Bird and insect pollinators differ in specialization and potential pollination services along disturbance and resource gradients

Combined studies of the communities and interaction networks of bird and insect pollinators are rare, especially along environmental gradients. Here, we determined how disturbance by fire and variation in sugar resources shape pollinator communities and interactions between plants and their pollinating insects and birds. We recorded insect and bird visits to 21 Protea species across 21 study sites and for 2 years in Fynbos ecosystems in the Western Cape, South Africa. We recorded morphological traits of all pollinator species (41 insect and nine bird species). For each site, we obtained estimates of the time since the last fire (range: 2–25 calendar years) and the Protea nectar sugar amount per hectare (range: 74–62 000 g/ha). We tested how post-fire age and sugar amount influence the total interaction frequency, species richness and functional diversity of pollinator communities, as well as pollinator specialization (the effective number of plant partners) and potential pollination services (pollination service index) of insects and birds. We found little variation in the total interaction frequency, species richness and functional diversity of insect and bird pollinator communities, but insect species richness increased with post-fire age. Pollinator specialization and potential pollination services of insects and birds varied differently along the environmental gradients. Bird pollinators visited fewer Protea species at sites with high sugar amount, while there was no such trend for insects. Potential pollination services of insect pollinators to Protea species decreased with increasing post-fire age and resource amounts, whereas potential pollination services of birds remained constant along the environmental gradients. Despite little changes in pollinator communities, our analyses reveal that insect and bird pollinators differ in their specialization on Protea species and show distinct responses to disturbance and resource gradients. Our comparative study of bird and insect pollinators demonstrates that birds may be able to provide more stable pollination services than insects.     

The pollination niche and its role in the diversification and maintenance of the southern African flora

The flora of southern Africa has exceptional species richness and endemism, making it an ideal system for studying the patterns and processes of evolutionary diversification. Using a wealth of recent case studies, I examine the evidence for pollinator-driven diversification in this flora. Pollination systems, which represent available niches for ecological diversification, are characterized in southern Africa by a high level of ecological and evolutionary specialization on the part of plants, and, in some cases, by pollinators as well. These systems are asymmetric, with entire plant guilds commonly specialized for a particular pollinator species or functional type, resulting in obvious convergent floral evolution among guild members. Identified modes of plant lineage diversification involving adaptation to pollinators in these guilds include (i) shifts between pollination systems, (ii) divergent use of the same pollinator, (iii) coevolution, (iv) trait tracking, and (v) floral mimicry of different model species. Microevolutionary studies confirm that pollinator shifts can be precipitated when a plant species encounters a novel pollinator fauna on its range margin, and macroevolutionary studies confirm frequent pollinator shifts associated with lineage diversification. As Darwin first noted, evolutionary specialization for particular pollinators, when resulting in ecological dependency, may increase the risk of plant extinction. I thus also consider the evidence that disturbance provokes pollination failure in some southern African plants with specialized pollination systems.     

Seasonal and annual variations in the pollination efficiency of a pollinator community of Dictamnus albus L.

The interplay between insect and plant traits outlines the patterns of pollen transfer and the subsequent plant reproductive fitness. We studied the factors that affect the pollination efficiency of a pollinator community of Dictamnus albus L. by evaluating insect behaviour and morphological characteristics in relation to flowering phenology. In order to extrapolate the pollinator importance of single taxa and of the whole pollinator guild, we calculated an index distinguishing between potential (PPI) and realized (RPI) pollinator importance. Although the pollinator species spectrum appeared rather constant, we found high intra‐ and inter‐annual variability of pollinator frequency and importance within the insect community. Flower visitation rate strictly depended on insect abundance and on the overlap between their flying period and flower blooming. All the pollinators visited flowers from the bottom to the top of the racemes, excluding intra‐plant geitonogamous pollination, and most of them showed high pollen fidelity. Only medium large‐sized bees could contact the upward bending stiles while feeding on nectar, highlighting a specialisation of the plant towards bigger pollinators. Moreover, we found evidence of functional specialisation, since all pollinators were restricted to a single taxonomic group (order: Hymenoptera; superfamily: Apoidea). Both the PPI and RPI indices indicate Habropoda tarsata as the most important pollinator of D. albus. Following hand cross‐pollination experiments we revealed the presence of pollination limitation in 1 of the 3 years of field study. We discuss this result in relation to flowering abundance and to possible mismatches of phenological periods between plants and insects.     

