Changes In Pollinator Visitation Research Articles

Soil moisture affects plant-pollinator interactions in an annual flowering plant.

Many environmental factors impact plant and pollinator communities. However, variation in soil moisture and how it mediates the plant-pollinator interactions has yet to be elucidated. We hypothesized that long-term variation in soil moisture can exert a strong selective pressure on the floral and vegetative traits of plants, leading to changes in pollinator visitation. We demonstrated that there are three phenotypic populations of Gentiana aristata in our study alpine region in the Qinghai-Tibetan Plateau that vary in floral colour and other traits. Pink (dry habitat) and blue (intermediate habitat) flower populations are visited primarily by bumblebees, and white (wet habitat) flower populations are visited by flies. These patterns of visitation are driven by vegetative and floral traits and are constant when non-endemic plants are placed in the intermediate habitats. Additionally, the floral communities in different habitats vary, with more insect-pollinated forbs in the dry and intermediate habitats versus the wet habitats. Through a common garden and reciprocal transplant experiment, we demonstrated that plant growth traits, pollinator attractiveness and seed production are highest when the plant population is raised in its endemic habitat. This suggests that these plant populations have evolved to pollinator communities associated with habitat differences. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Below-ground competition alters attractiveness of an insect-pollinated plant to pollinators.

Competitive interactions between plants can affect patterns of allocation to reproductive structures through modulation of resource availability. As floral traits involved in plant attractiveness to pollinators can be sensitive to these resources, competition with any neighbouring species may influence the attractiveness of insect-pollinated plants. While pollination research has primarily focused on above-ground interactions, this study aims at investigating if the presence of a competitor plant can modulate neighbouring insect-pollinated plant attractiveness to pollinators and resulting fecundity, especially through below-ground competitive interactions for soil resources. We set up a plot experiment in which we grew an insect-pollinated plant, Sinapis alba (Brassicaceae), in a mixture dominated by a wind-pollinated plant, Holcus lanatus (Poaceae). Individuals of S. alba were either subjected to or isolated from (with buried tubes in the soil) below-ground competition. Across the flowering season, floral traits involved in attractiveness of S. alba and pollinator visitation were followed at the plot and plant level to investigate different scales of attractiveness. At the end of the experiment, seeds were harvested to assess plant fecundity. Competition had a significant negative effect on plot and plant floral display size as well as flower size while nectar traits were not affected. When plants of S. alba were in competition, the time to first visit was altered: the proportion of plots that received a visit was smaller for a given time; in other words, it took more time for a given proportion of plots to be visited and some plots were even never visited. Moreover, pollinators made fewer visits per plots. The proportion of viable seeds produced by S. alba in competition was lower and probably linked to the competition itself rather than changes in pollinator visitation. This study suggests that competitive interactions between plants can modulate pollination interactions even when competing plant species are not insect-pollinated.

Pollinator visitation rate and effectiveness vary with flowering phenology.

Flowering time may influence pollination success and seed set through a variety of mechanisms, including seasonal changes in total pollinator visitation or the composition and effectiveness of pollinator visitors. We investigated mechanisms by which changes in flowering phenology influence pollination and reproductive success of Mertensia ciliata (Boraginaceae). We manipulated flowering onset of potted plants and assessed the frequency and composition of pollinator visitors, as well as seed set. We tested whether floral visitors differed in their effectiveness as pollinators by measuring pollen receipt and seed set resulting from single visits to virgin flowers. Despite a five-fold decrease in pollinator visitation over four weeks, we detected no significant difference in seed set among plants blooming at different times. On a per-visit basis, each bumblebee transferred more conspecific pollen than did a solitary bee or a fly. The proportion of visits by bumblebees increased over the season, countering the decrease in visitation rate so that flowering time had little net effect on seed set. This work illustrates the need to consider pollinator effectiveness, along with changes in pollinator visitation and species composition to understand the mechanisms by which phenology affects levels of pollination.

Do local conspecific density and floral display size influence fruit set via pollinator visitation in Orchis militaris?

