Native jewelweed,but not other native species,displays post‐invasion trait divergence

Invasive exotic plants reduce the diversity of native communities by displacing native species. According to the coexistence theory, native plants are able to coexist with invaders only when their fitness is not significantly smaller than that of the exotics or when they occupy a different niche. It has therefore been hypothesized that the survival of some native species at invaded sites is due to post‐invasion evolutionary changes in fitness and/or niche traits. In common garden experiments, we tested whether plants from invaded sites of two native species, Impatiens noli‐tangere and Galeopsis speciosa, outperform conspecifics from non‐invaded sites when grown in competition with the invader (Impatiens parviflora). We further examined whether the expected superior performance of the plants from the invaded sites is due to changes in the plant size (fitness proxy) and/or changes in the germination phenology and phenotypic plasticity (niche proxies). Invasion history did not influence the performance of any native species when grown with the exotic competitor. In I. noli‐tangere, however, we found significant trait divergence with regard to plant size, germination phenology and phenotypic plasticity. In the absence of a competitor, plants of I. noli‐tangere from invaded sites were larger than plants from non‐invaded sites. The former plants germinated earlier than inexperienced conspecifics or an exotic congener. Invasion experience was also associated with increased phenotypic plasticity and an improved shade‐avoidance syndrome. Although these changes indicate fitness and niche differentiation of I. noli‐tangere at invaded sites, future research should examine more closely the adaptive value of these changes and their genetic basis.     

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

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

Reproductive interactions mediated by flowering overlap in a temperate hummingbird–plant assemblage

Pollinator‐mediated competition through shared pollinators can lead to segregated flowering phenologies, but empirical evidence for the process responsible for this flowering pattern is sparse. During two flowering seasons, we examined whether increasing overlap in flowering phenology decreased conspecific pollination, increased heterospecific pollination, and depressed seed output in the seven species composing a hummingbird–plant assemblage from the temperate forest of southern South America. Overall trends were summarized using meta‐analysis. Despite prevailing negative associations, relations between phenological overlap and conspecific pollen receipt varied extensively among species and between years. Heterospecific pollen receipt was low and presumably of limited biological significance. However, our results supported the hypothesis that concurrent flowering promotes interspecific pollen transfer, after accounting for changes in the abundance of conspecific flowers. Seed output was consistently reduced during maximum phenological overlap during the first flowering season because of limited fruit set. Responses varied more during the second year, despite an overall negative trend among species. Relations between estimated effects of phenological overlap on pollination and seed output, however, provided mixed evidence that conspecific pollen loss during pollinator visits to foreign flowers increases pollen limitation. By flowering together, different plant species might benefit each other's pollination by increasing hummingbird recruitment at the landscape level. Nevertheless, our results are mostly consistent with the hypothesis of pollinator‐mediated competition shaping the segregated flowering pattern reported previously for this temperate plant assemblage. The mechanisms likely involve effects on male function, whereby pollen‐transport loss during heterospecific flower visits limit pollen export, and more variable effects on female function through pollen limitation.     

Relative impact of mate versus pollinator availability on pollen limitation and outcrossing rates in a mass‐flowering species

Plant mating systems are driven by several pre‐pollination factors, including pollinator availability, mate availability and reproductive traits. We investigated the relative contributions of these factors to pollination and to realized outcrossing rates in the patchily distributed mass‐flowering shrub Rhododendron ferrugineum. We jointly monitored pollen limitation (comparing seed set from intact and pollen‐supplemented flowers), reproductive traits (herkogamy, flower size and autofertility) and mating patterns (progeny array analysis) in 28 natural patches varying in the level of pollinator availability (flower visitation rates) and of mate availability (patch floral display estimated as the total number of inflorescences per patch). Our results showed that patch floral display was the strongest determinant of pollination and of the realized outcrossing rates in this mass‐flowering species. We found an increase in pollen limitation and in outcrossing rates with increasing patch floral display. Reproductive traits were not significantly related to patch floral display, while autofertility was negatively correlated to outcrossing rates. These findings suggest that mate limitation, arising from high flower visitation rates in small plant patches, resulted in low pollen limitation and high selfing rates, while pollinator limitation, arising from low flower visitation rates in large plant patches, resulted in higher pollen limitation and outcrossing rates. Pollinator‐mediated selfing and geitonogamy likely alleviates pollen limitation in the case of reduced mate availability, while reduced pollinator availability (intraspecific competition for pollinator services) may result in the maintenance of high outcrossing rates despite reduced seed production.     