Dynamics of plant–pollinator–robber systems

Plant–pollinator–robber systems are considered, where the plants and pollinators are mutualists, the plants and nectar robbers are in a parasitic relation, and the pollinators and nectar robbers consume a common limiting resource without interfering competition. My aim is to show a mechanism by which pollination–mutualism could persist when there exist nectar robbers. Through the dynamics of a plant–pollinator–robber model, it is shown that (i) when the plants alone (i.e., without pollination–mutualism) cannot provide sufficient resources for the robbers’ survival but pollination–mutualism can persist in the plant–pollinator system, the pollination–mutualism may lead to invasion of the robbers, while the pollinators will not be driven into extinction by the robbers’ invasion. (ii) When the plants alone cannot support the robbers’ survival but persistence of pollination–mutualism in the plant–pollinator system is density-dependent, the pollinators and robbers could coexist if the robbers’ efficiency in translating the plant–robber interactions into fitness is intermediate and the initial densities of the three species are in an appropriate region. (iii) When the plants alone can support the robbers’ survival, the pollinators will not be driven into extinction by the robbers if their efficiency in translating the plant–pollinator interactions into fitness is relatively larger than that of the robbers. The analysis leads to an explanation for the persistence of pollination–mutualism in the presence of nectar robbers in real situations.     

Relative effectiveness of insects versus hummingbirds as pollinators of Rubiaceae plants across elevation in Dominica,Caribbean

• Most angiosperms rely on animal pollination for reproduction, but the dependence on specific pollinator groups varies greatly between species and localities. Notably, such dependence may be influenced by both floral traits and environmental conditions. Despite its importance, their joint contribution has rarely been studied at the assemblage level.
• At two elevations on the Caribbean island of Dominica, we measured the floral traits and the relative contributions of insects versus hummingbirds as pollinators of plants in the Rubiaceae family. Pollinator importance was measured as visitation rate (VR) and single visit pollen deposition (SVD), which were combined to assess overall pollinator effectiveness (PE).
• In the wet and cool Dominican highland, we found that hummingbirds were relatively more frequent and effective pollinators than insects, whereas insects and hummingbirds were equally frequent and effective pollinators at the warmer and less rainy midelevation. Furthermore, floral traits correlated independently of environment with the relative importance of pollinators, hummingbirds being more important in plant species having flowers with long and wide corollas producing higher volumes of dilute nectar.
• Our findings show that both environmental conditions and floral traits influence whether insects or hummingbirds are the most important pollinators of plants in the Rubiaceae family, highlighting the complexity of plant–pollinator systems.

     

Noncorrelated effects of seed predation and pollination on the perennial herb Ruellia nudiflora remain spatially consistent

By simultaneously manipulating both seed predator and pollinator effects on the perennial herb Ruellia nudiflora at two sites in Yucatan (Mexico), the present study evaluated (1) whether a correlation (interaction) existed between seed predator and pollinator effects on R. nudiflora seed production and (2) whether such an interaction varied geographically. We used three populations per site, and a total of 20 plants per population ( N  = 120). Groups of five plants were randomly chosen at each population to simultaneously receive one of two seed predator and pollinator exclosure levels (present or excluded in each case). These two factors were fully crossed, resulting in each group being subjected to one of four possible combinations: pollinators excluded/herbivores present; herbivores excluded/pollinators present; herbivores excluded/pollinators excluded; or control (neither excluded). Response variables were the number of seeds produced per plant and the proportion of attacked fruits by seed predators per plant. Seed predators had a large impact on R. nudiflora seed production but did not show any preference for fruits from plants not excluded from pollinators. In addition, the pollination treatment was not significant, indicating no effect of pollinators on reproductive success. These findings resulted in a nonsignificant herbivory × pollination interaction, which was consistent across sites, indicating lack of correlated selection of these two guilds on R. nudiflora seed production.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 800–807.     

