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Interaction networks are widely used as tools to understand plant–pollinator communities, and to examine potential threats to plant diversity and food security if the ecosystem service provided by pollinating animals declines. However, most networks to date are based on recording visits to flowers, rather than recording clearly defined effective pollination events. Here we provide the first networks that explicitly incorporate measures of pollinator effectiveness (PE) from pollen deposition on stigmas per visit, and pollinator importance (PI) as the product of PE and visit frequency. These more informative networks, here produced for a low diversity heathland habitat, reveal that plant–pollinator interactions are more specialized than shown in most previous studies. At the studied site, the specialization index was lower for the visitation network than the PE network, which was in turn lower than for the PI network. Our study shows that collecting PE data is feasible for community-level studies in low diversity communities and that including information about PE can change the structure of interaction networks. This could have important consequences for our understanding of threats to pollination systems.

Flower form is one of many floral features thought to be shaped by pollinator-mediated selection. Although the drivers of variation in flower shape have often been examined in microevolutionary studies, relatively few have tested the relationship between shape evolution and shifts in pollination system across clades. In the present study, we use morphometric approaches to quantify shape variation across the Andean clade Iochrominae and estimate the relationship between changes in shape and shifts in pollination system using phylogenetic comparative methods. We infer multiple shifts from an ancestral state of narrow, tubular flowers toward open, bowl-shaped, or campanulate flowers as well as one reversal to the tubular form. These transitions in flower shape are significantly correlated with changes in pollination system. Specifically, tubular forms tend to be hummingbird-pollinated and the open forms tend to be insect-pollinated, a pattern consistent with experimental work as well as classical floral syndromes. Nonetheless, our study provides one of the few empirical demonstrations of the relationship between flower shape and pollination system at a macroevolutionary scale.

The large majority of angiosperm species depend on animals for pollination, including many agricultural crops, and plant-pollinator interactions have been extensively studied. However, not all floral visitors actually transfer pollen, and efforts to distinguish true pollinators from mere visitors are particularly scarce among the bat pollination literature. To determine whether Old World bat species are equally effective pollinators in mixed-agricultural areas of southern Thailand, we examined six night-blooming plant taxa and quantified pollinator importance (PI) of seven common nectarivorous bat species. PI was calculated as the product of nightly bat visitation rate (obtained from mist-netting data) and pollen transfer efficiency (estimated from bat pollen loads). We found that PI varied by both bat species and plant species. In general, the nectar-specialist bat species were more important pollinators, yet their order of importance differed across our focal plant species. In addition, PI was dictated more by pollen transfer effectiveness than visitation rate. Our findings highlight the importance of Old World bat pollinators within southern Thailand’s mixed-agricultural landscape and illustrate how seemingly similar floral visitors can have very different contributions toward plant pollination success.

Insufficient investigation of nocturnal floral visitors may bias our understanding of floral diversification in many plant lineages. Here we re-examined the pollination ecology of Rhododendron excellens , which lacks a narrow floral tube characteristic of many hawkmoth flowers and has been considered a bee specialist with daytime observations alone. We used five temporally sequential proxies (i.e. visitation rate, pollen deposition, fruit production, seed production, and seed viability) covering the entire process of reproduction to track the relative importance of the two floral visitor groups that were active by day and by night respectively. We then quantified the floral syndrome and tested the mating system with hand pollination treatments. Both bumblebees and hawkmoths regularly visited R . excellens in two flowering seasons. Hawkmoths’ relative importance increased step-by-step from being inferior to bumblebees (floral visitation and pollen deposition) to making over three times the contribution by bumblebees (seed viability). This is probably because they differ in the ability to deliver outcross pollen for this partially self-compatible species. Correspondingly, R . excellens exhibits a floral syndrome associated with a particular subdivision of sphingophily. We revealed a distinctive pollination mode that mainly involves nocturnal pollinators and is likely to have repeatedly evolved in Rhododendron . Our findings also highlight the need to consider the ‘quality’ of pollen deposited onto stigmas, whose effect on plant fecundity may significantly expand over the course of sexual reproduction.