THE EFFECT OF DELPHINIUM NELSONII POLLEN ON SEED SET IN IPOMOPSIS AGGREGATA,A COMPETITOR FOR HUMMINGBIRD POLLINATION

Sympatric plant species can compete for pollination services in several ways. For example, pollinators may move between species and deposit heterospecific pollen on stigmas, which in turn may reduce the efficacy of conspecific pollen. We explored this possibility by determining the effect of Delphinium nelsonii pollen on seed set in Ipomopsis aggregata. These montane herbs are pollinated by hummingbirds, experience heterospecific pollen deposition in nature, and suffer reduced seed set in each other's presence. We hand-pollinated flowers of I. aggregata with either pure conspecific pollen or a mixture of pollen of the two species. Resulting pollen loads on stigmas ranged from 0–865 D. nelsonii grains and from 10–336 I. aggregata grains; mean seed set per flower was 11.3. There was no detectable effect of D. nelsonii pollen load on I. aggregata seed set. It is possible that seed set reductions seen in previous studies of competition for pollination between these species were caused by pollen wastage, pollen layering on the pollinator, or the temporal sequence of pollen arrival at the stigma.     

Pollinator‐mediated interactions between cultivated papaya and co‐flowering plant species

Many modern crop varieties rely on animal pollination to set fruit and seeds. Intensive crop plantations usually do not provide suitable habitats for pollinators so crop yield may depend on the surrounding vegetation to maintain pollination services. However, little is known about the effect of pollinator‐mediated interactions among co‐flowering plants on crop yield or the underlying mechanisms. Plant reproductive success is complex, involving several pre‐ and post‐pollination events; however, the current literature has mainly focused on pre‐pollination events in natural plant communities. We assessed pollinator sharing and the contribution to pollinator diet in a community of wild and cultivated plants that co‐flower with a focal papaya plantation. In addition, we assessed heterospecific pollen transfer to the stigmatic loads of papaya and its effect on fruit and seed production. We found that papaya shared at least one pollinator species with the majority of the co‐flowering plants. Despite this, heterospecific pollen transfer in cultivated papaya was low in open‐pollinated flowers. Hand‐pollination experiments suggest that heterospecific pollen transfer has no negative effect on fruit production or weight, but does reduce seed production. These results suggest that co‐flowering plants offer valuable floral resources to pollinators that are shared with cultivated papaya with little or no cost in terms of heterospecific pollen transfer. Although HP reduced seed production, a reduced number of seeds per se are not negative, given that from an agronomic perspective the number of seeds does not affect the monetary value of the papaya fruit.     

Asymmetric and frequency‐dependent pollinator‐mediated interactions may influence competitive displacement in two vernal pool plants

A plant species immigrating into a community may experience a rarity disadvantage due to competition for the services of pollinators. These negative reproductive interactions have the potential to lead to competitive displacement or exclusion of a species from a site. In this study, we used one‐ and two‐species arrays of potted plants to test for density and frequency dependence in pollinator‐mediated and above‐ground intraspecific and interspecific competition between two species of Limnanthes that have overlapping ranges, but rarely occur in close sympatry. There were asymmetric competitive effects; the species responded differently to their frequency within 16‐plant replacement series arrays. Limnanthes douglasii rosea experienced stronger reductions in lifetime and per‐flower fertility, likely due to pollinator‐mediated competition with Limnanthes alba. This effect may be linked to asymmetrical competition through heterospecific pollen transfer. This study demonstrates that pollinator‐mediated competition may discourage establishment of L. d. rosea in sites already occupied by its congener.     

