Pollination by ants: consequences of the quantitative effects on a mutualistic system

Summary We have analysed the importance of worker ants (Proformica longiseta, Formicidae) as pollinators of a mass-flowering woody plant (Hormathophylla spinosa, Cruciferae) in the high-mountain area of the Sierra Nevada (southern Spain). We have quantified the abundance and foraging behavior of P. longiseta in comparison with winged flower visitors. We have also examined, by means of selective exclusion experiments, the role of ants as true pollinators, comparing them with the winged flower visitors. A total of 39 species belonging to 18 families visited the flowers of H. spinosa. All the visitors were winged insects, except P. longiseta, a species which alone made up more than 80% of the total number of insects found on the flowers. All pollinators of H. spinosa had similar foraging patterns, with 98% of total movements made between flowers within the same plant. Ants always made contact with the plant reproductive organs when foraging for nectar, and transferred large numbers of pollen grains. However, pollen exposed to ants for brief periods exhibited reduced percentage of germination. P. longiseta is both the most abundant and spatio-temporally predictable flower visitor of H. spinosa. These characteristics, weighted by their flower visitation rate, make worker ants the pollinator that maintains the strongest mutualistic interaction with H. spinosa. The exclusion experiments show that workers behave as true pollinators, since they contribute to increase the number of viable seeds produced by H. spinosa. The key factor of this interaction is mainly the great density of workers throughout the flowering period. In short, the H. spinosa-P. longiseta mutualistic interaction mainly depends on its high probability of occurrence.     

Relative importance of diurnal and nocturnal pollinators for reproduction in the early spring flowering shrub Stachyurus praecox (Stachyuraceae)

Generalized pollination systems may be favored in early spring flowering plants, as during this period pollinator activity is unpredictable. Many previous studies have concentrated on the importance of diurnal visitors in pollination, and consequently, information on the contribution of nocturnal visitors to pollination in early spring is limited. This study was conducted to evaluate the relative importance of diurnal and nocturnal pollinators in the early spring flowering dioecious shrub Stachyurus praecox (Stachyuraceae), in two temperate forests in central Japan. Visitors to the female and male flowers were observed during day and night, and their relative contributions to seed set were compared. The pollinator observations revealed that the diurnal and nocturnal insects visited both male and female flowers, and that the main flower visitors were diurnal small bees and flies as well as nocturnal settling moths. The diurnal and nocturnal flower visitors also acted as pollinators, as the pollen grains of S. praecox were attached to the insects collected from the female flowers. Pollination experiments demonstrated that the contributions of diurnal pollinators to the seed set were higher than those of the nocturnal pollinators. The results of this study indicate that S. praecox has a generalized pollination system, comprising both diurnal insects and nocturnal settling moths. Although the roles of diurnal insects are more important in the pollination of S. praecox, nocturnal settling moths may have a complementary role in early spring.     

Breeding system of the critically endangered Lakela’s Mint and influence of plant height on pollinators and seed output

Understanding reproductive systems of rare plants is critical for conservation efforts. Lakela's Mint, Dicerandra immaculata Lakela var. immaculata, is an endangered plant endemic to an approximately 4.8-km long area in Florida, USA. We used an experimental garden and three populations of Lakela's Mint to determine: (1) what is the breeding system (autonomous, asexual, self-fertile, cross-fertile) and are insects necessary for reproduction; (2) which native and nonnative insect species visit flowers and is the frequency of visits to a plant influenced by its height; (3) does the number of flowers visited within a plant by individual insects differ among native and nonnative insect species and due to plant height; and (4) is seed output influenced by plant height? Our results indicate that the breeding system of Lakela's Mint was facultative outcrossing. Insect-pollinated flowers produced more seeds than flowers that reproduced autonomously or asexually. The honey bee Apis mellifera L., a nonnative species, was the most frequent visitor to plants and visited more flowers within plants than native pollinators, but its behavior was not influenced by plant height. Native pollinators such as Bombus impatiens Cresson were attracted more frequently to shorter plants, but visited fewer flowers than on taller plants. Despite having fewer total and pollinated flowers, shorter plants had a higher output of intact seeds than taller plants, which could be due to differences in efficiency between native and nonnative pollinators or other factors. Our results add insight into factors influencing seed output and interactions between pollinators and rare plants.     

