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Opuntia is the most diverse and widespread genus in the cactus family. In spite of the strong relationship with biotic pollination, little is known about their floral nectaries. We present the first deep structural and ultrastructural description of the floral nectaries of two Opuntia species (Opuntia anacantha var. retrorsa; Opuntia elata var. elata), linking these attributes to their functioning. A characterization of their floral visitors is also made. Both nectaries are very similar and represent the hypanthial type, with the presence of surface undulations that favor nectar retention. In O. elata, the style dilatation forms a nectar chamber. Three tissues are distinguished in these nectaries: an epidermis consisting of a single layer of cells with a thin cuticle, a nectariferous parenchyma with numerous vascular bundles with xylem and phloem and, a subnectariferous parenchyma with numerous amyloplasts. Pre-nectar may come from the starch hydrolysis of these plastids as well as from other green tissues of the plant. The exudation occurs through stomata and cuticle microchannels. O. anacantha may be a melittophilous plant, while in O. elata the presence of a nectar chamber and the orange color of the perianth suggest an incipient ornithophily. [ABSTRACT FROM AUTHOR]

Agricultural land management modifies ecosystem structure and functioning in natural landscapes. Pollinators are a key functional group that may suffer from such intensification. Here we evaluate how agricultural land management influences the diversity of pollen transported by pollinators and the pollination niche overlap among plants. We described pollen transport networks (which allow assessing the contribution of pollinators to the flow of pollen among plants) in agricultural and restored fragments in three sites representative of the pampas region of Argentina. We analyzed diversity of pollen transported by pollinators and the pollination niche overlap among plants in both types of fragments with general and generalized linear mixed models. The agricultural fragments were associated to increased diversity of transported pollen and pollination niche overlap among plants. Greater pollination niche overlap in agricultural fragments was associated with increased abundance of exotic plants. Our results indicate that agricultural intensification has significantly increased the diversity of pollen and the pollination niche overlap in natural communities by promoting exotic plants and generalized plant-pollinator interactions. Strategies to encourage improvements in the quality of pollination in agroecosystems could range from controlling the levels of exotic species to mechanisms that promote increased diversity of native plants. [ABSTRACT FROM AUTHOR]

Gomesa bifolia (Orchidaceae, Cymbidieae, Oncidiinae) is an epiphytic orchid that presents showy yellow flowers with brown markings on sepals and petals, fragranceless and with oil as reward. The floral biology of this species was studied in cultivated individuals to describe phenology, flower longevity, reproductive system (using manipulative treatment of cross-pollination vs. self-pollination) and floral pollinators. Moreover, direct observations were made on plants exposed outdoors to identify pollinators, describing their foraging behavior and the duration of visits. The results showed that G. bifolia is a mainly self-incompatible, non-autogamous and pollinator-dependent species. The fruit set in manually cross-pollination flowers (100%) was higher than the manually self-pollinated flowers (30.8%), whereas the control flowers did not set fruits. Females of Centris trigonoides (Apidae, Centridini) were the exclusive pollinators, and the visit rate was low. Several Neotropical orchids of the subtribe Oncidiinae offer floral oils to the pollinators, like the Malpighiaceae familiy. Our results allow inferring that both groups of plants would be part of a guild of species pollinated by oil-collecting bees and that the deceit/pollination syndrome should not be applied to all Oncidiinae flowers. [ABSTRACT FROM AUTHOR]

Sunflower (Helianthus annuus L.) is an important oilseed crop in Argentina. During three agricultural years, the diversity and abundance of diurnal floral visitors of sunflower heads were determined in eight sites spanning much of this crop's cultivation area in Argentina. Seventy-six morpho-species of floral visitors, belonging to eight orders, were captured on sunflower. The principal order was Hymenoptera, with 37 species or morpho-species, of which 32 were bees (Apoidea). The most represented bee families were Apidae (13), Megachilidae (11) and Halictidae (7). The domestic bee (Apis mellifera I.) accounted for 93% of the visits. Floral visitor composition did not show an identifiable variation pattern either throughout the day or with respect to the distance to the edge of the crop, but varied among sites. It is concluded that the domestic bee is the main sunflower pollinator in Argentina, although several native bee species (Melissodes tintinnans (Holmberg), M. rufithorax Brèthes, Melissoptila tandilensis Holmberg, and Megachilespp.) could be considered as potential crop pollinators. [ABSTRACT FROM AUTHOR]

In this work we analyse the pollination community in a South American forest known as ‘talar’. This is a vegetal woody community that inhabits fossil coastal banks characterized by seasonal temperate weather and calcareous soil, at the coast of the Río de la Plata, in the province of Buenos Aires, Argentina. We obtained data of the interactions between anthophylous insects and entomophylous flowering plants over an extensive period of time. We showed that pollination system parameters, such as partners’ identity, system size, and connectance, fluctuated among months, when sampled year-long. Maximal network size occurred in early spring and early autumn, when both the number of mutualistic species and the number of interactions peaked, and this was also when network asymmetry was higher than average. Monthly connectance of the plant-flower visitor matrix decreased to its lowest values at these peaks. Available data suggest that cumulative traditional connectance (i.e. the connectance calculated as the whole number of interactions registered in the community divided by the full size system) underestimates actual connectance values by a factor of c. 3 ×. Monthly values of connectance decreased exponentially as system size increased, and the distribution of interactions per species followed power-law regimes for animals, and truncated power-law regimes for plants, in accordance with patterns previously deduced from among-network cumulative communities studies. We think that either within or and among pollination networks, systems that are organized as power-law regimes may be a basic property of these webs, and provide examples of the fact. Both seasonal changes and interactions between mutualists like competition, and some degree of facilitation, may be very important to understand the performance of the system as a whole, and the role and importance of different species in the community. We suggest that communities of plant – pollinators that exhibit extended activity, such as temperate or tropical seasonal ones, should be studied through consecutive plant-pollinator webs rather than cumulative ones. The partition of the system into smaller serial parts allows us to obtain outstanding information of every short period. This information is flattened by the average effect when we considered the combined analysis of the whole data. [ABSTRACT FROM AUTHOR]

