Floral Density Research Articles

Integrating effects of neighbor interactions for pollination and abiotic resources on coffee yield in a multi‐strata agroforest

AbstractAnimal‐pollinated plants interact with neighbors for both abiotic resources and pollination, with consequences for reproduction and yield. Yet few studies have compared the relative magnitude of these effects, particularly in agroecosystems. In vertically stratified communities, such as agroforests, neighbor effects may be stratum‐dependent. Understanding the net effects of neighbors on crop yield is important for managing multifunctional agroecosystems to simultaneously support production and biodiversity. This study evaluated the effects of neighboring plants on pollen deposition, fertilization, and yield in Coffea arabica in a shaded organic coffee farm with high non‐crop plant abundance and diversity in Chiapas, Mexico. We assessed the impact of (1) floral resources at three vertical strata (herbs, coffee bushes, and canopy trees) on stigma pollen load (a measure of interaction for pollination), and (2) floral density and canopy cover (proxies for competition for abiotic resources) on yield (final fruit set and per‐fruit weight), using structural equation modeling to evaluate the relative effect of each interaction type. Coffee competed for pollination with neighbors (conspecifics and heterospecifics) across strata. Pollen load influenced final fruit set, but the effect of neighbor competition for pollination was weaker than effects mediated by interactions for abiotic resources. Effects of interactions for abiotic resources were heterogeneous across strata, with negligible effects of herb‐layer or coffee flower density but net positive effects of canopy trees on final fruit set. Overall effects of neighbors on coffee yield were weak, suggesting that coffee agroecosystems can be managed to maintain high plant density and diversity without sacrificing yield.Abstract in Spanish is available with online material.

Orchid fruiting success is unrelated to surrounding floral resources in South Australian plant communities

AbstractTo maintain recruitment in orchid populations in an ecosystem setting, we must understand how surrounding floral resources affect fruiting success. We studied fruiting success in two endemic Australian species, Diuris pardina and Glossodia major, in relation to surrounding floral resources. Diuris pardina has a visually deceptive pollination strategy via mimicry of pea flowers, attracting pollinators associated with co-flowering plants of Pultenaea. Glossodia major displays dummy anthers and has a more generalist pollination strategy. We expected fruiting success of both species to positively correlate to conspecific and heterospecific floral density because orchid pollination should be enhanced by the attraction of higher densities of native bees. We expected fruiting success of D. pardina to positively correlate with abundance of Pultenaea flowers. Surveying 18 plots in South Australia, we counted species, individuals and flowers of conspecifics and heterospecifics and returned to count flowers that set fruit. We conducted Pearson correlations between fruiting success and density of conspecific flowers, richness, abundance and Shannon index of surrounding floral resources and floral abundance of individual species. Fruiting success was correlated with conspecific floral density for D. pardina but not G. major. No relationship was found between fruiting success and heterospecific floral resources. Fruiting success of D. pardina was not correlated with abundance of Pultenaea; instead it was positively correlated with the invasive species Lavandula stoechas.

Effects of short‐term managed honey bee deployment in a native ecosystem on wild bee foraging and plant–pollinator networks

Abstract Honey bees (Apis mellifera L.) are important agricultural pollinators, and there is increasing demand for forage habitat for managed colonies. However, there is also evidence that pasturing honey bee colonies within natural landscapes may negatively affect wild bees through resource competition. To assess resource competition between managed honey bees and wild bees, we conducted repeated, short‐term deployments of honey bee colonies within Florida forests coinciding with seasonal wildflower bloom, and compared wild bee foraging with and without honey bee colonies present over multiple seasons. We recorded over 2000 bee visits including 196 pairwise bee–plant interactions. Deploying honey bee colonies was associated with a reduction in wild bee foraging rates, and honey bee and wild bee foraging rates were significantly, negatively correlated. Honey bees disproportionately visited resources with high floral density. Honey bee foraging preferences differed significantly from genera with small‐bodied (Lasioglossum, Perdita, Augochlorella), and/or specialist species (Perdita, Andrena), as well as with Megachile, but overlapped with genera including larger‐bodied (Bombus, Habropoda, Osmia, Xylocopa) and/or generalist species (Bombus, Xylocopa, Agapostemon). Deploying honey bee colonies did not significantly affect plant–pollinator network metrics. These results illustrate that short‐term honey bee colony deployment can negatively affect wild bee foraging and that competition may be greater for certain genera, particularly larger‐bodied bees or those with generalist diets though less for smaller‐bodied and/or specialist bees. Our short‐term, low‐density deployment treatments may have precluded significant effects on network metrics and likely underestimate the effects of typical higher density and longer‐term honey bee deployment.