Floral specialization for different pollinators and divergent use of the same pollinator among co‐occurring Impatiens species (Balsaminaceae) from Southeast Asia

Floral variation among closely related species is thought to often reflect differences in pollination systems. Flowers of the large genus Impatiens are characterized by extensive variation in colour, shape and size and in anther and stigma positioning, but studies of their pollination ecology are scarce and most lack a comparative context. Consequently, the function of floral diversity in Impatiens remains enigmatic. This study documents floral variation and pollination of seven co‐occurring Impatiens spp. in the Southeast Asian diversity hotspot. To assess whether floral trait variation reflects specialization for different pollination systems, we tested whether species depend on pollinators for reproduction, identified animals that visit flowers, determined whether these visitors play a role in pollination and quantified and compared key floral traits, including floral dimensions and nectar characteristics. Experimental exclusion of insects decreased fruit and seed set significantly for all species except I. muscicola, which also received almost no visits from animals. Most species received visits from several animals, including bees, birds, butterflies and hawkmoths, only a subset of which were effective pollinators. Impatiens psittacina, I. kerriae, I. racemosa and I. daraneenae were pollinated by bees, primarily Bombus haemorrhoidalis. Impatiens chiangdaoensis and I. santisukii had bimodal pollination systems which combined bee and lepidopteran pollination. Floral traits differed significantly among species with different pollination systems. Autogamous flowers were small and spurless, and did not produce nectar; bee‐pollinated flowers had short spurs and large floral chambers with a wide entrance; and bimodally bee‐ and lepidopteran‐pollinated species had long spurs and a small floral chamber with a narrow entrance. Nectar‐producing species with different pollination systems did not differ in nectar volume and sugar concentration. Despite the high frequency of bee pollination in co‐occurring species, individuals with a morphology suggestive of hybrid origin were rare. Variation in floral architecture, including various forms of corolla asymmetry, facilitates distinct, species‐specific pollen‐placement on visiting bees. Our results show that floral morphological diversity among Impatiens spp. is associated with both differences in functional pollinator groups and divergent use of the same pollinator. Non‐homologous mechanisms of floral asymmetry are consistent with repeated independent evolution, suggesting that competitive interactions among species with the same pollination system have been an important driver of floral variation among Impatiens spp.     

Three pollination morphs in Cimicifuga simplex; incipient speciation due to inferiority in competition

Cimicifuga simplex, a Japanese herb, has three different pollinator guilds. At high altitudes it is pollinated by bumblebees. In lowlands, Impatiens textori, a much superior nectar-producer, outcompetes it for bumblebees. In sunny lowland habitats, flowers of a second morph produce a two-component fragrance which facilitates pollination by fritillaries. In experiments, the two components separately had limited effect on alighting rate, while application of both together elevated the attractiveness of non-fragrant flowers to the same as for naturally fragrant flowers. In heavily shaded lowland habitats, a third morph with nonfragrant flowers does not blom until after I. textori, and is pollinated by occasional bumblebees. The data strongly suggest that inferiority in competition has induced incipient speciation in this plant species.     

Intrinsic and extrinsic factors acting on the reproductive process in alpine‐snowbed plants: roles of phenology,biological interaction,and breeding system

Pollinator activity and competition for pollinators lead to quantitative and qualitative pollen limitations on seed production and affect the reproductive success of plant species, depending on their breeding system (e.g., self‐compatibility and heterospecific compatibility) and genetic load (e.g., inbreeding depression and hybrid inviability). In alpine ecosystems, snowmelt regimes determine the distribution and phenology of plant communities. Plant species growing widely along a snowmelt gradient often grow with different species among local populations. Their pollinators also vary in their abundance, activity, and behavior during the season. These variations may modify plant–pollinator and plant–plant interactions. We integrated a series of our studies on the alpine dwarf shrub, Phyllodoce aleutica (Ericaceae), to elucidate the full set of intrinsic (species‐specific breeding system) and extrinsic factors (snow condition, pollinator activity, and interspecific competition) acting on their reproductive process. Seasonality of pollinator activity led to quantitative pollen limitation in the early‐blooming populations, whereas in the late‐blooming populations, high pollinator activity ensured pollination service, but interspecific competition for pollinators led to qualitative and quantitative pollen limitation in less competitive species. However, negative effects of illegitimate pollen receipt on seed‐set success might be reduced when cryptic incompatibility systems (i.e., outcross pollen grains took priority over self‐ and heterospecific pollen grains) could effectively prevent ovule and seed discounting. Our studies highlight the importance of species‐specific responses of plant reproduction to changing pollinator availability along environmental gradients to understand the general features of pollination networks in alpine ecosystems.     