Urbanization increases floral specialization of pollinators

Understanding how urbanization alters functional interactions among pollinators and plants is critically important given increasing anthropogenic land use and declines in pollinator populations. Pollinators often exhibit short‐term specialization and visit plants of the same species during one foraging trip. This facilitates plant receipt of conspecific pollen—pollen on a pollinator that is the same species as the plant on which the pollinator was foraging. Conspecific pollen receipt facilitates plant reproductive success and is thus important to plant and pollinator persistence. We investigated how urbanization affects short‐term specialization of insect pollinators by examining pollen loads on insects’ bodies and identifying the number and species of pollen grains on insects caught in urban habitat fragments and natural areas. We assessed possible drivers of differences between urban and natural areas, including frequency dependence in foraging, species richness and diversity of the plant and pollinator communities, floral abundance, and the presence of invasive plant species. Pollinators were more specialized in urban fragments than in natural areas, despite no differences in the species richness of plant communities across site types. These differences were likely driven by higher specialization of common pollinators, which were more abundant in urban sites. In addition, pollinators preferred to forage on invasive plants at urban sites and native plants at natural sites. Our findings reveal indirect effects of urbanization on pollinator fidelity to individual plant species and have implications for the maintenance of plant species diversity in small habitat fragments. Higher preference of pollinators for invasive plants at urban sites suggests that native species may receive fewer visits by pollinators. Therefore, native plant species diversity may decline in urban sites without continued augmentation of urban flora or removal of invasive species.     

Forest fragments influence pollination and yield of soybean crops in Chaco landscapes

Habitat remnants act as a source of pollinators potentially relevant for crop pollination and yield. This work analyzes how habitat loss influences pollinators, effective pollination and yield of soybean crops. The study area comprises ten sites adjacent to forest patches surrounded by a soybean matrix in central Argentina (eight sites in the season 2014–2015 and two sites in the season 2015–2016). Pollination was estimated by pollen deposition and frequency of flower visitors. Pollen deposition on stigmas and seed set were measured comparing open plants and plants with pollinator exclusion. These response variables were compared considering increasing distance to the forest edge and an increasing gradient of forest patch size. Bees were the most frequent visitors of soybean flowers, especially honeybees, but also at least three native bee species were recorded. Open plants showed higher rates of stigmatic pollen deposition than plants with pollinator exclusion, but seed set was similar. Total insect visitation rates, especially of native insects, decreased with distance to the forest edge and so did pollen deposition. Pollen deposition and seed set increased with increasing forest patch size for plants located near and far from the forest edge, respectively. Overall, our results suggest that the contribution of native pollinators from local forest patches is important for effective pollination across the landscape. Small patches of forest (approximately 1 ha.) guarantee pollinators to ensure plant yields similar to the yields of plants growing close to large patches, but only at short distances; while larger forest patches provide better pollination services for the crop at larger distances from the forest edge. However, we encourage further studies because results suggest that other factors may also influence soybean pollination and production.     