Co-Flowering Plant Densities Affect Bee Visitation to a Focal Plant Species, but Bee Genera Differ in Their Response

The relative densities of co-flowering plant species can affect pollinator foraging behavior, i.e., bee choice of which plants to visit in a foraging path. Because animal-pollinated plants depend on sequential pollinator visits to conspecifics for successful reproduction, understanding the relationship between relative densities of co-flowering plants and bee visits within a foraging path is necessary to determine whether co-flowering species facilitate or compete for bee visitation. We examined the role of relative co-flowering plant species densities in determining bee visits to two focal plant species (Astragalus scaphoides and Penstemon albertinus) within foraging paths by following individual bees in natural multispecies co-flowering plant communities. We found that conspecific and heterospecific plant density influenced the proportion of visits to the focal plant species during foraging bouts, but effects of density differed among bee groups. Considering all bees, Lupinus spp. facilitated visits to A. scaphoides while Castilleja spp. competed with A. scaphoides for visits. When three bee groups, Bombus, Anthophora/Eucera, and “small bees,” were modeled separately, Bombus consistently visited A. scaphoides more than expected regardless of its density and P. albertinus less than expected at intermediate densities. Anthophora/Eucera exhibited a nonlinear response to A. scaphoides floral density but visited P. albertinus in direct proportion to its density. “Small bees” visited both plants in direct proportion to their densities. Our results highlight that bee groups should be treated separately in models of pollinator visitation behavior, and that the composition and density of the co-flowering plant community may impact reproductive success of focal plant species.

Invader complexes or generalist interactions? Seasonal effects of a disturbance gradient on plants and floral visitors

Disturbances can facilitate the spread of exotic plants, which establish mutualisms with exotic bees, constituting invader complexes. However, a disturbance-mediated increase in flower resources can also promote native floral visitors due to the fact that plant-pollinator interactions are generalist. We experimentally tested these ideas in northern Patagonian woodlands. In three sites, we examined the effect of harvesting intensity (HI) (0%, 30%, 50%, 70% of biomass removal) on flower density (exotic: herbs; native: herbs, shrubs, trees) and floral visitor density (exotic: honey bees, bumblebees; native: bees and wasps, hoverflies, other flies, beetles, ants). For four years, we made observations throughout the flowering period to test seasonality. HI had the most substantial positive effect on native herbs, followed by shrubs, and it was null on trees. The effects on the floral density of exotic herbs depended on site. HI enhanced the density of exotic bumblebees, native bees and wasps, and hoverflies; but it had no effect on honey bees, and it showed variable effects on the other floral-visitor groups. Although seasonality was relevant, there was no strong interaction with HI. In general, HI enhanced the density of native and exotic floral visitors, which is better explained by generalist interactions rather than by invader complexes.

Testing competing hypotheses about the behaviour of floral visitors in mixed‐flower stands

1. Plants occur in mixed stands and flower simultaneously. Most hypotheses about how flowers affect the visitation rates of conspecific or heterospecific neighbours involve attractiveness. Attractive flowers are thought to monopolise visitors, but also to act as magnets increasing visits to unattractive neighbours. Attractiveness may be density‐dependent: Visitors could profit from focusing on abundant species, sometimes forming search images that make rare flowers less detectable.2. We tested four hypotheses using data on visitation rates by honeybees (Apis melifera) and bee flies (Bombyliidae) to 1072 stands comprising six plant species in different densities. Hypotheses were compared through multi‐model inference using the disc equation (a functional response model) modified for co‐flowering species and density‐dependent attractiveness.3. We found that the most attractive flowers had attributes (colour, position, mobility) that elicit strong responses in the sensory apparatus of each type of visitor.4. Attractiveness usually increased with conspecific floral density, indicating that visitors focus on abundant species. The most attractive flowers strongly reduced the attractiveness of their neighbours, reinforcing visitor monopolisation. However, mildly attractive species increased the attractiveness of less alluring flowers, acting as magnets.5. In bombyliids, flowers increased the attractiveness of same‐coloured heterospecifics but reduced it when colours differed, suggesting search‐image formation. The results were similar for both types of visitors but were weaker and less consistent in honeybees. This suggests that Apis behaviour is not as rapidly adjusted to local densities as that of bombyliids, perhaps because of longer‐term memory or a strong role of sociality in determining individual behaviour.

