Floral Density Research Articles

Effects of an agri-environment scheme on bumblebee reproduction at local and landscape scales

Agri-environment schemes (AES) have been implemented across Europe, aiming to mitigate effects of habitat loss in agro-ecosystems for a range of declining species. These include pollinating insects such as bumblebees, for which positive effects of AES on abundance and species richness have been shown. However, there is a lack of evidence for effects of AES on reproduction of target species, at either local or landscape scales. We conducted a large-scale study across landscapes exhibiting a gradient of agricultural intensity to investigate the effects of a targeted flower mixture, sown in patches of three different sizes, on an index of the total biomass of bumblebee sexuals (males and queens) on replicated transects within each landscape. We used this index (MQ) as a measure of bumblebee reproduction. After controlling for floral density on transects, we found that MQ was significantly higher on sown flower patches than on conventionally managed control patches at local scales throughout the three-year study. While sown flower patches did not significantly increase MQ in surrounding landscapes, MQ was higher in landscapes surrounding larger (1ha) than smaller (0.25ha) sown patches. Our results suggest that, while responses of different bee species may vary depending on the plant species sown, targeted flower mixtures can enhance bumblebee reproduction by providing locally attractive forage resources to bumblebees of all castes and sexes from nests within foraging distance. If established at large enough scales, sown flower patches may lead to a detectable spill-over of reproductives into surrounding landscapes. Furthermore, effects of sown patches on MQ were moderated by landscape context, the strongest positive responses being detected at sites with high proportions of arable land. This supports previous findings that AES can deliver greater net benefits for pollinators in more intensively farmed landscapes.

Species richness of wild bees, but not the use of managed honeybees, increases fruit set of a pollinator‐dependent crop

Summary Native, wild bees are important pollinators for both crop and wild plants. With concerns over the availability and cost of managed honeybees, attention has turned to native, wild bees as crop pollinators. However, the ability of native, wild bees to provide sufficient pollination may depend on their populations at local scales. Therefore, at the farm scale, we examined the pollination contribution of both native, wild bees and managed honeybees to apples and assessed the relative importance of bee abundance vs. species richness. Over three growing seasons, apple fruit set, bee abundance and bee species richness were measured at orchards in Wisconsin, half of which used managed honeybees, thus allowing us to independently examine the contribution of native, wild bees to fruit set. We additionally conducted observations of honeybees and wild bees foraging on apple blossoms in order to examine functional complementarity. We found that apples are highly dependent on animal pollinators. However, fruit set was not significantly higher at orchards with managed honeybees, nor did it increase with the number of honeybees per orchard. Instead, fruit set significantly increased with the species richness of native, wild bees during bloom. Honeybees and wild bees showed different foraging preferences: honeybees more frequently visited apple flowers on densely blooming trees, while wild bees showed no preference for floral density, thereby evenly visiting trees throughout the orchard. Synthesis and applications. Our results show that native, wild bees play a significant and unique role in apple pollination within our region and cannot therefore be replaced by managed bees. Moreover, our findings suggest that bee conservation efforts should focus specifically on maintaining or increasing bee species richness in order to improve pollination and crop yields.

The association between nectar availability and nectarivore density in urban and natural environments

In some Australian and South American cities, nectarivorous birds have become a conspicuous component of the avifaunal community. One mechanism that has been proposed to explain their success is an increased availability of nectar that is provided by suburban gardens and other ornamental plantings. To determine whether the amount of nectar within the suburban landscape is comparable to that of the natural environment, we measured floral density and nectar concentrations in 24 sites in Sydney, Australia. We also measured the density of four species of nectarivores and available nectar energy over a period of 18 months within the suburban landscape (streetscapes and remnant vegetation) and the natural environment (forest and heathland) to determine whether nectar availability explains nectarivore density. Two species of nectarivorous parrot, the rainbow lorikeet and musk lorikeet were present in higher densities within the suburban landscapes compared to the natural environment, and the suburban landscapes provided significantly more nectar than the natural environment during spring and winter. Both the rainbow lorikeet and musk lorikeet were associated with flowering of Eucalyptus spp. and red wattlebirds with the flowering of Grevillea and Callistemon spp. within streetscapes. Suburban landscapes appear to provide a constant supply of nectar for large-bodied nectarivores and provide for more efficient foraging, explaining the success of these species in urban environments.