Most plants attract multiple flower visitors that may vary widely in their effectiveness as pollinators. Floral evolution is expected to reflect interactions with the most important pollinators, but few studies have quantified the contribution of different pollinators to current selection on floral traits. To compare selection mediated by diurnal and nocturnal pollinators on floral display and spur length in the rewarding orchid Gymnadenia conopsea , we manipulated the environment by conducting supplemental hand-pollinations and selective pollinator exclusions in two populations in central Norway. In both populations, the exclusion of diurnal pollinators significantly reduced seed production compared to open pollination, whereas the exclusion of nocturnal pollinators did not. There was significant selection on traits expected to influence pollinator attraction and pollination efficiency in both the diurnal and nocturnal pollination treatment. The relative strength of selection among plants exposed to diurnal and nocturnal visitors varied among traits and populations, but the direction of selection was consistent. The results suggest that diurnal pollinators are more important than nocturnal pollinators for seed production in the study populations, but that both categories contribute to selection on floral morphology. The study illustrates how experimental manipulations can link specific categories of pollinators to observed selection on floral traits, and thus improve our understanding of how species interactions shape patterns of selection.

Community and biogeographic surveys often conclude that plant-pollinator interactions are highly generalized. Thus, a central implication of the pollination syndrome concept, that floral trait evolution occurs primarily via specialized interactions of plants with their pollinators, has been questioned. However, broad surveys may not distinguish whether flower visitors are actual pollen vectors and hence lack power to assess the relationship between syndrome traits and the pollinators responsible for their evolution. Here we address whether the floral traits of three closely related hermaphroditic Silene spp. native to eastern North America (S. caroliniana, S. virginica, and S. stellata) correspond to predicted specialized pollination based on floral differences among the three species and the congruence of these floral features with recognized pollination syndromes. A nocturnal/diurnal pollinator exclusion experiment demonstrated that all three Silene spp. have diurnal pollinators, and only S. stellata has nocturnal pollinators. Multiyear studies of visitation rates demonstrated that large bees, hummingbirds, and nocturnal moths were the most frequent pollinators of S. caroliniana, S. virginica, and S. stellata, respectively. Estimates of pollen grains deposited and removed per visit generally corroborated the visitation rate results for all three species. However, the relatively infrequent diurnal hawkmoth pollinators of S. caroliniana were equally effective and more efficient than the most frequent large bee visitors. Pollinator importance (visitation × deposition) of each of the animal visitors to each species was estimated and demonstrated that in most years large bees and nocturnal moths were the most important pollinators of S. caroliniana and S. stellata, respectively. By quantifying comprehensive aspects of the pollination process we determined that S. virginica and S. stellata were specialized on hummingbirds and nocturnal moths, respectively, and S. caroliniana was the least specialized with diurnal hawkmoth and large bee pollinators. Compared across the Silene species, divergent floral character states are consistent with increasing the attraction and/or pollen transfer efficiency of their respective major pollinators, which suggests that the pollinators are past and/or contemporary selective agents for floral trait evolution in these three Silene species. We conclude that the pollination syndrome concept allows us to effectively relate the functional significance of floral morphology to the major pollinators of these Silene species.

Floral divergence among congeners may relate to differential utilization of pollinators and contribute to reducing overlap in pollination niches. To investigate whether and how floral differences are associated with differential utilization of pollinators in three sympatric Adenophora species, we analyzed floral traits and evaluated the contribution of different visitors to pollination. We compared visitation rates of different pollinator categories in different years and sites. A suite of floral traits differed among the three Adenophora species, suggesting adaptation to diurnal versus nocturnal pollination and an intermediate condition. However, many visitor species were shared among the three plant species, suggesting that floral traits did not rigorously filter visitors. Effective pollinators were large bees and moths. The importance of large bees as pollinators decreased whereas that of moths increased along the gradient from typically bee-pollinated to moth-pollinated flowers. The intermediate species (A. khasiana) differed substantially from the other two species in pollinator species but not in pollinator categories. The principal pollinator category of each species was constant across years and sites except in the intermediate species where it differed between two sites. Overall, the three sympatric species of Adenophora partition pollinators by floral divergence and the principal pollinators coincide with the predictions based on floral syndromes.