Wild bee visitors and their association with sown and unsown floral resources in reconstructed pollinator habitats within an agriculture landscape

AbstractIn the Midwestern United States where the landscape has been largely converted from tallgrass prairie to row crops, thousands of private land parcels have been enrolled in federal pollinator habitat reconstruction programmes.To examine the outcome of pollinator habitat reconstruction, we randomly selected 19 sites that were in the third growing season post restoration and surveyed plant, floral and wild bee richness and abundance. Floral sources were divided into three categories: species from the seed mix (sown), species from the soil seed bank or surrounding landscape (unsown), and species that were likely unintentionally sown (contamination).Seventy‐two percent of captured bees were collected from sown flowers. The majority of oligolectic bees were Asteraceae specialists. Using linear regressions with bee abundance and richness as response variables and floral density and diversity as predictor variables, we showed that sown floral density was positively correlated with the total abundance and richness of bees collected from sown flowers, indicating that the selected pollinator friendly flowers provide support for wild bee communities. Conversely, total plant stem density or richness or the total floral density or richness was not significantly correlated with bee abundance or richness.The plant–pollinator network nestedness and modularity were positively correlated with the ratio of the unsown floral richness at each site, which suggests that the association between unsown flowers and wild bees can increase the long‐term stability of the plant‐pollinator network, even though the unsown species do not directly increase the bee abundance or richness.

Facilitation and competition shape a geographical mosaic of flower colour polymorphisms

Abstract Flower colour differs dramatically between populations for some plant species; yet, we know little about what drives this variation. Such polymorphisms can be influenced by plant–pollinator interactions, but whether they are also influenced by pollinator‐mediated plant–plant interactions is unknown. We test whether flower colour polymorphisms can arise through convergence (facilitation) or divergence (competition) of flower phenotypes resulting from plant–plant interactions mediated by the shared, and only, pollinator (orange‐breasted sunbird) of 10 Erica communities in South Africa. Sunbird visitation rates to the less‐preferred Erica species in communities increased with colour similarity to the most‐visited species, suggesting that polymorphisms can be maintained by selection for colour convergence within communities, which promotes shared signals through rewarding mimicry (i.e. facilitation). Colour similarity was lowest when risk of reproductive interference was greatest: that is, when reproductive morphology (stigma–anther distance) and flowering phenology was most similar, and when floral density differed greatly between species. This implies that polymorphisms can also be maintained by selection for colour divergence, since this promotes assortative pollinator foraging (i.e. competition). Different populations of a species may experience opposite selective pressures, depending on their community context. This is the first evidence that a geographic mosaic of plant–plant pollination interactions could maintain colour polymorphisms in congeneric species sharing a single pollinator. A free Plain Language Summary can be found within the Supporting Information of this article.

Effects of different interstock length on vegetative growth and flowering in peach cv. Pavie Catherine

Peach production needs appropriate management strategies to control canopy size, to increase tree density and yield, and to facilitate orchard operations. We evaluated compatibility, vegetative growth and flowering in peach trees cv. Pavie Catherine grafted on seven interstocks, at two interstock lengths (5 and 35 cm). Citation, Damas GF 1869, Hiawatha, Krymsk® 1, Monpol, MrS 2/5 and Myran were used as interstock. Rootstock for those interstocks was Monegro, which was used as a Control and also grafted on cv. Pavie Catherine but without interstock. The trial was carried out from 2017 to 2019 in Mendoza, Argentina, during the first three years of plantation establishment. The Hiawatha, Krymsk® 1 and Citation interstocks reduced in average 56% total shoot length, 33% shoot number per plant and 136% canopy volume compared to the Control. In contrast, interstocks Damas GF 1869, Monpol, MrS 2/5 and Myran showed similar vegetative growth to the Control. Among interstock treatments, the number and density of the flowers were 30% and 42% higher than the Control respectively. In the high grafted plants total shoot length, number of shoots and canopy volume decreased significantly, by 50%, 38% and 61% respectively, in relation to the low grafted ones. Reproductive characteristics followed a similar pattern, thus flowering shoot lengths were reduced in high than in low grafted plants and floral density increased in high grafted plants. In addition, bloom date was delayed in high grafted plants. Interstocks and interstock length influenced plant size significantly, providing an opportunity to increment tree density and improve canopy management.