Reproductive biology of the South American cycad Zamia boliviana,involving brood-site pollination

Brood-site pollination mutualisms represent extreme levels of reciprocal specialization between plants and insects. Several studies have revealed intimate associations between cycads and their insect pollinators across different continents. However, the reproductive biology of South American cycads is still poorly understood, with limited empirical data available. Here, we investigated the plant–pollinator interactions of Zamia boliviana (Cycadales, Zamiaceae), which is distributed on savanna patches in Brazil and Bolivia. Our aim was to examine reproductive biology (strobili development and their visitors) to identify potential pollinators from the perspective of dioecy and expectations of obligatory mutualism. We monitored reproductive cohorts and registered the strobiliʼs visitors. We classified the visitorsʼ activities and described the behavior of the most active and recurring ones in both reproductive structures to ascertain the potential pollinators and their relationship with the Z. boliviana. In situ experiments were used to demonstrate efficacy in seed production. Although other insects visit the strobili, only one Coleoptera (Erotylidae: Pharaxonotha) was present seasonally. Pharaxonotha cerradensis was the single species capable of transporting pollen and developing a life cycle associated with polleniferous strobili, while concomitantly visiting ovuliferous strobili. Experimental tests associated with observational evidence in the cycad indicated brood-site pollination mutualism with the Erotylidae beetle, a potential pollinator of Z. boliviana. Our investigation confirmed a new case of obligatory mutualism between cycads and P. cerradensis in the New World. Future research on Zamia from South America should address pollination systems of this genus to better understand the ecology and evolution of plant–pollinator interactions.     

Floral syndromes accurately predict pollination by a specialized oil-collecting bee (Rediviva peringueyi, Melittidae) in a guild of South African orchids (Coryciinae)

The long-standing notion of pollination syndromes, which postulates that plants form recognizable groups according to pollinator type, has been challenged recently on the basis of apparent widespread generalization in pollination systems. As a test of the pollination syndrome concept, I examined the pollination biology of a group of 15 orchids that share a recognizable syndrome of floral features that includes yellow-green coloration, oil secretion, pungent scent, shallow flowers, and a September peak in flowering. The orchids occur in sympatry in the Cape Floral Region of South Africa. According to the pollination syndrome concept, the similar floral features of this group indicate a shared pollinator. To test this prediction, I observed pollinators on Pterygodium alatum, P. caffrum, P. catholicum, P. volucris, Corycium orobanchoides, and Disperis bolusiana subsp. bolusiana. They shared a single species of pollinator, the oil-collecting bee, Rediviva peringueyi. Female bees collected oil from the lip appendage using modified front tarsi. The orchids reduce interspecific reproductive interference through differences in pollinarium length or the use of mutually exclusive pollinarium attachment sites on the body of the bee. The results are contrary to the expectation of generalization in pollination systems and suggest that pollinators play an important role in mediating selection on floral traits.     

The functional consequences of diversity in plant–pollinator interactions

The role of biological diversity in maintaining ecosystem functioning is a central issue in ecology. Most studies on diversity–functioning relationships have focused on ecosystem and community levels, leaving the extension of those relationships to other organization levels, such as populations, as a challenging and unsolved issue. Empirical studies have shown links between pollinator diversity and plant fecundity, suggesting that a diversity–functioning relationship at the population level may occur in pollination systems. We theoretically explored the effect of pollinator diversity on plant reproduction. We found that low pollinator diversity is beneficial when the most abundant pollinators are the most effective. In contrast, when the most effective pollinators are not the most abundant, we found an optimal value of pollinator diversity at which plant fecundity is maximized. When we parametrized our model with real data, we obtained that an increase in pollinator diversity was beneficial for the reproduction of some plants whereas it was harmful for other plants, the outcome depending exclusively on the differences in effectiveness among pollinators. Consequently, our theoretical approach suggests that in pollination systems the diversity–function relationship may be explained as the consequence of the interaction between among-pollinator differences in effectiveness and frequency of interaction, without the need to invoke additional ecological mechanisms.     

Genetic structure and hybridization in the species group of Ficus auriculata: can closely related sympatric Ficus species retain their genetic identity while sharing pollinators?

