Functional diversity of plant-pollinator interaction webs enhances the persistence of plant communities

Pollination is exclusively or mainly animal mediated for 70% to 90% of angiosperm species. Thus, pollinators provide an essential ecosystem service to humankind. However, the impact of human-induced biodiversity loss on the functioning of plant–pollinator interactions has not been tested experimentally. To understand how plant communities respond to diversity changes in their pollinating fauna, we manipulated the functional diversity of both plants and pollinators under natural conditions. Increasing the functional diversity of both plants and pollinators led to the recruitment of more diverse plant communities. After two years the plant communities pollinated by the most functionally diverse pollinator assemblage contained about 50% more plant species than did plant communities pollinated by less-diverse pollinator assemblages. Moreover, the positive effect of functional diversity was explained by a complementarity between functional groups of pollinators and plants. Thus, the functional diversity of pollination networks may be critical to ecosystem sustainability.     

Functional Diversity of Plant–Pollinator Interaction Webs Enhances the Persistence of Plant Communities

Pollination is exclusively or mainly animal mediated for 70% to 90% of angiosperm species. Thus, pollinators provide an essential ecosystem service to humankind. However, the impact of human-induced biodiversity loss on the functioning of plant–pollinator interactions has not been tested experimentally. To understand how plant communities respond to diversity changes in their pollinating fauna, we manipulated the functional diversity of both plants and pollinators under natural conditions. Increasing the functional diversity of both plants and pollinators led to the recruitment of more diverse plant communities. After two years the plant communities pollinated by the most functionally diverse pollinator assemblage contained about 50% more plant species than did plant communities pollinated by less-diverse pollinator assemblages. Moreover, the positive effect of functional diversity was explained by a complementarity between functional groups of pollinators and plants. Thus, the functional diversity of pollination networks may be critical to ecosystem sustainability.     

The evolution of pollen heteromorphism inViola: A phylogenetic approach

Pollen heteromorphism, defined here as the production within all flowers of a plant of several pollen morphs differing in aperture number, is common in angiosperms. We have focused on the evolution of pollen heteromorphism in the genusViola, in which about 1/3 of the species are pollen-heteromorphic. We have studied the distribution of pollen heteromorphism in the genusViola using a molecular phylogeny based on ITS sequence data. We show that pollen heteromorphism has evolved independently at least six times inViola. A comparative analysis shows that the occurrence of pollen heteromorphism is correlated with sporophytic polyploidy in all sections of the genus apart from sectionMelanium. This section differs from all other sections on several aspects such as flower morphology, absence of cleistogamous flowers, and a high proportion of heteromorphic species. We discuss the possible adaptiveness of pollen heteromorphism in this section.     

Pollen aperture heteromorphism is not due to unreduced gametophytes

Variations in ploidy level of the sporophyte are usually acknowledged as the main cause of polymorphism in pollen aperture number and pollen size. In heteromorphic species (two or more pollen morphs in all flowers of all plants) the different pollen types could thus be due to the presence of reduced and unreduced gametes. But data from the fossil record indicate that pollen aperture number has increased, suggesting that this trait may have evolved through selection. Our studies of Viola diversifolia have shown that the different aperture morphs have different selective values, also indicating that this character could have evolved and is not always due to ploidy level variations. To test this hypothesis, we measured DNA content of the reproductive nucleus as a test of gametophytic reduction in four heteromorphic species of Viola and in diploid and tetraploid individuals of a Petunia hybrida clone (which also served as a control to test for accuracy of the measurements). Our results show that aperture polymorphism is not due to gametophytic ploidy variations within plants and that variations in the proportions of the different pollen morphs are not correlated with variations of the amount of DNA in the sporophyte.     

Relationships among pansies (Viola section Melanium) investigated using ITS and ISSR markers

Sequences of the nuclear region ITS and the variable molecular markers ISSR were used to estimate the phylogeny of the section Melanium of the genus Viola. We confirm that the so-called pansies form a derived and monophyletic group. Two floral characters, the upturned side petals and the large size of pollen grains appear to be synapomorphies in Melanium. The Melanium species are very closely related, as shown by the reduced genetic variation compared to the other sections of Viola. Our analyses suggest x=5 or x=7 as the possible base chromosome number of the section Melanium. Polyploidy and hybridization would have played an important role in the evolutionary history of this clade resulting in a wide range of chromosome number. The low genetic differentiation and the complex cytological evolution suggest that diversification in Melanium is the result of a reticulate evolution and rapid radiation in Europe and Northern Africa.     