Mapping bee flora in honey producing areas of the Alto Médio Canindé microregion in Piauí state, Brazil

The Brazilian state of Piauí is one of the largest producers of organic honey in the country. Although the product is made from nectar collected from natural vegetation, including several species endemic to the Caatinga, knowledge of the beekeeping potential of semiarid vegetation remains scarce, particularly in Piauí. As such, the present study aimed to map the flora of honey-producing areas of the Alto Médio microregion in Piauí. A survey was conducted of apiaries in the region and bee production in four beekeeping communities, using data provided by the Simplício Mendes Microregion Beekeeping Cooperative (COMAPI). Geographic coordinates were used to generate a map to analyze vegetation in the region and excursions were carried out in previously defined areas to survey plants visited by honeybees. The beekeeping communities in areas with greater floral density had the highest honey production. Forty plant species were collected, with the most representative family being Fabaceae, followed by Euphorbiaceae and Anacardiaceae. Most of the plants surveyed were visited by bees with the sole purpose of collecting nectar and were flowering in months with the highest honey production. Thus, creating a flowering calendar could help beekeepers make better use of the local flora.

Bark beetle outbreak enhances biodiversity and foraging habitat of native bees in alpine landscapes of the southern Rocky Mountains

Landscape-scale bark beetle outbreaks alter forest structure with direct and indirect effects on plants and animals in forest ecosystems. Using alpine spruce forest and a native bee community as a study system, we tested how tree mortality from bark beetles impacts bee foraging habitats and populations. Bees were collected across the growing season (early-, middle-, and late-season) for two years using passive trapping methods, and collections were used to analyze patterns in species abundances and diversity. Three important findings emerged: (1) forest stands that were post-outbreak had 62% higher floral density and 68% more floral species during peak bloom, respectively, than non-affected stands; (2) bee captures were highest early-season (June) and were not strongly affected by bark beetle outbreak; however, mean number of bee species and Shannon–Weiner diversity were significantly higher in post-outbreak stands and this effect was pronounced early in the growing season. Corresponding analysis of β-diversity indicated higher accumulation of bee biodiversity in post-outbreak stands and a turnover in the ratio of Bombus: Osmia; (3) bee captures were linked to variation in foraging habitat, but number of bee species and diversity were more strongly predicted by forest structure. Our results provide evidence of increased alpine bee biodiversity in post-outbreak stands and increased availability of floral resources. We conclude that large-scale disturbance from bark beetle outbreaks may drive shifts in pollinator community composition through cascading effects on floral resources, mediated via mortality of overstory trees.

Wild Bee Visitation Rates Exceed Pollination Thresholds in Commercial Cucurbita Agroecosystems.

Wild bees supply sufficient pollination in Cucurbita agroecosystems in certain settings; however, some growers continue to stock fields with managed pollinators due to uncertainties of temporal and spatial variation on pollination services supplied by wild bees. Here, we evaluate wild bee pollination activity in wholesale, commercial pumpkin fields over 3 yr. We identified 37 species of bees foraging in commercial pumpkin fields. Honey bees (Apis mellifera L. [Hymenoptera: Apidae]), squash bees (Eucera (Peponapis) Say, Dorchin [Hymenoptera: Apidae]), and bumble bees (Bombus spp., primarily B. impatiens Cresson [Hymenoptera: Apidae]) were the most active pollinator taxa, responsible for over 95% of all pollination visits. Preference for female flowers decreased as distance from field edge increased for several bee taxa. Visitation rates from one key pollinator was negatively affected by field size. Visitation rates for multiple taxa exhibited a curvilinear response as the growing season progressed and responded positively to increasing floral density. We synthesized existing literature to estimate minimum 'pollination thresholds' per taxa and determined that each of the most active pollinator taxa exceeded these thresholds independently. Under current conditions, renting honey bee hives may be superfluous in this system. These results can aid growers when executing pollination management strategies and further highlights the importance of monitoring and conserving wild pollinator populations.