Obligate mutualistic nursery pollination systems between insects and plants have led to substantial codiversification involving at least some parallel cladogenesis, as documented in Yucca, Ficus and Phyllanthaceae. In such systems, pollinators are generally species specific thus limiting hybridization and introgression among interfertile host species. Nevertheless, in the three systems, cases of one insect pollinating several plant species are reported. In most cases, host plants sharing pollinators are allopatric. However, in the case of the species group of Ficus auriculata, forms may co‐occur over large parts of their range. We show here that the species group of F. auriculata is constituted by four well‐defined genetic entities that share pollinators. We detected hybrids in nature mainly when both parental forms were growing nearby. Controlled crosses showed that F1 offspring could be successfully backcrossed. Hence, despite sharing pollinators and despite hybrid viability, the different forms have preserved their genetic and morphological identity. We propose that ecological differentiation among forms coupled with limited overlap of reproductive season has facilitated the maintenance of these interfertile forms. As such, establishment of pollinator host specificity may not be a prerequisite for sympatric diversification in Ficus.     

Diversification and coevolution in brood pollination mutualisms: Windows into the role of biotic interactions in generating biological diversity

Brood pollination mutualisms—interactions in which specialized insects are both the pollinators (as adults) and seed predators (as larvae) of their host plants—have been influential study systems for coevolutionary biology. These mutualisms include those between figs and fig wasps, yuccas and yucca moths, leafflowers and leafflower moths, globeflowers and globeflower flies, Silene plants and Hadena and Perizoma moths, saxifrages and Greya moths, and senita cacti and senita moths. The high reciprocal diversity and species‐specificity of some of these mutualisms have been cited as evidence that coevolution between plants and pollinators drives their mutual diversification. However, the mechanisms by which these mutualisms diversify have received less attention. In this paper, we review key hypotheses about how these mutualisms diversify and what role coevolution between plants and pollinators may play in this process. We find that most species‐rich brood pollination mutualisms show significant phylogenetic congruence at high taxonomic scales, but there is limited evidence for the processes of both cospeciation and duplication, and there are no unambiguous examples known of strict‐sense contemporaneous cospeciation. Allopatric speciation appears important across multiple systems, particularly in the insects. Host‐shifts appear to be common, and widespread host‐shifts by pollinators may displace other pollinator lineages. There is relatively little evidence for a “coevolution through cospeciation” model or that coevolution promotes speciation in these systems. Although we have made great progress in understanding the mechanisms by which brood pollination mutualisms diversify, many opportunities remain to use these intriguing symbioses to understand the role of biotic interactions in generating biological diversity.     

Plant–pollinator network assembly along the chronosequence of a glacier foreland

Forelands of retreating glaciers offer an ideal model system to study community assembly processes during primary succession. As plants colonize the area that is freed from ice they should be accompanied by their pollinators to successfully reproduce and spread. However, little is known about the assembly of plant–pollinator networks. We therefore used quantitative network analysis to study the structure of plant–pollinator interactions at seven sites representing a chronosequence from 8 to 130 years since deglaciation on the foreland of the Morteratsch glacier (southeastern Switzerland). At these sites, individual visits of plant flowers by insects were recorded throughout the flowering season. Species richness of insect‐pollinated plants and plant‐pollinating insects, together with measures of interaction diversity and evenness, increased along the chronosequence at least for the first 80 years after deglaciation. Bees were the most frequent flower visitors at the two youngest sites, whereas flies dominated in mature communities. Pollinator generalization (the number of visited plant species weighted by interaction strength), but not plant generalization, strongly increased during the primary succession. This was reflected in a pronounced decline in network level specialization (measured as Blüthgen's H₂’) and interaction strength asymmetry during the first 60 years along the chronosequence, while nestedness increased along the chronosequence. Thus, our findings contradict niche‐theoretical predictions of increasing specialization of pollination systems during succession, but are in agreement with expectations from optimal foraging theory, predicting an increase in pollinator generalization with higher plant diversity but similar flower abundance, and an increase in diet breadth at higher pollinator densities during primary succession.     