The effect of limited visibility on vigilance behaviour and speed of predator detection: implications for the conservation of granivorous passerines

Ants have been traditionally considered either as predators or dispersers of seeds, but not both. That is, ant dispersal is restricted to myrmecochorous seeds, while almost all seeds removed by seed‐harvesting ants are eaten. However, harvesting ants might be simultaneously antagonistic and mutualistic towards seeds. This study analyzes the predation–dispersal relationship between seed‐harvesting ants and seeds of Lobularia maritima, a non‐myrmechorous perennial herb, in order to disentangle the dual role of ants as dispersers and predators of L. maritima seeds. The results obtained confirm the role of harvesting ants as both predators and dispersers of the non‐myrmechorous seeds of L. maritima. The removal activity of Messor bouvieri on L. maritima seeds is very important, particularly in autumn, which is the flowering and fruiting peak of this plant. It can be estimated that harvesting ants collect more than 85% of seeds, and almost 70% of them are effectively lost to predation. However, these granivorous ants also have drawbacks as seed dispersers. There is a relatively small percent of seeds collected by ants that escape predation, either because they are dropped on the way to the nest (16.4% of seeds harvested), or because they are mistakenly rejected on the refuse pile (0.9%). Abiotic dispersal of L. maritima seeds in the absence of ants occurs over very short distances from the plant stem. As seeds dispersed by ants reach a considerably greater distance than that obtained by gravity, this might represent a real advantage for the species, because it reduces intraspecific adult competition for seedlings, which directly influences seedling survivorship. These results challenge the generalization that seed removal by ants generally leads to successful seed dispersal if done by legitimate seed dispersers, or seed loss if done by seed consumers that eat them, and confirm that harvesting ants might have a dual role as both predators and dispersers of nonmyrmechorous seeds.     

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.     

Sources of variation in pollinator contribution within a guild: the effects of plant and pollinator factors

Among plants visited by many pollinator species, the relative contribution of each pollinator to plant reproduction is determined by variation in both pollinator and plant traits. Here we evaluate how pollinator movement among plants, apparent pollen carryover, ovule number, resource limitation of seed set, and pollen output affect variation in contribution of individual pollinator species to seed set in Lithophragma parviflorum (Saxifragaceae), a species visited by a broad spectrum of visitors, including beeflies, bees and a moth species. A previous study demonstrated differences among visitor species in their single-visit pollination efficacy but did not evaluate how differences in visitation patterns and pollen carryover affect pollinator efficacy. Incorporation of differential visitation patterns and pollen carryover effects —commonly cited as potentially important in evaluating pollinator guilds — had minor effects (0–0.6% change) on the estimates of relative contribution based on visit frequency and single-visit efficacy alone. Beeflies visited significantly more flowers per inflorescence than the bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating that apparent pollen carryover did not reduce per-visit efficacy of these taxa. In contrast, Greya moth visits showed a decrease in seed set by 55.4% and the smaller bees by 45.4% from first to second flower. The larger carryover effects in smaller bees and Greya were diminished in importance by their small overall contribution to seed set. Three variable plant traits may affect seed set: ovule number, resource limitation on seed maturation, and pollen output. Ovule number per flower declined strongly with later position within inflorescences. Numbers were much higher in first-year greenhouse-grown plants than in field populations, and differences increased during 3 years of study. Mean pollen count by position varied 7-fold among flowers; it paralleled ovule number variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowering, with seed set in hand-pollinated flowers ranging from 66% in early flowers to 0% in the last two flowers of all plants. Variation in ovule number and resource limitation of seed maturation jointly had a strong effect on the number of seeds per flower. Visitation to early flowers had the potential to cause more seed set than visitation to later flowers. Overall, the most important sources of variation to seed production contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potentially differential phenology among visitor species. These effects are likely to vary among populations and years.     

Ant pollination of the palaeoendemic dioeciousBorderea pyrenaica (Dioscoreaceae)

The importance of ants for pollination in the dioeciousBorderea pyrenaica (Dioscoreaceae), a Tertiary palaeoendemic plant of the Pyrenees (NE Iberian Peninsula) was studied. The frequency of different visitors (ants, lady beetles, andDiptera) to staminate and pistillate flowers was quantified, and their effectiveness as pollinators was examined by means of fruit and seed set in selective experimental exclusions. Although ants were less abundant on flowers than other visitors, they were the most effective pollinators. Some qualitative factors of this mutualistic ant-plant interaction may account for their effectiveness: the small size of the ants, their high visitation rate to pistillate flowers, and the lack of reduction in viability of the pollen transported on the integument. In addition, the sedentary nature of ants assures their presence during the flowering period. The most abundant floral visitors ofB. pyrenaica were therefore not the most effective pollinators.     