Plant density varies naturally, from isolated plants to clumped individuals, and this can influence pollinator foraging behaviour and plant reproductive success. In addition, the effect of conspecific density on reproduction may depend on the pollination system, and deceptive species differ from rewarding ones in this regard, a high density being often associated with low fruit set in deceptive plants. In our study, we aimed to determine how local conspecific density and floral display size (i.e. number of flowers per plant) affect fruit set in a deceptive orchid (Orchis militaris) through changes in pollinator visitation. We measured fruit set in a natural population and recorded pollinator abundance and foraging behaviour within plots of different O. militaris densities. Detailed data were recorded for the most abundant potential pollinators of O. militaris, i.e. solitary bees. Floral display size was negatively correlated to fruit set in medium‐density plots, but uncorrelated in low‐ and high‐density plots. Plot density had no effect on solitary bee abundance and visitation, which may be due to low pollinator abundance within the study site. The proportion of visited flowers per inflorescence was negatively influenced by floral display size, which is in line with previous studies. In addition, solitary bees spent decreasing time in successive flowers within an inflorescence, and the time spent per flower was negatively affected by ambient temperature. Our results suggest that pollinator behaviour during visitation is poorly linked to pollen deposition and reproductive success in O. militaris.

Climate and synchrony with conspecifics determine the effects of flowering phenology on reproductive success in Silene acaulis

ABSTRACT Changes in flowering phenology resulting from climate change could impact individual plant fitness and population viability. Flowering phenology can mediate plant reproductive success in several ways, including pollinator interactions, flowering synchrony with conspecifics, and timing of suitable abiotic conditions. We explored factors that control phenology and reproductive success for an alpine cushion plant, Silene acaulis, across two years and four sites, totaling 1,123 plants, in Colorado, USA. We investigated relationships between flowering time, flowering synchrony, and reproductive success with local abiotic conditions and pollinator behavior. Mean flowering phenology was strongly correlated with the timing of snowmelt across sites and years. Relative to mean flowering times, earlier flowering plants generally produced more flowers and experienced greater soil moisture during flowering but reduced synchrony with conspecifics. Fruit set tended to increase with greater soil moisture, synchrony during flowering, and earlier flowering times. Pollinator visitation increased with local Silene flower density. Earlier snowmelt and drier conditions later in the season favor earlier flowering, but these effects are partially counteracted by the positive effects of synchrony, perhaps because of changes in pollinator visitation. Overall, while both biotic and abiotic effects influence reproductive success, late-season drought may outweigh the benefits of flowering synchrony to increasingly favor earlier flowering.

Direct and indirect effects of episodic frost on plant growth and reproduction in subalpine wildflowers.

Frost is an important episodic event that damages plant tissues through the formation of ice crystals at or below freezing temperatures. In montane regions, where climate change is expected to cause earlier snow melt but may not change the last frost-free day of the year, plants that bud earlier might be directly impacted by frost through damage to flower buds and reproductive structures. However, the indirect effects of frost mediated through changes in plant-pollinator interactions have rarely been explored. We examined the direct and pollinator-mediated indirect effects of frost on three wildflower species in southwestern Colorado, USA, Delphinium barbeyi (Ranunculaceae), Erigeron speciosus (Asteraceae), and Polemonium foliosissimum (Polemoniaceae), by simulating moderate (-1 to -5°C) frost events in early spring in plants insitu. Subsequently, we measured plant growth, and upon flowering measured flower morphology and phenology. Throughout the flowering season, we monitored pollinator visitation and collected seeds to measure plant reproduction. We found that frost had species-specific direct and indirect effects. Frost had direct effects on two of the three species. Frost significantly reduced flower size, total flowers produced, and seed production of Erigeron. Furthermore, frost reduced aboveground plant survival and seed production for Polemonium. However, we found no direct effects of frost on Delphinium. When we considered the indirect impacts of frost mediated through changes in pollinator visitation, one species, Erigeron, incurred indirect, negative effects of frost on plant reproduction through changes in floral traits and pollinator visitation, along with direct effects. Overall, we found that flowering plants exhibited species-specific direct and pollinator-mediated indirect responses to frost, thus suggesting that frost may play an important role in affecting plant communities under climate change.