New insights into plants co‐existence in species‐rich communities: The pollination interaction perspective

Questions

In animal‐mediated pollination, pollinators can be regarded as a limiting resource for which entomophilous plant species might interact to assure pollination, an event pivotal for their reproduction and population maintenance. At community level, spatially aggregated co‐flowering species can thus be expected to exhibit suitable suites of traits to avoid competition and ensure pollination. We explored the problem by answering the following questions: (1) are co‐flowering species specialized on different guilds of pollinators; (2) do co‐flowering pollinator‐sharing species segregate spatially; and (3) do co‐flowering pollinator‐sharing species that diverge in anther position spatially aggregate more than those that converge in anther position?

Study Site

Euganean Hills, NE Italy.

Methods

Plant composition, flowering phenology and interactions between each entomophilous plant species and pollinating insects were monitored every 15 days in 40 permanent plots placed in an area of 16 ha. We quantified the degree of flowering synchrony, pollinator‐sharing and spatial aggregation between each pair of entomophilous species. We then tested the relationship between the degree of co‐flowering, pollinator‐sharing and spatial aggregation, and between spatial aggregation and anther position.

Results

Entomophilous species converged, at least partially in flowering time, and the phenological synchronization of flowering was significantly associated with the sharing of pollinator guilds. Co‐flowering pollinator‐sharing species segregated spatially. Furthermore, co‐flowering pollinator‐sharing species that diverged in anther position aggregated more than those that converged in anther position.

Conclusions

Reproductive traits that facilitate the co‐existence of co‐flowering species include specialization on different pollinator guilds and a phenological displacement of the flowering time. Furthermore, in circumstances of increased competition due to phenological synchronization, pollinator‐sharing and spatial aggregation, the chance of effective pollination might depend on differences in anther position, resulting in a divergent pollen placement on pollinator bodies. One of the most interesting results we obtained is that the presence of one mechanism does not preclude the operation of others, and each plant species can simultaneously exhibit different strategies. Although more studies are needed, our results can provide additional information about plant–plant interactions and provide new insights into mechanisms allowing the co‐existence of a high number of plant species in local communities.

     

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.     

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.     

Breeding system and bumblebee drone pollination of an explosively pollen‐releasing plant,Meliosma tenuis (Sabiaceae)

• Explosive pollen release is a mechanism used by some angiosperms that serves to attach pollen to a pollinator's body. It is usually adopted by species with zygomorphic tubular flowers and pollinated by birds and bees. The tree genus Meliosma (Sabiaceae, Proteales) has unique disc‐like flowers that are externally actinomorphic, but internally zygomorphic, and release pollen explosively.
• To elucidate the adaptive significance of explosive pollen release, we observed flowering behaviour, the breeding system and pollinator visits to flowers of the Japanese species Meliosma tenuis in a temperate forest. Flowers bloomed in June and were nectariferous and protandrous. Explosive pollen release was triggered by slight tactile stimuli to anther filaments or staminodes in male‐stage flowers.
• Because pollen cannot come into contact with the pistils enclosed by staminodes, M. tenuis is functionally protandrous. Artificial pollination treatments revealed that M. tenuis is allogamous. The dominant flower visitors were nectar‐seeking drones of the bumblebee species Bombus ardens (Apidae). The drones’ behaviour, pollen attachment on their bodies and fruit set of visit‐restricted flowers suggest that they are the only agent triggering the explosive pollen release mechanism, and are the main pollinator of M. tenuis.
• The finding that bumblebee workers rarely visit these flowers suggests that the explosive pollen release has another function, namely to discourage pollen‐harvesting bumblebee workers.

     

Relationships between the floral neighborhood and individual pollen limitation in two self-incompatible herbs

Local flower density can affect pollen limitation and plant reproductive success through changes in pollinator visitation and availability of compatible pollen. Many studies have investigated the relationship between conspecific density and pollen limitation among populations, but less is known about within-population relationships and the effect of heterospecific flower density. In addition, few studies have explicitly assessed how the spatial scales at which flowers are monitored affect relationships. We investigated the effect of floral neighborhood on pollen limitation at four spatial scales in the self-incompatible herbs Armeria maritima spp. maritima and Ranunculus acris spp. acris. Moreover, we measured pollen deposition in Armeria and pollinator visits to Ranunculus. There was substantial variation in pollen limitation among Armeria individuals, and 25% of this variation was explained by the density of compatible and heterospecific flowers within a 3 m circle. Deposition of compatible pollen was affected by the density of compatible and incompatible inflorescences within a 0.5 m circle, and deposition of heterospecific pollen was affected by the density of heterospecific flowers within a 2 m circle. In Ranunculus, the number of pollinator visits was affected by both conspecific and heterospecific flower densities. This did not, however, result in effects of the floral neighborhood on pollen limitation, probably due to an absence of pollen limitation at the population level. Our study shows that considerable variation in pollen limitation may occur among individuals of a population, and that this variation is partly explained by floral neighborhood density. Such individual-based measures provide an important link between pollen limitation theory, which predicts ecological and evolutionary causes and consequences for individual plants, and studies of the effects of landscape fragmentation on plant species persistence. Our study also highlights the importance of considering multiple spatial scales to understand the spatial extent of pollination processes within a population.     