Flowering seasonality and flower characteristics ofLoranthus acaciae Zucc. (Loranthaceae): Implications for advertisement and bird-pollination

The flowering biology and pollination ecology ofLoranthus acaciae was studied at Hazeva in the northern Arava Valley in Israel. Flowers at anthesis had red anthers, a red stigma and a green corolla which turned red as a postfloral phenomenon. Their flowering period was approximately 10 months long (from mid-June until mid-April) during which time two main flowering patterns were distinguished. Some plants flowered twice a year, with separate summer and winter flowering periods; other plants flowered continuously, with two peaks, one in the summer and one in the winter. Several significant differences between summer and winter flowering and fruiting were found: (1) the summer flowering period was shorter than that of winter, (2) flowering synchrony between individual plants was lower in summer than in winter, (3) in summer the plants produced a larger proportion of female flowers, whereas in winter most of the plants produced a larger proportion of hermaphrodites, (4) in summer a limited number of plants produced smaller flowers while the majority produced normal-sized flowers, whereas in winter the entire population produced only normal-sized flowers, and (5) fruit set percentage was lower in summer than in winter.L. acaciae was found to be self-compatible, but, since it was not spontaneously self-pollinated, it showed high dependence on pollinator activity. In summer the flowers were visited by a wide spectrum of pollinators, both birds and insects, while in winter flowers were visited almost exclusively by the orange-tufted sunbird (Nectarinia osea osea, Nectariniidae). These seasonal changes in flowering characteristics and pollinator activity could explain why reproductive success is higher in winter than in summer.     

A non‐pollinating moth inflicts higher seed predation than two co‐pollinators in an obligate pollination mutualism

1. Mutualisms are relationships of mutual exploitation, in which interacting species receive a net benefit from their association. In obligate pollination mutualisms (OPMs), female pollinators move pollen between the flowers of a single plant species and oviposit eggs within the female flowers that they visit. 2. Competition between co‐occurring pollinator species is predicted to increase pollinator virulence, i.e. laying more eggs or consuming more seeds per fruit. Plants involved in OPMs frequently host various non‐pollinating seed parasites and parasitoids that may influence the outcome of the mutualism. Quantifying the prevalence of parasites and parasitoids and competition between pollinators is important for understanding the factors that influence OPM evolutionary stability. 3. This study investigated the pollination mutualism occurring between the leaf flower plant, Breynia oblongifolia, and its co‐pollinating Epicephala moths. A third moth, Herpystis, also occurs in B. oblongifolia fruits as a non‐pollinating seed parasite. 4. Breynia oblongifolia fruits were collected to quantify seed predation and compare seed predation costs between the three moth species. Results showed that the larvae of the two pollinator species consume similar numbers of seeds, and that adults deposit similar numbers of eggs per flower. As such, no evidence of increases in virulent behaviours was detected as a result of competition between co‐pollinators. 5. By contrast, the seed parasite Herpystis consumed more seeds than either pollinator species, and fruit crops with a high proportion of Herpystis had significantly lower net seed production. 6. This work adds to the growing understanding of the ecology and dynamics of plant–pollinator mutualisms.     

Seasonal and annual variations in the pollination efficiency of a pollinator community of Dictamnus albus L.

The interplay between insect and plant traits outlines the patterns of pollen transfer and the subsequent plant reproductive fitness. We studied the factors that affect the pollination efficiency of a pollinator community of Dictamnus albus L. by evaluating insect behaviour and morphological characteristics in relation to flowering phenology. In order to extrapolate the pollinator importance of single taxa and of the whole pollinator guild, we calculated an index distinguishing between potential (PPI) and realized (RPI) pollinator importance. Although the pollinator species spectrum appeared rather constant, we found high intra‐ and inter‐annual variability of pollinator frequency and importance within the insect community. Flower visitation rate strictly depended on insect abundance and on the overlap between their flying period and flower blooming. All the pollinators visited flowers from the bottom to the top of the racemes, excluding intra‐plant geitonogamous pollination, and most of them showed high pollen fidelity. Only medium large‐sized bees could contact the upward bending stiles while feeding on nectar, highlighting a specialisation of the plant towards bigger pollinators. Moreover, we found evidence of functional specialisation, since all pollinators were restricted to a single taxonomic group (order: Hymenoptera; superfamily: Apoidea). Both the PPI and RPI indices indicate Habropoda tarsata as the most important pollinator of D. albus. Following hand cross‐pollination experiments we revealed the presence of pollination limitation in 1 of the 3 years of field study. We discuss this result in relation to flowering abundance and to possible mismatches of phenological periods between plants and insects.     