Temporal‐ and density‐dependent impacts of an invasive plant on pollinators and pollination services to a native plant

AbstractPollinators and pollination services are under threat globally, and invasive plants have been implicated in their decline. Results of previous studies suggest that consequences of invasion for pollinators and plant–pollinator interactions are context specific. Investigating factors such as the density of an invasive plant and its phenology may provide a nuanced understanding of invasive species impacts. We conducted a 2‐yr study in Montana to investigate how local pollinator abundance, richness, community composition, and visitation patterns varied with invasive Centaura stoebe density and phenology, and whether C. stoebe altered the reproduction of a co‐flowering native plant, Heterotheca villosa, through changes in pollinator visitation. In an observational study, we found that during its peak bloom in August, Centaurea stoebe provided abundant floral resources to late‐season pollinators. However, prior to C. stoebe bloom, native floral density and pollinator abundance and richness of these plots were lower compared to plots where C. stoebe was low or absent. Pollinator community composition in plots without C. stoebe was different compared to plots with C. stoebe (both high and low C. stoebe density), and these differences in pollinator composition strongly depended on the time of season. In an experimental study, we found that there was little evidence of competition between C. stoebe and H. villosa for pollinators at low relative densities of C. stoebe. Using experimental pollen supplementation, we observed no evidence of pollen limitation of seed set in H. villosa with increasing density of experimentally added C. stoebe. Our results suggest that the impact of an invasive plant on pollinators and plant–pollinator interactions depends on the relative density of the invasive plant and the timing of its bloom. Differences in pollinator visitation patterns over the growing season suggest that although C. stoebe provides abundant resources to late‐season pollinators, displacement of native plants at high C. stoebe density may indirectly harm pollinators that are active before C. stoebe blooms or that prefer native plants. Based on our results, restricting C. stoebe to low densities may help mitigate negative repercussions to native plant reproduction and may even be beneficial to some pollinators.

Root herbivory indirectly affects above‐ and below‐ground community members and directly reduces plant performance

Summary There is a widespread recognition that above‐ and below‐ground organisms are linked through their interactions with host plants that span terrestrial subsystems. In addition to direct effects on plants, soil organisms such as root herbivores can indirectly alter interactions between plants and other community members, with potentially important effects on plant growth and fitness. We manipulated root herbivory by Acalymma vittatum in Cucumis sativus to determine indirect effects on arbuscular mycorrhizal fungi, leaf herbivory, the leaf pathogen downy mildew and pollinators. We also manipulated pollen receipt by plants to determine whether root herbivory reduced plant reproduction through changes in pollinator visitation. Overall, root herbivory had strong net negative effects on plant growth and fitness, with 34% reductions in both leaf and fruit production by high root damage levels relative to control, despite reduced infection by downy mildew. High root herbivory also reduced floral visitation by 39%, apparently due to lower flower production, as flower size and scent were unaffected. Above‐ground herbivory was not affected by root herbivores. Although root herbivory reduced pollinator visits, pollen receipt manipulations had no effect on fruit set, indicating that reduced pollinator service did not affect plant reproduction. Synthesis. Root herbivory had indirect effects on a range of community members, including mutualists and antagonists both above‐ and below‐ground. Although reduced pathogen infection associated with root herbivory would be expected to benefit plants, root herbivory had an overall strong negative effect on plant growth and reproduction, indicating that direct negative effects over‐rode any potential indirect benefits.

Pollinator response to flower color polymorphism and floral display in a plant with a single‐locus floral color polymorphism: consequences for plant reproduction