Neighborhood‐contingent indirect interactions between native and exotic plants: multiple shared pollinators mediate reproductive success during invasions

Native and exotic plants can influence one another's fecundity through their influence on shared pollinators. Specifically, invasion may alter abundance and composition of local floral resources, affecting pollinator visitation and ultimately causing seedset of natives in more‐invaded and less‐invaded floral neighborhoods to differ. Such pollinator‐mediated effects of exotic plants on natives are common, but native and exotic plants often share multiple pollinators, which may differ in their responses to altered floral neighborhoods. We quantified pollinator‐mediated interactions between three common forbs of western Washington prairies (native Microseris laciniata and Eriophyllum lanatum and European Hypochaeris radicata) in three floral neighborhoods: 1) high native and low exotic floral density, 2) high exotic floral density and low native density, and 3) experimentally manipulated low exotic floral density. Pollinator visitation rates varied by floral neighborhood, plant species identity, and their interaction for all three plant species. Similarly, pollinator functional groups (eusocial bees, solitary bees, and syrphid flies) contributed differing proportions of total visitation to each species depending upon neighborhood context. Consequently, in exotic neighborhoods H. radicata competed with native M. laciniata, reducing seed set, while simultaneously facilitating visitation and seed set for native E. lanatum. Seed set of H. radicata was also highest in exotic neighborhoods (with high densities of conspecifics), raising the possibility of a positive feedback between exotic abundance and success. Our results suggest that the outcome of indirect interactions between native and exotic plants depends on the density and the composition of the floral neighborhood and of the pollinator fauna, and on context‐dependent pollinator foraging.

Floral neighbourhood effects on pollination success in red clover are scale‐dependent

Summary Plant‐pollinator interactions are highly important because of its direct link to plant fitness and because such interactions involve species at different trophic levels. Our understanding of competitive and facilitative interactions among plants for pollination is of large importance to govern conservation of species and management of crop production. Still we know little about the scale dependency of plant–plant interactions for pollinator visitation which is central to both basic and applied ecology. Here, I tested whether plant–plant interactions for bumblebee visitation and subsequent reproduction were scale‐dependent by studying the ecologically and economically important red clover (Trifolium pratense) and its floral neighbourhood. I expected that high floral densities at small scale could trigger facilitative interactions due to concentration effects of pollinators. This was expected to change into competitive interactions when floral densities became higher at coarser scales because of dilution effects. I also expected that increased bumblebee visitation would increase fruit production in red clover. The interaction for pollination success in red clover appeared to be scale‐dependent in the landscape under study. At plot scale, the bumblebee visitation rate to red clover inflorescences increased with both intra‐ and interspecific floral density, indicating facilitative interaction for pollinator visitation among red clover flowers and its nearest floral neighbourhood. Increased visitation of bumblebees led to subsequent increased fruit set, but there was a saturating effect as fruit set did not increase when visitation rates exceeded a certain level. As the floral neighbourhood became denser at a coarser scale, the interactions for pollinator visitation changed towards competition. This study shows that key ecological processes such as plant–pollinator interactions may indeed be scale‐dependent and that the relative importance of competition and facilitation among plants for attracting pollinators may change with scale. The density of the floral neighbourhood of a focal species (e.g. key‐, rare‐, crop species) may thus be of large importance for its pollination and fruit production. My findings suggest that increasing floral densities at fine scales may have facilitative effects on pollinator visitation to red clover, whereas pollinators may be drawn away when floral resources become denser at coarser scale.

