or nectar volume, or by removing anthers, to examine the effect of these primary and secondary attractants (i.e. rewards and advertisements) on attractiveness. A change in corolla size and shape reduced visiting frequency and pollen load. Removal of anthers did not affect visiting rates, but significantly reduced pollination rates and stigmatic pollen load. A decrease in the nectar volume of a flower was associated with a reduction in handling time and pollen loads on stigmas. These results show that corolla size is an important advertisement to pollinators (particularly at greater distance), which associate hermaphrodite flowers with a larger corolla and a larger volume of nectar than female flowers. We found that artificial changes in population structure affected the behavior of pollinators as well as the pollination rates of flowers. We suggest that the pattern of distribution of hermaphrodite and female clones in a population may serve to avoid pollen limitation in a female clone or patch. This effect may ensure female reproductive success and allow for the maintenance of female individuals in natural populations of this gynodioecious plant.     

FLORAL BIOLOGY,HUMMINGBIRD POLLINATION AND FRUIT PRODUCTION OF TRUMPET CREEPER (CAMPSIS RADICANS,BIGNONIACEAE)

Trumpet creeper is self-incompatible and bears long, tubular, orange flowers from June to September. Flowering peaks rapidly, then declines and continues at low levels for several weeks. The initial burst of flowering may attract pollinators that return even during subsequent reduced flowering. Most flowers open before noon and nectar production totals 110 μl of 26% sucrose equivalents per flower, an exceptionally high production for a temperate zone plant. Production ceases within 20–30 hr of flower opening, but corollas persist for several days and may serve to attract pollinators. Effective pollination reduces the period of stigma receptivity and speeds closing of stigma lobes. Only 1–9% of flowers produced mature fruits at four sites in Illinois and Missouri. Roughly 400 pollen grains had to be deposited on a receptive stigma to cause fruit development beyond an initial period of high abortion. At two sites, 17% and 89% of stigmas received over 400 pollen grains. Assuming 50% of deposited grains were from the same plant, fruit production at one site was clearly pollinator limited, that at the second site may have been. Ruby-throated Hummingbirds (Archilochus colubris) deposited ten times as much pollen per stigma per visit as honeybees (Apis mellifera) and bumblebees (Bombus spp.). Fruit set was highest where rubythroat visitation was most frequent. Trumpet creeper appears primarily adapted for hummingbird pollination, but can also be adequately pollinated by honeybees and bumblebees. This is one of the first attempts to relate pollen-depositing capabilities of pollinators of any plant to pollen requirements for fruit production. Several characteristics suggest that trumpet creeper may be adapted to pollination at low densities (often called traplining) in its presumed original, woodland, habitat.     

Flower visitation by birds in Europe

Most flowering plants depend on animal pollination. Several animal groups, including many birds, have specialized in exploiting floral nectar, while simultaneously pollinating the flowers they visit. These specialized pollinators are present in all continents except Europe and Antarctica, and thus, insects are often considered the only ecologically relevant pollinators in Europe. Nevertheless, generalist birds are also known to visit flowers, and several reports of flower visitation by birds in this continent prompted us to review available information in order to estimate its prevalence. We retrieved reports of flower–bird interactions from 62 publications. Forty‐six bird species visited the flowers of 95 plant species, 26 of these being exotic to Europe, yielding a total of 243 specific interactions. The ecological importance of bird–flower visitation in Europe is still unknown, particularly in terms of plant reproductive output, but effective pollination has been confirmed for several native and exotic plant species. We suggest nectar and pollen to be important food resources for several bird species, especially tits Cyanistes and Sylvia and Phylloscopus warblers during winter and spring. The prevalence of bird flower‐visitation, and thus potential bird pollination, is slightly more common in the Mediterranean basin, which is a stopover to many migrant bird species, which might actually increase their effectiveness as pollinators by promoting long‐distance pollen flow. We argue that research on bird pollination in Europe deserves further attention to explore its ecological and evolutionary relevance.     

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.     

One for all and all for one: retention of colour‐unchanged old flowers increases pollinator attraction in a hermaphroditic plant

• Long‐lived flowers increase pollen transfer rates, but these entail high water and carbon maintenance costs. The retention of pollinated and reward‐free old flowers enhances pollinator visitation to young receptive flowers by increasing floral display size. This mechanism is associated with acropetal inflorescences or changes in flower colour and openness, but the retention of unchanging solitary flowers remains overlooked.
• We examined pollination‐dependent variation in floral longevity and determined stigmatic receptivity, pollen viability and pollen removal rates among flower ages in Kielmeyera regalis, a Neotropical savanna shrub. We also evaluated the effects of floral display size on pollinator visitation rates. Lastly, we determined whether old flowers are unvisited and exclusively increase pollinator attraction to young flowers through flower removal experiments.
• Regardless of pollination treatment, flowers lasted fully open with no detectable physical changes for 3 days. Over time, stigmas remained receptive but >95% of pollen was removed. Pollinator visitation significantly increased with floral display size and intermediate percentages (15–30%) of newly opened flowers. Accordingly, the retention of reward‐free and unvisited old flowers increased young flower–pollinator interaction.
• Our results reveal the importance of a prolonged floral longevity in increasing pollinator attraction toward newly opened receptive flowers without changes in flower colour and form. We conclude that the retention of pollinated, reward‐free and unvisited colour‐unchanged old flowers in K. regalis is a strategy that counteracts the water use costs associated with the maintenance of large flowers with increased mate opportunities in a pollen‐limited scenario.

     

The relative importance of birds and bees in the pollination of Metrosideros excelsa (Myrtaceae)

Exclusion experiments were used to assess the effect of different pollinator groups on outcrossing and seed production in Metrosideros excelsa. The main study site was Little Barrier Island, New Zealand where indigenous bird and native solitary bees are the main flower visitors. Our results showed that native birds were more important pollinators of M. excelsa than native bees. Seed production was much higher in open pollination than in two exclusion experiments where either birds were excluded and native bees only had access to flowers, or where all pollinators had been excluded. The number of fertile seeds per capsule was 45% higher after open pollination than in treatments with bee visitation only and 28% higher than in treatments where all flower visitors were excluded. Estimated outcrossing rates were significantly higher (t_m = 0.71) for open pollination in the upper canopy (>4 m above‐ground level) where bird visitation is presumed to be more frequent than for a treatment with native bee access only (t_m = 0.40). Our results also suggest that a large proportion of seeds (66%) arise from autonomous self‐pollination when all pollinators are excluded. In four trees of a modified mainland population with predominantly introduced birds and a mixture of introduced and native bees there was no decrease in seed production for the treatment allowing bee access only, indicating that – in contrast to native bees – honeybees may be more efficient pollinators of M. excelsa. Observation of the foraging behaviour of both groups of bees showed that native bees contact the stigma of flowers less frequently than honeybees. This is likely to be a consequence of their smaller body size relative to honeybees.