Floral orientation may affect pollinator attraction and pollination effectiveness, and its influences may differ among pollinator species. We, therefore, hypothesized that, for plant species with a generalized pollination system, changes in floral orientation would affect the composition of pollinators and their relative contribution to pollination. Geranium refractum, an alpine plant with downward floral orientation was used in this study. We created upward-facing flowers by altering the flower angle. We compared the pollinator diversity, pollination effectiveness, and pollinator importance, as well as female reproductive success between flowers with downward- and upward-facing orientation. Results indicated that the upward-facing flowers were visited by a wider spectrum of pollinators (classified into functional groups), with higher pollinator diversity than natural flowers. Moreover, due to influences on visitation number and pollen removal, the pollinator importance exhibited by the main pollinator groups differed between flower types. Compared with natural flowers, the pollination contribution of principal pollinators (i.e., bumblebees) decreased in upward-facing flowers and other infrequent pollinators, such as solitary bees and muscoid flies, removed more pollen. Consequently, stigmatic pollen loads were lower in upward- than in downward-facing flowers. These findings reveal that floral orientation may affect the level of generalization of a pollination system and the relative importance of diverse pollinators. In this species, the natural downward-facing floral orientation may increase pollen transfer by effective pollinators and reduce interference by inferior pollinators.

In a 3-yr study, we examined the pollinator guild and intersexual floral characteristics of the dioecious, perennial cloudberry (Rubus chamaemorus), which flowers in early spring. The findings contribute to our general understanding of pollination ecology at high latitudes and provide important information for the commercialization of cloudberry. Female flowers were smaller than males but provided more nectar, although this resource was low in both sexes. Insects from 43 families visited cloudberry flowers, yet four families (Apidae, Halictidae, Muscidae, Syrphidae) represented ca. 87% of all visitors observed. Introduction experiments revealed that apids and muscids are significantly poorer pollinators (based on fruit production) than halictids and syrphids, but when fruit mass or seed set was considered, there were no significant differences between families. Pollinator importance, a product of flower visitation frequency and seed set effectiveness, revealed that the dipterans were of paramount importance to the pollination of cloudberry. Furthermore, they are limited to cloudberry because their lapping mouthparts exclude them from accessing the nutritional rewards of competing Ericaceae flowers. While the total number of pollinator families observed suggest a generalist pollination system, if one considers the dominant pollinators (flies) as a functional group, then this insect-flower relationship could be considered a specialized one.

Despite the development of diversity indices in community ecology that incorporate both richness and evenness, pollination biologists commonly use only pollinator richness to estimate generalization. Similarly, while pollination biologists have stressed the utility of pollinator importance, incorporating both pollinator abundance and effectiveness, importance values have not been included in estimates of generalization in pollination systems. In this study, we estimated pollinator generalization for 17 plant species using Simpson's diversity index, which includes richness and evenness. We compared these estimates with estimates based on only pollinator richness, and compared diversity estimates calculated using importance data with those using only visitation data. We found that pollinator richness explains only 57-65% of the variation in diversity, and that, for most plant species, pollinator importance was determined primarily by differences in visitation rather than by differences in effectiveness. While simple richness may suffice for broad comparisons of pollinator generalization, measures that incorporate evenness will provide a much more accurate understanding of generalization. Although incorporating labor-intensive measurements of pollinator effectiveness are less necessary for broad surveys, effectiveness estimates will be important for detailed studies of some plant species. Unfortunately, at this point it is impossible to predict a priori which species these are.