Relative floral density of an invasive plant affects pollinator foraging behaviour on a native plant

Interactions between invasive and native plants for pollinators vary from competition to facilitation of pollination of native plants. Theory predicts that relative floral densities should account for some of this variation in outcomes, with facilitation at low floral densities and competition at high floral densities of the invader. We tested this prediction by quantifying pollination and female reproductive success of a native herb, Geranium maculatum, in three experimental arrays that varied in floral density of the invasive shrub Lonicera maackii: control (no L. maackii), low floral density of L. maackii, and high floral density of L. maackii. A low density of L. maackii flowers was associated with an increase in pollinator visitation rate to G. maculatum flowers and an increase in conspecific pollen deposition compared to controls and high density arrays. Increased visitation rates were not associated with an increase in the number of visitors to low density arrays, suggesting instead that a behavioural switch in visitation within the array accounted for increased pollen deposition. In contrast, the only evidence of competition in high density arrays was a shorter duration of visits to G. maculatum flowers relative to the other treatments. The number of seeds per flower did not vary among treatments, although trends in seeds per flower were consistent with patterns of pollinator foraging behaviour. Given increased pollinator visits and pollen deposition at a low density of the invader, our study indicates that complete eradication of invasives as a management or restoration technique may have unintended negative consequences for pollination of native plants.

Does competition for pollinators contribute to structuring Erica communities?

AbstractQuestionPollinators are often implicated in the origin of plant species, but their role during the assembly of diverse plant communities has only rarely been investigated. Intra‐specific competition for pollinators can theoretically facilitate plant species co‐occurrence by limiting the density of any one species, while inter‐specific competition can potentially structure communities through the exclusion of inferior competitors. We analyse plant community composition and use field experiments to test whether intra‐ and inter‐specific competition for pollinators structures communities of co‐occurring Erica species.LocationCape Floral Region, southwestern South Africa (ca. 34° S, 18° E).Methods and ResultsIn the study area, the genus Erica has diversified into ca. 680 species that differ primarily in their floral morphologies, reflecting insect, bird and wind pollination syndromes. Vegetation plot data from nine regions throughout the Fynbos Biome were used to test whether the frequency of co‐occurrence of species with the same pollination syndrome differs from a null model of random community assembly. We found that Erica communities were assembled randomly with respect to pollination in six regions, but that pollination syndrome co‐occurrence was significantly lower than expected in three regions. This over‐dispersion of syndromes across communities is consistent with communities being structured by inter‐specific competition for pollinators. To better understand the processes generating these patterns, we conducted competition experiments with six Erica species that share the Orange‐breasted sunbird (Anthobaphes violacea) as pollinator. We transplanted inflorescences of a focal Erica species into communities containing another resident Orange‐breasted sunbird bird‐pollinated Erica species and recorded pollination and nectar robbing rates in both the resident and the transplanted species in relation to the floral density of the resident species. Consistent with the observed community structure, pollination rate in both the resident and the introduced species declined with increasing density of the resident species, but the effect was not consistent among all populations. Nectar robbing rates of the resident species were negatively related to its own density.ConclusionOur analyses concur that both intra‐ and inter‐specific competition for pollination can contribute to structuring plant communities, but it is not the dominant factor structuring Erica communities.

Miscanthus Clones Display Large Variation in Floral Biology and Different Environmental Sensitivities Useful for Breeding

A wider range of Miscanthus varieties is required to develop Miscanthus clones that are suitable for bioenergy production. For this reason, breeding programs need to be initiated using knowledge regarding the genetic influence on floral biological traits. The objective of the present study was to characterize the genotypic variation in flowering and panicle architecture traits in Miscanthus by studying (i) the clone effect on these traits and (ii) the clone sensitivity to environmental conditions. The flowering traits characterized were date of panicle emergence, date of flowering onset, and interval between these two traits. The panicle architecture traits characterized were total panicle length, longest panicle raceme size, raceme number per panicle, floral density, and total flower number per panicle. Eight clones were studied in a greenhouse under four environmental conditions including two day lengths (an 8-h short day length and a natural day length) and two temperature treatments (warm and cool). Miscanthus clones showed large differences in flowering and panicle architecture traits. Moreover, day length appeared to be the most important environmental factor creating differential clone sensitivities for the panicle emergence and the onset of flowering in contrast to temperature factor for the total flower number per panicle. In addition, the behavior of the clone Sacc was in contrast with that of the other clones for most of the traits studied. This knowledge will be useful to optimize the synchronization of flowering between Miscanthus clones for more successful breeding programs.