Pollen Heteromorphism in Nicotiana tabacum (Solanaceae)

Pollen heteromorphism is defined as the production by a single plant of different fertile pollen types in all its anthers, and thus all flowers, throughout its life cycle. Eight cultivars of Nicotiana tabacum, as well as its ancestors (N. tabacum is an amphiploid hybrid 4 × from a cross between N. sylvestris and N. tomentosiformis) and recent hybrids were analyzed. Most cultivars and the hybrids are heteromorphic (producing 3- and 4-aperturate pollen grains), whereas both parent species are homomorphic (3-aperturate). Heteromorphism is a common consequence of polyploidization and these results agree with this interpretation. There is a significant variation in the proportions of the two pollen types among cultivars (genetic component), but also (with a much lower component of variance) within each cultivar, between plants (genets), flowers of a plant, and even anthers of a flower. This is interpreted as a release of the selective pressure: the cultivars of N. tabacum were obtained after several generations of selfing and are themselves selfers. Selfing, by removing pollen mixtures on a stigma, removes pollen competition, which is the drive for heteromorphism, and allows for a large variation of the proportions of the different pollen types.     

The bee fauna of large parks in downtown Paris,France

In recent years, a growing number of studies have focused on the potential interest of urban green areas for supporting biodiversity. Private gardens, urban parks or green roofs may support relatively high densities of diverse wild bees. Knowledge is lacking regarding bee assemblages in Paris, the French capital, and one of the most densely populated part of France. In this context, we here provide a first assessment of the taxonomic and functional composition of bee assemblages in three urban parks in downtown Paris. Bees were sampled with a set of three coloured pan traps per park. Fifteen 24-hour sessions were performed from April to July 2011. We captured 425 specimens from 44 species representing five families and 11 genera. The assemblage was dominated by Halictidae (15 species representing 70.1% of total abundance), especially the eusocial species Lasioglossum morio that made 25.2% of total abundance. From a functional point of view, there were only two species of parasitic bees that made 1.2% of total abundance. Most non-parasitic species were polylectic and below-ground nesting. This study highlights the importance of preserving and managing urban parks and other green areas to promote the conservation of wild bees and ultimately the functionality of pollination service in urban ecosystems.     

Conspecifics as informers and competitors: an experimental study in foraging bumble-bees

Conspecifics are usually considered competitors negatively affecting food intake rates. However, their presence can also inform about resource quality by providing inadvertent social information. Few studies have investigated whether foragers perceive conspecifics as informers or competitors. Here, we experimentally tested whether variation in the density of demonstrators ('none', 'low' and 'high'), whose location indicated flower profitability, affected decision-making of bumble-bees Bombus terrestris. Bumble-bees foraged on either 'simple' (two colours) or 'complex' (four colours) artificial floral communities. We found that conspecifics at low density may be used as sources of information in first flower choices, whereas they appeared as competitors over the whole foraging sequence. Low conspecific densities improved foragers' first-visit success rate in the simple environment, and decreased time to first landing, especially in the complex environment. High conspecific densities did not affect these behavioural parameters, but reduced flower constancy in both floral communities, which may alter the efficiency of pollinating visits. These results suggest that the balance of the costs and benefits of conspecific presence varies with foraging experience, floral community and density. Spatio-temporal scales could thus be an important determinant of social information use. This behavioural flexibility should allow bumble-bees to better exploit their environment.     

Flowering Phenology and Gender Variation in Pennisetum typhoides

Because of the modular structure of pearl millet (an annual grass crop, Poaceae), different tillers of a plant share the same genotype but are subjected to different environmental conditions during their maturation. This allows investigation of the effects of tiller flowering phenology on allocation to resource-producing photosynthetic biomass, sexual functions, and thus tiller gender. All tillers of plants of two families collected from individual maternal plants (represented by 33 and 31 plants each) were analyzed. In both families, allocation to aboveground vegetative biomass decreased as flowering was delayed. On average, late-flowering tillers were 65% smaller than the first ones to flower. The proportion of biomass allocated to reproduction significantly increased with the flowering rank of the tillers, suggesting that translocations of assimilates occurred between early- and late-flowering tillers. In both families, late-flowering tillers produced significantly fewer pollen grains per stamen than early-flowering ones, and female reproductive allocation (expressed as seed mass per tiller) was also affected by flowering phenology. Tillers became increasingly female as flowering phenology progressed. This gender variation is possibly adaptive because pollination efficiency is maximized by plant height. Natural selection may favor a shift toward femaleness to maximize reproductive fitness in small, late-developing tillers.