Self-pollination in Euphrasia willkommii Freyn (Scrophulariaceae), an endemic species from the alpine of the Sierra Nevada (Spain)

 The reproductive ecology of Euphrasia willkommii (Scrophulariaceae), an endemic species from the Mediterranean alpine environments of the SE Spain, has been experimentally studied during two reproductive seasons. The flowers of this plant species were visited by very few insects belonging only to two generalist taxa, thrips and ants. Nevertheless, reproduction is not pollen limited in E. willkommii. Hand-pollinations demonstrated that this plant species is capable of selfing, reproductive success being similar in autogamous and allogamous crosses. Moreover, pollinator-exclusion experiments also showed that, under natural conditions, this plant relies predominantly on selfing, seed production being similar in presence or absence of pollinators. Selfing in E. willkommii is presumably an ecological mechanism to ensure successful reproduction in a harsh environment where pollinator availability is extremely low. Received May 2, 2001 Accepted December 6, 2001     

Pollination and reproduction enhance the invasive potential of an early invader: the case of Lythrum salicaria (purple loosetrife) in South Africa

The potential of an alien plant to spread rapidly and colonize new habitat may be related to the mode of reproduction and the ability to attract pollinators. Most studies focus on widespread invasive plants, in which pollinators are rarely limiting. Here, we assess the ability of a recent invader in South Africa, the tristylous Lythrum salicaria to self-reproduce and whether this can explain the delay between introduction and spread. This study was conducted in one of the largest known populations (a total of 7 populations in South Africa) of L. salicaria in the Liesbeek river in the fynbos biome. We assessed the importance of pollinators and autonomous selfing in L. salicaria by comparing seed set between pollinator excluded and naturally pollinated flowers. Overall, 5 pollinators (4 native and 1 alien) were recorded with Cape honeybees and Africa Monarch butterflies the most prominent. Seed and fruit set were significantly higher in open pollinated flowers compared to pollinator excluded flowers. Also, seed and fruit set in pollinator excluded flowers were higher in long and medium morphs compared to short morphs. Germination was high for seeds from pollinator, but also from pollinator excluded treatments. This shows that L. salicaria in South Africa is self-compatible to some extent, but it is frequented by pollinators, significantly increasing seed production. Despite L. salicaria being tristylous, all 3 morphs are present in South Africa and with a huge seed production, this species has the potential to become a major invader of rivers and wetlands in South Africa.

     

Effects of season and population size on pollination and reproductive output in a Mediterranean shrub

• Pollinator guilds may change throughout extended flowering periods, affecting plant reproductive output, especially in seasonal climates. We hypothesised a seasonal shift in pollinator guild and an autumn reduction in pollinator abundance, especially in small and sparse populations.
• We recorded pollinator identity, abundance and behaviour in relation to flower density from plant to population throughout the extended flowering of Ononis tridentata. We evaluated female reproductive output by recording pollination success and pre‐dispersal seed predation in eight populations of contrasting size and density. Offspring quality was also characterised through seed weight and germination.
• A diverse guild of insects visited O. tridentata in spring, while only Apis mellifera was observed in autumn. Visitation frequency did not vary seasonally, but the number of flowers per foraging bout was lower, and seeds were heavier and had a higher germination rate in autumn. Plant and neighbourhood flowering display were not related to pollinator visitation frequency or behaviour. However, the rate of fertilised ovules, seed set and autumn flowering display size were positively related to population density.
• The maintenance of pollination in autumn enhances the reproductive performance of O. tridentata due to higher quality of autumn seed, and to a large reduction in seed predator pressure. We also suggest that observed changes in pollinator behaviour could be one of the processes behind seasonal variation in seed performance, since geitonogamous crosses were less likely to occur in autumn.