Ecological factors affecting the fruiting success of a Tibouchina trichopoda (DC.) Baill. (Melastomataceae) flower

The abundance and distribution of resources for pollinators modulate the reproduction of plants. This study attempted to determine whether and how the fruiting success of Tibouchina trichopoda flowers varies due to plant size, number of flowers produced, distance between individuals and cardinal orientation of the flowers. From July 2009 to February 2010, we conducted monthly evaluations of 21 plants in an area of regenerating forest within the Atlantic Forest biome, located near the municipality of Antonina, in southern Brazil. Floral density was monitored weekly. During the study period, the species bloomed twice: once during the cool, dry season, showing an extended flowering period with scattered flowering individuals; and once during the hot, rainy season, showing a brief flowering period with a high density of flowering individuals. Plant size was not found to influence fruiting success. There was greater production of flowers and fruits during the brief flowering period. The extent of the floral display positively affected the fruiting success of the flowers, the effect being greater when the density of flowers was low and tending to stabilize at higher densities. The northern-facing portions of tree crowns produced more flowers and therefore had greater fruiting success than did the southern-facing portions. However, the proportion of fruits produced per flower was the same.

Explaining the Effects of Floral Density on Flower Visitor Species Composition

Floral density often influences the species composition of flower visitors. This variation in visitor species composition could have significant effects on pollination success and plant fitness but is poorly understood, especially in the many pollination guilds dominated by nonterritorial species. This article presents a foraging model that explores how flower visitors with diverse traits should distribute themselves across resource patches differing in floral density. The model predicts that species with low flower search speeds and low flower handling costs compared to those of competitors will usually dominate dense flower patches. In addition, among flower visitors that have lower energy expenditure rates while handling flowers than while traveling, species maximizing energetic efficiency are typically associated with dense flower patches, whereas those maximizing net rate of energy intake are associated with sparse patches. The model is able to predict some key aspects of a previously observed effect of floral density on species composition of flower visitors to the yellowflower tarweed (Holocarpha virgata). By providing insights into how flower visitors' traits shape the effects of floral density on the species composition of flower visitors, this study makes an important step towards understanding how pollinator diversity influences relationships between plant density and plant fitness.

Negative Effects of Conspecific Floral Density on Fruit Set of Two Neotropical Understory Plants

AbstractPlant reproductive success is usually positively related to conspecific floral density, but neutral or negative effects of floral density on reproduction have also been reported. Differences in the relationship between reproduction and floral density largely originate from a trade‐off between increasing attractiveness versus increasing competition for pollinators at high floral densities. Although floral densities strongly vary in the understory of tropical forests, for instance, due to variation in light availability, little is known about the density dependence of reproduction in tropical understory plants. We used path analyses to disentangle direct and indirect effects of canopy openness and floral density on fruit set and analyzed the relationship between pollen load and floral density for two Neotropical understory plants, Heliconia metallica and Besleria melancholica. In both species, fruit set was not directly related to canopy openness, but decreased with increasing floral density. In H. metallica, canopy openness had an indirect negative effect on reproduction mediated by its effects on floral density. Effects of floral density on pollen loads were species‐specific. In B. melancholica, pollen loads linearly decreased with increasing floral density, indicating competition for pollinators at high densities. In H. metallica, pollen loads were reduced at both low and high densities, indicating an interplay of facilitative and competitive effects of floral density on pollen deposition. In contrast to other studies, we found negative density dependence of reproduction in both understory species. Negative effects of floral density on reproduction appear to be related to pollinator‐mediated effects on reproduction rather than to variation in abiotic conditions.