AbstractVariation in flower color, particularly polymorphism, in which two or more different flower color phenotypes occur in the same population or species, may be affected or maintained by mechanisms that depend on pollinators. Furthermore, variation in floral display may affect pollinator response and plant reproductive success through changes in pollinator visitation and availability of compatible pollen. To asses if flower color polymorphism and floral display influences pollinator preferences and movements within and among plants and fitness‐related variables we used the self‐incompatible species Cosmos bipinnatus Cav. (Asteraceae), a model system with single‐locus flower color polymorphism that comprises three morphs: white (recessive homozygous), pink (heterozygous co‐dominate), and purple (dominant homozygous) flowers. We measured the preferences of pollinators for each morph and constancy index for each pollinator species, pollination visitation rate, floral traits, and female fitness measures. Flower color morphs differed in floral trait measures and seed production. Pollinators foraged nonrandomly with respect to flower color. The most frequent morph, the pink morph, was the most visited and pollinators exhibited the highest constancy for this morph. Moreover, this morph exhibited the highest female fitness. Pollinators responded strongly to floral display size, while probed more capitulums from plants with large total display sizes, they left a great proportion of them unvisited. Furthermore, total pollinator visitation showed a positive relation with female fitness. Results suggest that although pollinators preferred the heterozygous morph, they alternate indiscriminately among morphs making this polymorphism stable.

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.

Pollinator responses to variation in floral display and flower size in dioecious Sagittaria latifolia (Alismataceae)

In animal-pollinated plants with unisexual flowers, sexual dimorphism in floral traits may be the consequence of pollinator-mediated selection. Experimental investigations of the effects of variation in flower size and floral display on pollinator visitation can provide insights into the evolution of floral dimorphism in dioecious plants. Here, we investigated pollinator responses to experimental arrays of dioecious Sagittaria latifolia in which we manipulated floral display and flower size. We also examined whether there were changes in pollinator visitation with increasing dimorphism in flower size. In S. latifolia, males have larger flowers and smaller floral displays than females. Visitation by pollinators, mainly flies and bees, was more frequent for male than for female inflorescences and increased with increasing flower size, regardless of sex. The number of insect visits per flower decreased with increasing floral display in males but remained constant in females. Greater sexual dimorphism in flower size increased visits to male inflorescences but had no influence on the number of visits to female inflorescences. These results suggest that larger flower sizes would be advantageous to both females and males, and no evidence was found that females suffer from increased flower-size dimorphism. Small daily floral displays may benefit males by allowing extended flowering periods and greater opportunities for effective pollen dispersal.

Direct and indirect effects of habitat area and matrix composition on species interactions among flower‐visiting insects

Traditionally, ecologists have focused on direct effects of habitat area and arrangement on individual species or pairwise species interactions. Indirect effects of habitat heterogeneity on multiple interacting species are often neglected or lack experimental support. In a factorial field experiment, we explored the direct and indirect effects of habitat area, fragmentation, and matrix composition on a community of flower‐visiting insects in red clover. Habitat area, fragmentation, and density of inflorescences of red clover all influenced the insect pollinators and, in turn, the production of clover seeds. The strongest direct effect was on pollinator visitation, which was substantially higher in small clover patches surrounded by bare‐ground than in larger patches embedded within grass. Structural equation modeling indicated that the observed matrix‐dependent changes in pollinator visitation propagated across a tri‐trophic system: higher visitation rates positively correlated with a higher seed set, which in turn was positively correlated with abundances of seed predators and their parasitoids. Therefore, this study suggests that habitat area and matrix composition can strongly influence the structure of species interaction webs through indirect effects, and also emphasizes that these effects can be propagated through mutualistic as well as trophic interactions.

Seasonal changes in pollen-packaging schedules in the protandrous plant Chamerion angustifolium.

The rate at which pollen is presented varies widely both among and within species of flowering plants. Although several studies have tried to explain between-species variation in this trait, few explanations exist for the variation that exists among individuals of the same species. I used protandrous fireweed, Chamerion angustifolium (Onagraceae), to examine seasonal changes in pollinator visitation, population sex ratio, and pollen presentation schedules. Most studies that make predictions about optimal pollen presentation assume conditions are constant throughout the flowering season. However, data presented here show that pollen presentation schedules vary seasonally: in early season flowers, anthers dehisce over a number of days, while late season flowers present all anthers simultaneously. I show that pollinator visitation rates to individual plants are higher but more variable early in the season than late in the season. Furthermore, per-male availability of female-phase flowers increases over the course of the flowering season in this population. I suggest that seasonal differences in the variability of pollinator visitation and the availability of female flowers has played an important role in selection for pollen presentation schedules.