Climate change and invasion may synergistically affect native plant reproduction.

Global change drivers can interact in synergistic ways, yet the interactive effect of global change drivers, such as climatic warming and species invasions, on plant pollination are poorly represented in experimental studies. We paired manipulative experiments to probe two mechanistic pathways through which plant invasion and warming may alter phenology and reproduction of native plant species. In the first, we tested how experimental warming (+1.7°C) modulated flowering phenology and how this affected flowering overlap between a native plant (Dracophyllum subulatum) and an invasive plant (Calluna vulgaris L.). In the second experiment, we explored how variation in the ratio of native to invasive flowers, and the overall quantity of resources in a floral patch, affected the reproduction of the native species. We hypothesized that the flowering overlap of native and invasive plants would be altered by warming, given that invading plants typically exhibit greater phenological plasticity than native plants. Further, we hypothesized that pollination of native plant flowers would decrease in floral patches dominated by invasive plant flowers, but that this effect would depend on total floral density in the patch. As predicted, the invasive plant had a stronger phenological response to experimental warming than the native plant, resulting in increased flowering overlap between the native the invasive plants. There was a four-fold increase in the number of native flowers co-flowering with high densities of invasive flowers suggesting native plant competition for pollinators with invasive plants under a warmed climate. In the second experiment, we found depressed seed masses of the native species in high density floral patches that were dominated by invasive flowers relative to high density floral patches dominated by native flowers. At low floral densities, seed mass of native plants was unaffected by invasion. Together, these results demonstrate that by increasing their phenological overlap, warming may enhance the magnitude of existing competition for pollination exerted by an invasive plant on a native plant, particularly in plant patches with high floral density. Our results illustrate a novel pathway through which global change drivers can operate synergistically to alter an important ecosystem service: pollination.

Comparing Insect Pollinator Visitation for Six Native Shrub Species and Their Cultivars

Interest in native landscape plants to support pollinators has increased. Most native plants sold by nurseries are cultivars, and some consumer and conservation groups question the suitability of native cultivars to support pollinators. In 2017 and 2018, insect pollinator visitation was quantified for six native shrub species and one or more cultivars of each species (Aronia melanocarpa, A. melanocarpa ‘UCONNAM012’ Ground Hog®, A. melanocarpa ‘UCONNAM165’ Low Scape Mound®, Clethra alnifolia, C. alnifolia ‘Hummingbird’, C. alnifolia ‘Ruby Spice’, Dasiphora fruticosa, D. fruticosa ‘Goldfinger’, D. fruticosa ‘Pink Beauty’, Hydrangea arborescens, H. arborescens ‘Annabelle’, Kalmia latifolia, K. latifolia ‘Sarah’, Physocarpus opulifolius, and P. opulifolius ‘Monlo’ Diabolo®). Insects were identified into 12 categories (Apis mellifera, Bombus spp., Andrenidae, Halictidae, Megachilidae, other bees, Lepidoptera, Syrphidae, other flies, wasps, Coleoptera, and other insects). The number of inflorescences and insect visitation was similar for C. alnifolia and its cultivars, and the compact cultivar Hummingbird had the greatest floral density. A. melanocarpa had more total visitors of Andrenidae than both of its compact cultivars because it was larger and produced more inflorescences. Compact Aronia cultivars and the straight species were mostly similar for Andrenidae visitation when compared on a per-inflorescence basis. D. fruticosa had more visitors of Bombus spp. and Megachilidae than both of its cultivars. These insects may have been less attracted to ‘Pink Beauty’ because of its pink flower color and ‘Goldfinger’ because of its wider flowers, which result from it being a tetraploid. H. arborescens ‘Annabelle’ had one-third the number of Bombus spp. visitors as H. arborescens because ‘Annabelle’ produces >50% fewer fertile florets. P. opulifolius ‘Monlo’ attracted more syrphids than P. opulifolius possibly because flowers contrasted more strongly with the reddish purple foliage of ‘Monlo’ than with the green foliage of the straight species. Insect visitation was similar for K. latifolia and K. latifolia ‘Sarah’. Based on this work, we determined that native shrub cultivars are not universally less or more attractive to pollinators and must be evaluated on a case-by-case basis.