Evaluating pollen flow indicators for an insect-pollinated plant species

For insect-pollinated plant species, reproductive success and genetic exchange via the transfer of pollen between flowers depends (i.a.) on the efficiency, abundance and behaviour of floral visitors. These in turn are expected to respond to plant population size and flower density. High floral densities for example usually attract large numbers of pollinators that visit more flowers per plant or patch, which increases pollen deposition at short distances. Thus, population characteristics might serve as indicators for pollen dispersal patterns and help to identify suitable habitat size and quality for conservation measures. To test this hypothesis, we observed floral visitors of a generalist, entomophilous species, Comarum palustre, and compared their abundance and visitation rates in populations of different sizes and flower densities. At the same time, we mimicked pollen flow using fluorescent dye. In the large and dense populations, pollinator abundance and visitation rates were high and dye was dispersed to the edges of the populations (up to 200m). In the medium-sized population with high flower density, insect abundance and visitation rates were unexpectedly low and dye dispersal declined very quickly. On the contrary, in the smallest population with scattered flowers, especially bumble bee abundance was similar to the large populations and dye dispersal mirrored this high bumble bee activity. Thus, our results indicate that in smaller habitat fragments, the mere size of a population might be insufficient to suggest pollen flow for a plant species. Instead, the abundance of its major pollinators should be considered.

Scale-dependent shifts in the species composition of flower visitors with changing floral density

Responses of flower-visiting animals to floral density can alter interactions between plants, influencing a variety of biological processes, including plant population dynamics and the evolution of flowering phenology. Many studies have found effects of floral or plant density on pollinator visitation rates at patch scales, but little is known about responses of flower visitors to floral densities at larger scales. Here, I present data from an observational field study in which I measured the effects of floral density on visitation to the annual composite Holocarpha virgata at both patch (4 m(2)) and site (12.6 ha) spatial scales. The species composition of flower visitors changed with floral density, and did so in different ways at the two scales. At the site scale, average floral density within patches of H. virgata or within patches of all summer-flowering species combined had a significant positive effect on per-flowerhead visitation by the long-horned bee Melissodes lupina and no significant effects on visitation by any other taxa. At the patch scale, per-flowerhead visitation by honeybees significantly increased whereas visitation by M. lupina often decreased with increasing floral density. For both species, responses to patch-scale floral density were strongest when site-scale floral density was high. The scale-dependence of flower visitor responses to floral density and the interactions between site- and patch-scale effects of floral density observed in this study underscore the importance of improving our understanding of pollinators' responses to floral density at population scales.

Explaining Variation in the Effect of Floral Density on Pollinator Visitation

Pollinator responses to floral density have important implications for plant biology. In particular, a decline in pollinator visitation at low density can cause an Allee effect (a positive relation of fitness to density) in the plant population, which heightens that population's vulnerability to extinction. Empiricists have reported a variety of relations between flower or plant density and pollinator visitation rates. Here I develop and test a model that provides explanations for this diversity. The model assumes that pollinators distribute themselves between a focal patch of flowers and the surrounding environment so as to maximize foraging success. The resulting relation of per-flower visitation rate to focal-patch floral density is nonlinear, with positive effects at low floral densities and weaker or negative effects at higher densities. The relation is influenced by floral density in the surrounding environment and traits of both the plants and their pollinators. In a field experiment, floral density of Holocarpha virgata ssp. virgata had a nonlinear effect on per-flower visitation that was largely consistent with the model's predictions. By producing testable hypotheses based on biologically reasonable assumptions, this model serves as a starting point for explaining an important facet of plant-pollinator mutualisms.

Effects of an invasive plant transcend ecosystem boundaries through a dragonfly-mediated trophic pathway

Trophic interactions can strongly influence the structure and function of terrestrial and aquatic communities through top-down and bottom-up processes. Species with life stages in both terrestrial and aquatic systems may be particularly likely to link the effects of trophic interactions across ecosystem boundaries. Using experimental wetlands planted with purple loosestrife (Lythrum salicaria), we tested the degree to which the bottom-up effects of floral density of this invasive plant could trigger a chain of interactions, changing the behavior of terrestrial flying insect prey and predators and ultimately cascading through top-down interactions to alter lower trophic levels in the aquatic community. The results of our experiment support the linkage of terrestrial and aquatic food webs through this hypothesized pathway, with high loosestrife floral density treatments attracting high levels of visiting insect pollinators and predatory adult dragonflies. High floral densities were also associated with increased adult dragonfly oviposition and subsequently high larval dragonfly abundance in the aquatic community. Finally, high-flower treatments were coupled with changes in zooplankton species richness and shifts in the composition of zooplankton communities. Through changes in animal behavior and trophic interactions in terrestrial and aquatic systems, this work illustrates the broad and potentially cryptic effects of invasive species, and provides additional compelling motivation for ecologists to conduct investigations that cross traditional ecosystem boundaries.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-012-2357-1) contains supplementary material, which is available to authorized users.