     

Relative contributions to seed production by floral visitors of slickspot peppergrass, Lepidium papilliferum (Brassicaceae)

Assessing the relative contributions to seed production made by different types of floral visitors is fundamental to understanding the evolution of floral morphology and the influence that particular pollinator taxa have on plant fitness and reproduction. This 3-year study examined the pollinator activity and the seed production in three populations of Lepidium papilliferum, a threatened mustard endemic to sagebrush-steppe habitat in southwest Idaho. Relative amounts of time visitor taxa spent foraging on flowers, visitation rates (number of flowers visited per unit time during a foraging bout), and pollination effectiveness (fruit set per single visit to a virgin flower) were recorded for each of 12 insect taxa that visited L. papilliferum flowers. Relative contributions to seed production were calculated as the product of relative interaction frequencies (the relative number of flowers visited by each taxon—the “quantity” component of pollination) and pollination effectiveness (fruit set per single visit to a virgin flower—the “quality” component of pollination). Despite the superiority of some insect taxa in terms of pollination effectiveness, estimates of relative pollinator contributions to seed production were influenced primarily by an insect taxon’s interaction frequency with flowers. Pollinator assemblages varied widely both spatially and temporally, which suggest that L. papilliferum is not under strong selective pressure to shift from its generalist pollination strategy toward greater specialization. For this threatened plant, reliance on a diverse assemblage of insect pollinators may serve as an important buffer against disruption in reproductive success caused by fluctuations in population sizes of individual pollinator taxa.     

Spatial and temporal dispersion patterns of pollinators and their relationship to the flowering strategy of Yucca whipplei (Agavaceae)

Summary A field investigation of the mutualistic interaction between a monocarpic perennial plant, Yucca whipplei, and its host-specific pollinator and seed predator, Tegeticula maculata (Lepidoptera: Prodoxidae), was conducted to determine how the resource utilization pattern and population dynamics of the pollinator have influenced the evolution of the flowering and fruiting pattern of the plant. Although the temporal pattern of emergence of pollinators results in a relatively close tracking of flower abundance within a season, the ratio of pollinators to open flowers does vary significantly within a season, as well as between seasons. At any point in time during the flowering season, the population of adult yucca moths is distributed evenly among the available flowers, so that the number of pollinators on an inflorescence is directly proportional to the number of open flowers available. The relative isolation of individual flowering plants appears to have little effect on the distribution of pollinators among inflorescences. The number of fruits initiated on a plant is directly proportional to the number of flowers produced, and is also partially determined by the time of flowering. Yucca whipplei always produces many more flowers than fruits. Most flowers are not fertilized, and the plants also generally abort and abscise immature fruits after flowering. Fruit production of at least some plants, however, appeared limited by pollination. It is also expected that in some years the relative abundance of pollinators will be low enough that most plants will be pollinator-limited. It is suggested that the pattern of flowering and fruiting of this species has evolved in response to the unpredictability of pollinator availability, both within and between seasons. Resource uncertainty and selection acting on the male component of fitness may also be involved.     

Interaction frequency and per-interaction effects as predictors of total effects in plant–pollinator mutualisms: a case study with the self-incompatible herb <Emphasis Type="Italic">Linaria lilacina</Emphasis>

It is widely recognized that pollinators vary in their effectiveness in pollination mutualisms, due both to differences in flower–pollinator morphological fit as well as pollinator behaviour. However, pollination webs typically treat all interactions as equal, and we contend that this method may provide misleading results. Using empirical and theoretical data, we present the case study of a self-incompatible herb in which the number of flowers visited by a pollinator cannot be used as a surrogate for the total effect of a pollinator on a plant due to differences in per-visit effectiveness at producing seeds. In self-incompatible species, the relationship between interaction frequency and per-interaction effect may become increasingly negative as more flowers per plant are visited due to geitonogamous pollen transfer. We found that pollinators making longer bouts (i.e. visiting more flowers per plant visit) had an overall higher pollination success per bout. However, per-interaction effects tended to decrease as the bout progressed, particularly for pollinators that cause higher pollen deposition. Since the same interaction frequency may result from different combinations of number of bouts (plant visits) and bout length (flowers visited/bout), pollinators making repeatedly shorter bouts may contribute more to plant reproduction for the same number of flowers visited. Consequently, the magnitude of the differences in number of interactions of different insect types may be overridden by the magnitude of the differences in effectiveness as pollinators, even if the same pollinators consistently interact more frequently. We discuss two predictions regarding the validity of using interaction frequency as a surrogate for plant seed production (as a measure of total effect), depending on the degree of self-compatibility, plant size and floral display. We suggest that the role of interaction frequency must be tested for different species, environments, and across wider scales to validate its use as a surrogate for total effect in plant–pollinator networks.     