Prezygotic barriers to gene flow between <Emphasis Type="Italic">Taraxacum ceratophorum</Emphasis> and the invasive <Emphasis Type="Italic">Taraxacum officinale</Emphasis> (Asteraceae)

Prezygotic reproductive barriers limit interspecific gene flow between congeners. Here, I examine the strength of floral isolation and interspecific pollen-pistil barriers between an invasive apomictic, Taraxacum officinale, and the indigenous sexual alpine dandelion, Taraxacum ceratophorum. Experimental arrays of either native inflorescences or a mixture of native and exotic inflorescences were used to examine insect preference and to track movement of a pollen analog. Using hand-pollinations, conspecific and heterospecific pollen germination success on native stigmas was compared. To additionally test for interspecific pollen competition, T. ceratophorum plants received one of three possible hand-pollinations: control conspecific pollination, concomitant conspecific and heterospecific pollination (mixed), or conspecific pollen followed by heterospecific pollen 15 min later (staggered). Floral isolation was negligible as no insect preference was detected. On a presence/absence basis, florets on native inflorescences received slightly less pollen analog from heterospecific donors than from conspecific donors; however, the amount of dye particles transferred from either Taraxacum species to stigmas on recipient T. ceratophorum inflorescences was equivalent. In contrast to weak floral isolation, strong pollen germination and pollen competition barriers should reduce the potential for hybridization. Heterospecific T. officinale pollen exhibited reduced germination success on T. ceratophorum stigmas in comparison to conspecific pollen. Furthermore, a significant pollen-competition effect on the percentage of hybrid offspring was detected only when T. officinale preceded T. ceratophorum pollen by 15 min. This result indicates that conspecific pollen out-competes heterospecific pollen but further suggests that biotic and abiotic factors reducing pollen accrual rates may partially remove barriers to natural hybridization.     

A ballistic pollen dispersal system influences pollination success and fruit‐set pattern in pollinator‐excluded environments for the endangered species Synaphea stenoloba (Proteaceae)

The critically endangered Synaphea stenoloba (Proteaceae) has numerous scentless flowers clustered in dense inflorescences and deploys a ballistic pollen ejection mechanism to release pollen. We examined the hypothesis that active pollen ejection and flowering patterns within an inflorescence influence the reproductive success (i.e. fruit formation) of individual flowers within or among inflorescences of S. stenoloba in a pollinator‐excluded environment. Our results showed that: (1) no pollen grains were observed deposited on the stigma of their own flower after the pollen ejection system was manually activated, indicating self‐pollination within an individual flower is improbable in S. stenoloba; (2) fruit set in the indoor open pollination treatment and the inflorescence‐closed pollination treatment indicated that S. stenoloba is self‐compatible and pollen ejection can potentially result in inter‐floral pollination success; (3) fruit set in the inflorescence‐closed pollination treatment was significantly lower than that of indoor open pollination, indicating within‐ and between‐flower pollination events in an inflorescence are most likely limited, with pollination between inflorescences providing the highest reproductive opportunity; and (4) analysis of the spatial distribution of cumulative fruit set on inflorescences showed that pollen could reach any flower within an inflorescence and there was no functional limitation on seed set among flowers located at various positions within the inflorescence. These data suggest that the pollen ejection mechanism in S. stenoloba can enhance inter‐plant pollination in pollinator‐excluded environments and may suggest adaptation to pollinator scarcity attributable to habitat disturbance or competition for pollinators in a diverse flora. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 170 , 59–68.     