Reproductive biology of a threatened Australian saltmarsh plant –Wilsonia backhousei

The reproductive biology of a threatened saltmarsh plant, Wilsonia backhousei Hook.f., was investigated with a view to improving conservation and restoration outcomes for the species. Population phenology was studied every two weeks, over two consecutive flowering seasons, in three to six 0.25m2 quadrats set in monocultures of the species at each of eight sites in New South Wales, Australia. Floral density (flowersm−2) ranged from 0 to 5800±1400m−2 and varied significantly among sites (P<0.05). Peak flowering occurred in mid-late October, during a period of no tidal inundation; fruit maturation coincided with inundation by extreme high tides in December, January and February. Sediment samples collected from each quadrat post-flowering were analysed for water content, salinity, texture (clay content) and pH. Ordinal logistic regression (OLR) of floral density against foliage volume and sediment variables showed a significant positive response to increasing foliage volume and negative responses to increasing salinity and clay content (P<0.05). Foliage volume, in turn, showed a significant positive response to sediment water content and again negative responses to increasing salinity and clay content (P<0.05). Fruit development occurred at all sites; however, little to no seed was produced at five of the sites. When quadrats not producing seed were excluded (to limit the influence of self-incompatibility), OLR of seed yield against floral density and sediment variables indicated sediment water content was the only significant predictor of yield (P=0.015). The morphology and phenology of individual flowers, potential pollination vectors, and general seed biology were also investigated. A combination of floral characteristics indicated that W. backhousei is wind-pollinated, likely to be outbreeding, and may be self-incompatible. The fruit is buoyant and, as maturation coincides with inundation by extreme high tides, may be dispersed within and among sites by tidal flow. The seed was found to be viable and long-lived (>70% germination in both fresh seed and seed aged for 6 years); however, seed germination is limited by physical dormancy and is significantly reduced by salinity levels ≥40dSm−1 (6±2.4% after 28 days, P<0.001). Based on these outcomes, recommendations are made for maximising the reproductive potential of remnant and restored populations.

The invasive Lespedeza cuneata attracts more insect pollinators than native congeners in tallgrass prairie with variable impacts

Invasive plant species can potentially exert competitive or facilitative effects on insect pollination services of native species. Factors that influence these effects include the degree of shared pollinator species, synchronous flowering phenology, similar flower morphology and color, relatedness of invasive and natives, and showiness and densities of flowers. We investigated such plant-pollinator dynamics by comparing the invasive Lespedeza cuneata and three native congeners, all sympatric with synchronous flowering, using in situ populations over 2 years during peak floral displays. Insect visitation rates of the invasive were significantly higher per plant in both years than on the native species. The invasive exerted a competitive effect on visitation of the two native species with fewer shared pollinators, and a facilitative effect on visitation of the native species with the highest degree of shared insect visitors. Positive correlations were found between floral density and visitation rate per plant in all the native species. Although no such correlation was found for the invasive, floral density in L. cuneata was at least twenty times higher than in the native species and likely saturated the response of the pollinator community. Analyses of insect visitor taxonomic data indicated the insect communities visiting each of the Lespedeza species were generally similar though with species-specific differences. The main exception was that the common honeybee, Apis mellifera, was a primary visitor to the invasive plant species, yet was never observed on the native Lespedeza species.