• Harvesting intensity had a strong positive effect on native herbs. • The effects on exotic herbs depended on the site. • Harvesting intensity enhanced the density of native and exotic floral visitors. • Although seasonality was relevant, there was no strong interaction with harvesting. • Our results are better explained by generalist interactions rather than by invader complexes. 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. [ABSTRACT FROM AUTHOR]

There is a growing consensus that the world is facing a pollination crisis. To mitigate crop pollination deficits, some management strategies include the massive introduction of managed bee species, yet quite often they are applied blindly, as information on crop pollination effectiveness for each single pollinator species of assemblages is usually not available. Therefore, the introduction on managed species is not always the best option to improve crop yields. Here, by using the highly pollinator-dependent alfalfa crop (Medicago sativa L.) as a case study, we propose the use of the effectiveness landscape framework to identify key crop pollinator species. According to this framework, in a mutualistic interaction, each species´ effectiveness is represented by the product of a quantitative component and a qualitative one, these being measures of the outcomes of this interaction. We applied this framework for two managed and four wild bee species that visit alfalfa in fields southwest of Buenos Aires province, Argentina. We dissected the quantity components of the pollinator effectiveness landscape by estimating two quantitative subcomponents: visitation rate and flower tripping rate. Also, we estimate pod set as a qualitative component without dissecting it in subcomponents. Our results showed that the contribution of both components and the resulting pollinator effectiveness varied among pollinator species, indicating a contrasting effectiveness of different bee species on alfalfa pollination. For example, pollinator effectiveness was higher for managed than for wild bees, as consequence of their very high visitation rate, however, wild bee flower tripping rate and pod set were as high as managed ones. In fact, wild bees were more effective in promoting flower tripping than one of the managed bees (A. mellifera). This approach allowed us to assess which effectiveness components and subcomponents make pollinator species more or less effective, thus providing valuable information to identify key species to be enhanced to help in closing yield gaps. We suggest that the application of the effectiveness landscape framework would be useful to develop strategies to improve crop pollination service in pollinator-dependent crop systems. • Identification of key crop pollinator species is essential. • Effectiveness landscape decomposes effectiveness into components and subcomponents. • Effectiveness landscape allows us to assess the effectiveness of each crop pollinator species. • This approach is useful for crop pollination management strategies. [ABSTRACT FROM AUTHOR]

The topological structure of interaction networks determines community dynamics and stability. Closeness centrality is a species-level attribute that measures the relative position and proximity of species in the interaction network. We assessed the link between closeness centrality of crops in plant-pollinator interaction networks and crop seed production. We worked with five crop species used for seed production distributed in 14 farms. At each farm we recorded pollinators' interactions with the crop and with the co-flowering plants at crop edges and built quantitative plant-pollinator interaction networks. For each network, we calculated weighted closeness centrality of crops and related it to crop seed set by using generalized linear mixed models. Our results show that crop centrality is positively associated with crop seed production, and suggest that the use of ecological interaction network metrics in agricultural systems could be useful to design agronomic management plans and enhance crop yields. • Crop pollination service increases when crops become more central in their interaction networks. • Crop centrality has a higher effect on crop pollination service than floral visitor richness. • The studies and application of ecological interaction networks can improve crop yields. [ABSTRACT FROM AUTHOR]

Biotic pollination is an essential ecosystem service for agricultural production and is reflected in the high number of crops that depend on insect pollination in order to produce profitable yields. Alfalfa (Medicago sativa) is a crop whose flowers need to be visited by a bee to be pollinated for seed production, making it highly pollinator-dependent. Two managed bee species are currently used to pollinate this crop: the alfalfa leafcutting bee (ALCB), one of the most efficient alfalfa pollinators, and the honey bee (HB), whose efficiencies could be highly variable among production sites. Besides, there are many other wild bee species (WBs) that are effective pollinators of alfalfa, but little attention has been placed on them, especially in Argentina, where alfalfa seed production is deficient. Here, we evaluate the contribution of both managed species and WBs on alfalfa pollination services in one of the most important alfalfa seed productive regions of Argentina. During the span of two years, we calculated pollen limitation in different pollinator managed scenarios: with and without ALCBs (ALCB+ and ALCB−, respectively), and we also evaluated the relationship between pollen limitation and all bee species visitation rates. Our results show that the ALCB is a very effective pollinator of alfalfa, since ALCB+ lots have less pollen limitation (32%) compared to ALCB− lots. In contrast, HBs seem to have a detrimental effect in pollination service in ALCB+ lots and a positive effect in ALCB− lots. This differential effect could be due to differences in foraging behavior caused by competition between HBs and ALCBs. Finally, in spite of their low abundances, we found that an increase in WB visitation rates substantially reduces pollen limitation. In spite of ALCBs being a good alfalfa pollinators, the difficulty and cost of managing them hinder its widespread use. The use of WBs seems to be a good alternative, and practices that improve their abundance and diversity should be implemented to improve alfalfa pollination service. • Alfalfa reproduction highly depends on cross pollination by a bee. • Alfalfa leafcutting bee management improve alfalfa pollination service. • Honey bee differs in its efficiency between alfalfa crops. • Increased wild bee abundance enhances pollination service of alfalfa crops. [ABSTRACT FROM AUTHOR]