Pollinator‐independent orchid attracts biotic pollinators due the production of lipoidal substances

• Flowering plants often depend on the attraction of biotic pollinators for sexual reproduction. Consequently, the emergence and maintenance of selected floral attributes related to pollinator attraction and rewarding are driven by pollinator pressure.
• In this paper we explore the effect of pollinators, rainfall, temperature and air humidity on the reproduction of a Brazilian terrestrial orchid, Cranichis candida based on data of phenology, flower resources, olfactory and visual attraction cues, pollinators and breeding system.
• The flowers of C. candida are strongly protandrous and pollinated by workers of the social native bee Tetragonisca angustula. The bees collect labellar lipoidal substances (wax scales), which are transported to the nest. The lipoidal substance is composed of sterols, hydrocarbons and terpenes. The last presumably protects the bees and their nests against pathogens and other arthropods. C. candida sets fruits through biotic self‐ and cross‐pollination, and spontaneously due the action of raindrops on flowers.
• Our results indicate that in C. candida, although rain‐mediated spontaneous self‐pollination happens, fructification mediated by biotic pollinations also occurs, which may result in fruit set by cross‐pollination. A mixed pollination system must result in higher genetic variability when compared to species whose fruits are produced entirely by self‐pollination. On the other hand, autogamy is a form of reproductive assurance, and has commonly evolved where pollination services are rare or absent.

     

Spatio‐temporal patterns in pollination of deceptive Aristolochia rotunda L. (Aristolochiaceae)

• Pollination success of highly specialised flowers is susceptible to fluctuations of the pollinator fauna. Mediterranean Aristolochia rotunda has deceptive trap flowers exhibiting a highly specialised pollination system. The sole pollinators are kleptoparasitic flies in search of food. This study investigates these pollinators on a spatio‐temporal scale and the impact of weather conditions on their availability. Two potential strategies of the plants to cope with pollinator limitation, i.e. autonomous selfing and an increased floral life span, were tested.
• A total of 6156 flowers were investigated for entrapped pollinators in 10 Croatian populations. Availability of the main pollinator was correlated to meteorological data. Artificial pollination experiments were conducted and the floral life span was recorded in two populations according to pollinator availability.
• Trachysiphonella ruficeps (Chloropidae) was identified as dominant pollinator, along with less abundant species of Chloropidae, Ceratopogonidae and Milichiidae. Pollinator compositions varied among populations. Weather conditions 15–30 days before pollination had a significant effect on availability of the main pollinator. Flowers were not autonomously selfing, and the floral life span exhibited considerable plasticity depending on pollinator availability.
• A. rotunda flowers rely on insect pollen vectors. Plants are specialised on a guild of kleptoparasitic flies, rather than on a single species. Pollinator variability may result in differing selection pressures among populations. The availability/abundance of pollinators depends on weather conditions during their larval development. Flowers show a prolonged trapping flower stage that likely increases outcrossing success during periods of pollinator limitation.

     

Pollination and reproduction of Psychotria homalosperma,an endangered distylous tree endemic to the oceanic Bonin (Ogasawara) Islands,Japan

Distyly is usually rare or not observed in species thriving on oceanic islands. The rarity of this breeding system is probably because of the difficulty of colonization for distylous plants and the paucity of pollinators on oceanic islands. However, the endemic and endangered tree Psychotria homalosperma has maintained its distylous nature in the oceanic Bonin Islands, Japan. To understand how the distylous breeding system of P. homalosperma has been maintained on these islands and to characterize the reproduction systems, we studied the pollination and reproductive biology of this species. Specifically, we observed current flower visitors and estimated their effects on plant reproduction. We also examined the floral traits and floral volatiles of P. homalosperma to infer its original pollinators, because plant–pollinator relationships in the Bonin Islands have recently been disrupted by anthropological activities. Finally, we examined the fruit set and pollen tube growth in the stigmas under hand and open pollination. Although original pollinators were presumed to be moths with long proboscises, the introduced honeybee, Apis mellifera, was the most common flower visitor. The honeybee carried pollen grains only unidirectionally, from the short‐ to long‐styled morphs, because it could not reach the hidden stigmas of the short‐styled flowers, and long‐styled flowers set fruits 1.7–38 times more than short‐styled ones. This case study indicates that the instability of pollinator fauna can cause distylous species to be rare on oceanic islands.