Plant–pollinator interactions within the urban environment

Urban ecosystems pose different environmental constraints on plant and animal communities than natural ecosystems, and this in turn might affect ecological interactions. We investigated plant–pollinator interactions in urban vegetation communities in the context of local community structure and two landscape metrics describing the surrounding urban matrix. We recorded plant species diversity, floral density, pollinator species and floral visits at 89 flowering patches within the urban matrix, and used Trifolium pratense as a focal plant species. We correlated visits by all species, by bees, by bees when excluding the main visitor Bombus pascuorum, and by the main visitor alone T. pratense with the landscape metrics “green area”, representing the total extent of vegetated areas, and “edge density of green areas”, representing the degree of habitat fragmentation, at scales from 20 to 200 m. Extent of green area was positively correlated with visits by all species, by bees, and by Bombus pascuorum, but negatively with bees excluding the main visitor. The correlation increased with scale for all four variables, and the strongest correlations were found for B. pascuorum. The direction of correlation between edge density and the visitation variables was similar as for green area, but showed considerable variation at scales below 100 m radius. Visits by all bees and all visitors to T. pratense were negatively affected by flowering plant species diversity. There was a positive interaction between extent of green area and plant diversity on bee visitation, and a negative interaction between the extent of green area and heterospecific flowers for bees (excluding B. pascuorum). Increasing floral abundance and plant diversity reduced pollinator visitation to T. pratense, suggesting competitive effects through both quantity and diversity of resources. Competition for flower visitors was mainly from Lotus corniculatus and Trifolium repens, both of which frequently co-occurred with T. pratense. We conclude that T. pratense competes for pollinators with other co-occurring plants, but that the nature of the surrounding urban matrix mediates interactions among plant species for pollinator services at different scales, particularly with respect to the proportion of green areas.

Effects of conspecific and heterospecific floral density on the pollination of two related rewarding orchids

Variation in within-population floral density can affect interactions between plants and pollinators, resulting in variable pollen export for plants. We investigated the effects of conspecific and heterospecific floral densities on pollination success both of two related, self-compatible, nectar-rewarding orchid species in Ireland, Spiranthesromanzoffiana (rare and listed as endangered) and its congener, S. spiralis (more abundant and not of conservation concern). Floral densities, insect visitation rates, and orchid pollen transport were recorded in multiple quadrats in four populations of both orchid species over their flowering season. We found that conspecific and heterospecific co-flowering plant density affected pollination in both orchid species. For S. romanzoffiana, higher heterospecific density increased pollen removal. For S. spiralis, higher conspecific visitation increased pollen removal and increased heterospecific density decreased pollen deposition. In addition, increased conspecific density increased pollen deposition in both species. This study shows that plants may interact to facilitate or compete for different components of the pollination process, namely; pollinator attraction, pollen removal and deposition. Such interactions have immediate consequences for endangered plant species, as increases in both conspecific and heterospecific coflowering density may ameliorate the negative effects of rarity on pollination, hence overall reproductive success.

The Effect of Urbanization on Bumble Bee Communities in Greater Philadelphia

AbstractBumble bees (Bombus spp.) are among the most important wild pollinators in temperate ecosystems in North America and Europe, and are believed to be vital to the functioning of the pollination networks in which they occur. Accordingly, evidence of their overall decline in Europe and more selective decline in the U.S. has raised concern about the long term persistence of many species. Human-induced changes in land use, including the loss of natural and semi-natural habitat and associated floral resources, are purported causes in several cases. Declines prompted an investigation of this potential trend in the urbanized landscape in and surrounding Philadelphia, PA. We surveyed the species richness and abundance of bumble bee communities in ten half-ha plots located in restored, managed meadows along a gradient of urban and suburban development in the Philadelphia metropolitan area from June 1 to August 15, 2006. In conjunction with collections, we measured floral density within each plot. We calculated the proportion of developed land at differing spatial scales ranging from 500 to 4000 m from survey sites. General linear models were used to test the effect of developed land, local meadow size, and floral resource density on overall bumble bee species richness and abundance.Development did not have any detectable effect on species richness at any tested spatial scale. Bee abundance was best explained by a model that included the proportion of developed land at the 2500 m scale. In contrast to our expectations, total bumble bee abundance was significantly higher in plots with a higher proportion of developed land surrounding the site (F1,5 = 8.13, P = 0.04). Local floral density did not significantly affect richness or abundance (F 1,5 = 0.93, P = 0.34), nor was local resource quality associated with development (r = 0.52, P = 0.12) . We hypothesize that gardens maintained in urban and suburban landscapes may provide an important and consistent food supply to bumble bees throughout the duration of their colony cycle. Forested habitats, which were the other dominate habitat type, may actually be relatively poor in bumble bee resources following closure of the canopy in late spring. This research indicates that restored meadows are excellent habitat for bumble bees and may promote pollination services in urbanized settings.