Phenotypic variability in Cleome viscosa L. growing in Jammu region (J&K): some interesting insights in this prospective crop

The present investigation was undertaken on C. viscosa, a lesser known taxon of economic importance, for its morphological characterization. A continuum of morphological variation in this herbaceous weedy taxon was found across district Jammu. The plants differed in their average life span and phenological events (germination, vegetative, reproductive and senescence). A critical analysis further reflected colossal variation in their overall appearance like growth pattern, branching habit, leaf and floral density. Taking these features into consideration the plants were differentiated into four phenotypic classes/groups. Two groups included branched plants with dense foliage and greater flowering intensity while the other two categories clubbed medium to short statured unbranched plants with low intensity of flowering. These were accordingly named big branched, small branched, unbranched and small and abbreviated respectively as Bbm, Sbm, Ubm and Sm. Differences manifested in various agro-economical traits of vegetative and reproductive nature and in biomass allocation patterning. Data were quantified and values of diversity indices and co-efficients of variances calculated. Despite overlaps in few characters, significant variability was encountered in all the morphological traits among the four groups. Results of statistical analysis, PCV, GCV and broad sense heritability provide evidence in favour of this variation.

Taxonomic and functional diversity of the co‐flowering community differentially affect Cakile edentula pollination at different spatial scales

Abstract The number of co‐flowering species, floral density and floral trait diversity can be major determinants of pollinator‐mediated plant–plant interactions in a community. However, evaluation of how each one of these co‐flowering components affects the pollination success of a single focal plant species, and how these effects vary at different spatial scales, is lacking. Here, we evaluated the effects of functional diversity (flower morphology and colour), taxonomic diversity (reflecting potential sampling effects) and flower density (conspecific and heterospecific), on the pollinator environment (i.e. visitation rate and pollinator diversity) and pollination success (i.e. pollen load size and number of pollen tubes per style) of Cakile edentula (Brassicaceae). We applied structural equation models (SEMs) at the floral‐neighbourhood (plot level) and community‐wide scales to uncover the factors that mediate co‐flowering community effects on C. edentula pollination success. We found that co‐flowering community effects at the community level are more important than fine‐scale floral‐neighbourhood differences in mediating plant pollination success in our study species. Increasing plant functional diversity decreased pollinator visitation rate but increased the diversity of pollinator functional groups visiting C. edentula flowers. Taxonomic diversity negatively affected pollinator diversity suggesting that other unmeasured floral traits may be relevant or that single‐species effects (sampling effects) may be important. Overall, our results suggest that functional floral trait diversity in a community may be the most important factor influencing pollination success of species in a community. We also found evidence for intra‐ and interspecific pollinator competition mediated by flower density, but none of these effects seemed to have a significant impact on pollination success. This study is an important step towards understanding the complexity of co‐flowering community effects on the pollination success of individual plant species at multiple spatial scales. This study further reveals the potential importance of plant functional diversity in a community in helping predict competitive and facilitative interactions in co‐flowering communities. Synthesis. Floral density and taxonomic and functional co‐flowering diversity are important drivers of pollination success in flowering plants. The effects of the co‐flowering diversity on the pollination success of plant species can largely depend on the spatial scale being studied. Only evaluating the outcomes of pollinator‐mediated plant–plant interactions at multiple stages of the pollination process can lead to a complete understanding of their ecological consequences in nature.

The effects of post-wildfire salvage logging on plant reproductive success and pollination in Symphoricarpos albus, a fire-tolerant shrub

Post-wildfire salvage logging is an increasingly used land management tool with poorly understood ecological consequences for understory flowering plants and their interactions with pollinators. Understanding these consequences of salvage logging is important because an essential aspect of post-wildfire forest succession involves pollination and plant reproduction. For snowberry (Symphoricarpos albus), an ecologically-important and fire-tolerant shrub, we tested the long-term effects of post-wildfire logging on plant reproduction and pollen limitation using a supplemental-pollen experiment coupled with pollinator observations of S. albus and of potential co-flowering competitors of S. albus. Nearly a quarter century after these disturbances, we found no effects of post-wildfire logging on the reproduction of naturally-pollinated plants. The reproduction of some S. albus individuals were pollen limited, but only in unlogged areas, suggesting that plants in unlogged areas have higher potential reproduction compared to those logged areas but are unable to achieve this higher level of reproduction due to lack of pollination. This pollen limitation of S. albus reproduction is consistent with the relatively high floral densities of potential competitors of S. albus and generally low pollinator visitation rates in unlogged areas. Together, these results suggest that legacies of post-wildfire logging may restrict the reproductive potential of this shrub for at least several decades after the logging is complete, but this restriction is likely due to altered abiotic conditions and not via lack of pollination.