Experimental study of pollination by ants in Mediterranean high mountain and arid habitats

In this paper, we report the results of an experimental study on ant pollination of three plant species inhabiting the Mediterranean high mountains (Alyssum purpureum, Arenaria tetraquetra and Sedum anglicum) and four species inhabiting the aridlands (Lepidium subulatum, Gypsophyla struthium, Frankenia thymifolia and Retama sphaerocarpa) of South-eastern Spain. We determined several plant and ant traits, as well as the composition and abundance of the pollinator assemblage. Insects belonging to 29 families and five orders visited the flowers of the plant species studied. In all but two, L. subulatum and G. struthium, the ants comprised 70–100% of the flower visitors. The results clearly show that five out of seven of these plant species were pollinated by ants. The role of the ants as pollinators seems to depend heavily on the relative abundance of the ants with respect to the other species of the pollinator assemblage, ant pollination becoming evident when ants outnumber other floral visitors. The ant-pollination systems analysed in this study may be the result of prevailing ecological conditions more than an evolutionary result of a specialized interaction.     

The effects of floral display on pollinator visitation vary among populations ofPhacelia linearis (Hydrophyllaceae)

Summary Individual plants in gynodioecious populations ofPhacelia linearis (Hydrophyllaceae) vary in flower gender, flower size, and flower number. This paper reports the effects of variation in floral display on the visitation behaviour of this species' pollinators (mainly pollen-collecting solitary bees) in several natural and three experimental plant populations, and discusses the results in terms of the consequences for plant fitness. The working hypotheses were: (1) that because female plants do not produce pollen, pollen-collecting insects would visit hermaphrodite plants at a higher rate than female plants and would visit more flowers per hermaphrodite than per female; and (2) that pollinator arrival rate would increase with flower size and flower number, the two main components of visual display. These hypotheses were generally supported, but the effects of floral display on pollinator visitation varied substantially among plant populations. Hermaphrodites received significantly higher rates of pollinator arrivals and significantly higher rates of visits to flowers than did females in all experimental populations. Flower size affected arrival rate and flower visit rate positively in natural populations and in two of the three experimental populations. The flower size effect was significant only among female plants in one experimental population, and only among hermaphrodites in another. The effect of flower number on arrival rate was positive and highly significant in natural populations and in all experimental populations. In two out of three experimental populations, insects visited significantly more flowers per hermaphrodite than per female and visited more flowers on many-flowered plants than on few-flowered plants, but neither effect was detected in the third experimental population. Because seed production is not pollen-limited in this species, variation in pollinator visitation behaviour should mainly affect the male reproductive success of hermaphrodite plants. These findings suggest that pollinator-mediated natural selection for floral display inP. linearis varies in space and time.     

First record of potential bird pollination in the holoparasitic genus Orobanche L.

Pollinators play an important role in the reproduction of zoophilous plants. A shift in pollinators has often been observed for oceanic island plants, probably because of the differences in fauna. In this study, we obtained data on pollinator shifts from insects to birds in Orobanche boninsimae (Orobanchaceae), a holoparasitic plant species endemic to the Bonin (Ogasawara) Islands, oceanic islands in the Pacific. We observed pollination and measured seed viability in O. boninsimae and its continental sister species O. coerulescens. We found that two passerine birds, the Japanese white-eye (Zosterops japonicus) and bulbul (Hypsipetes amaurotis squameiceps), visited the flowers and sucked the nectar of O. boninsimae, while only insects visited those of O. coerulescens. Viable seeds were produced under pollinator-excluded treatments in the two Orobanche species, indicating that the seeds were produced by automatic self-pollination and/or apomixis. These results suggest that O. boninsimae may be pollinated by birds and can produce seeds by automatic self-pollination/apomixis. This is the first record of visitation of the genus Orobanche by birds. Studies of pollination systems in native plants on the Bonin Islands are few compared to those on other oceanic islands, and O. boninsimae may provide a valuable example of pollinator shifts in the Bonin Islands.     

Wind pollination in high-mountain populations of Hormathophylla spinosa (Cruciferae)

In this paper we study aspects of the breeding system of Hormathophylla spinosa (Crucifcrae) to identify the factors responsible for seed production in the absence of insect pollinators. The pollinator-exclusion experiments show that H. spinosa, under natural conditions, does not produce seed by apomixis or spontaneous autogamy. H. spinosa appears to be self-incompatible but slightly geitonogamous. Thus, this plant species needs pollen vectors for reproduction. The results of the wind-exclusion experiments performed during two different years in two populations of H. spinosa support the hypothesis that the wind acts as a pollen vector; flowers excluded from the wind had a lower fruit set and female fertility than flowers excluded from all pollinator insects (winged and wingless). This generalist pollination system (insects and wind) permits this species to colonize and sustain viable populations in high mountains, where this species is the only woody shrub living above 3000 m above sea level in the Sierra Nevada.