The influence of floral morph ratios and low plant density on mating and fertility in a tristylous colonizing species

Sexual reproduction in heterostylous populations may be vulnerable to demographic conditions because of the small number of mating types in populations. Here, we investigate mating and fertility under natural and experimental conditions in tristylous Lythrum salicaria L., an invasive species that exhibits a wide range of floral morph ratios and demographic contexts. We grew 147 open-pollinated seed families from six populations with different morph structures to estimate intermorph mating (d). In a field experiment, we used progeny ratios from 47 spatially isolated individuals to estimate d, and measured the intensity of pollen limitation experienced by the morphs. The M- and S-morphs experienced high rates of d, regardless of population size or morph ratio. Estimates for the L-morph revealed low levels of intramorph mating in three dimorphic and two trimorphic populations, but near complete intramorph mating in a monomorphic population. Despite high levels of intermorph mating in the field experiment, the morphs experienced significant pollen limitation of fruit and seed set, but this did not differ in intensity among the morphs. Our field experiment demonstrates that although plant isolation was associated with pollen limitation of seed set, “long-distance” bee-mediated pollen flow served to maintain intermorph mating. Tristyly in L. salicaria is remarkably robust to the demographic variation associated with colonization.

Floral usage partitioning and competition between social (Apis mellifera, Bombus terrestris) and solitary bees in New Zealand: Niche partitioning via floral preferences?

AbstractWorldwide, studies of interactions between introduced and native bees have produced contradictory evidence regarding the potential for competition. Different resource requirements and the impact of tongue length on resource use are often overlooked aspects of these studies. Here, we examine floral resource use and niche overlap between introduced social bees and native solitary bees over two flowering seasons on The Remarkables mountain range, South Island, New Zealand. The native bee fauna of New Zealand is composed of short‐tongued solitary bees in the families Colletidae and Halictidae. Long‐tongued social bees were introduced 150 years ago, and Apis mellifera and Bombus species are now widespread. We analysed floral resource utilization by introduced bees (mainly Bombus terrestris) and native bees in relation to resource abundance across elevations in montane to alpine grassland communities. We modelled resource utilization and overlap between bee taxa using a novel index of floral resource availability, developed to more accurately describe floral density. Additionally, we sought to quantify the impact of honey bees by introducing hives, however, low densities in study plots prevented direct conclusions from being drawn. Native and introduced bees did, however, have distinctly different floral preferences over the flowering season with more niche overlap within, rather than between, native and introduced groups. Introduced bees showed a preference for introduced species in the family Fabaceae which native bees could not access. Native bees heavily utilized introduced Asteraceae as well as native floral resources that introduced bees did not prefer. The limited resource overlap between bee taxa indicated that bee communities in New Zealand may have reduced potential for competition, owing to the different resource requirements of solitary versus social bees and their differing abilities to access floral resources. These findings provide some potential explanations for contradictory evidence worldwide regarding the perceived detrimental impacts of introduced social bees.

Floral preferences of hoverflies (Diptera: Syrphidae) in response to the abundance and species richness of flowering plants

For understanding the relationship between floral resources and the abundance of hoverflies, five meadows were selected at different elevations in a forested region. At each site the number of flowering herbaceous plants was assessed along a transect at 10-m intervals. To measure floral density, we counted the number of open flowers per species in each plot. Hoverflies visiting flowering plants were captured in each plot during 10 minutes. Path analysis showed that there was a negative correlation between plant species richness and syrphid abundance. Flower abundance had a strong positive direct effect on syrphid abundance but had indirect effects on the number of syrphid species. Only altitude had a strong positive direct effect on the number of syrphid species and this may be explained by the different climatic conditions. Some plant species attracted more syrphids than others. There were significantly fewer species of hoverfly per plot with increasing numbers of flowers. We interpret this to mean that plant species with a high density of flowers attracted more syrphids of the same species. Analysis of pollen grains extracted from two species of hoverflies showed that flower constancy occurred in three examined sites.