Entomophily of the cloudberry (Rubus chamaemorus)

Cloudberry (Rubus chamaemorus L.), a dioecious perennial plant of boreal circumpolar distribution, is greatly prized for its berries. We crossed two treatments, pollinator exclusion and supplementary hand-pollination, to determine i) the relative importance of insects as pollinators, ii) if pollinator activity was a limiting factor for the sexual reproduction of the plant, and iii) the relative contribution of diurnal vs. nocturnal visitors to pollination. The activity of natural pollinators resulted in 97.5 and 88.5% fruit set, along with 76.7 and 62.5% seed set in 1998 and 1999, respectively. When insects were excluded, fruit-set dropped significantly to 18.4 (1998) and 12.8% (1999) and seed-set to 5.4 (1998) and 5.0% (1999) showing the importance of mid- and large-sized insects as pollinators. Natural levels of insect activity were sufficient to ensure complete pollination in both years as supplementary hand-pollination did not significantly increase either parameter in plots where pollinators had free access. Nocturnal insects may serve as pollinators (fruit-set = 41%), although they were less effective than diurnal pollinators (fruit-set = 93%).     

Community structure of pollination webs of Mauritian heathland habitats

Pollination webs have recently deepened our understanding of complex ecosystem functions and the susceptibility of biotic networks to anthropogenic disturbances. Extensive mutualistic networks from tropical species-rich communities, however, are extremely scarce. We present fully quantitative pollination webs of two plant–pollinator communities of natural heathland sites, one of which was in the process of being restored, on the oceanic island of Mauritius. The web interaction data cover a full flowering season from September 2003 to March 2004 and include all flowering plant and their pollinator species. Pollination webs at both sites were dominated by a few super-abundant, disproportionately well-connected species, and many rare and specialised species. The webs differed greatly in size, reflecting higher plant and pollinator species richness and abundance at the restored site. About one fifth of plant species at the smaller community received <3 visits. The main pollinators were insects from diverse taxonomic groups, while the few vertebrate pollinator species were abundant and highly linked. The difference in plant community composition between sites appeared to strongly affect the associated pollinator community and interactions with native plant species. Low visitation rate to introduced plant species suggested little indirect competition for pollinators with native plant species. Overall, our results indicated that the community structure was highly complex in comparison to temperate heathland communities. We discuss the observed differences in plant linkage and pollinator diversity and abundance between the sites with respect to habitat restoration management and its influence on pollination web structure and complexity. For habitat restoration to be successful in the long term, practitioners should aim to maintain structural diversity to support a species-rich and abundant pollinator assemblage which ensures native plant reproduction.     

Effect of habitat disturbance on pollination biology of the columnar cactus Stenocereus quevedonis at landscape‐level in central Mexico

Stenocereus quevedonis (‘pitire’) is a columnar cactus endemic to central Mexico, grown for its edible fruit. Phenology, pollination biology and behaviour of flower visitors of this species were compared in six conserved and disturbed sites, hypothesising that: (i) pitire pollination is self‐incompatible, requiring animal vectors; (ii) higher incidence of radiation on plants in cleared forest may lead to a higher number of flowers per pitire plant and longer blooming season, and disturbing and differential spatial availability of flower resources may determine differential attraction of pollinators to conserved and disturbed areas; (iii) if pitire pollination system is specialised, reproductive success would decrease with pollinator scarcity, or other species may substitute for main pollinators. In all sites, pitire reproduction started in January, flowering peak occurring in April, anthesis duration was 15 h and predominantly nocturnal (9 h), pollen was released at 23:00 h, nectar was produced throughout anthesis, and breeding system was self‐incompatible. Flower production per plant was similar in disturbed and conserved sites, but flower availability was higher (because of higher tree density) and longer in disturbed sites. Pollination is nocturnal, the most frequent legitimate pollinator being the bat Leptonycteris yerbabuenae; diurnal pollination is rare but possible, carried out by bee species. Fruit and seed set in control and nocturnal pollination treatments at disturbed sites were higher than in conserved sites. Frequency of L. yerbabuenae visits was similar among site types, but more visits of complementary nocturnal and diurnal pollinators were recorded in disturbed sites, which could explain differences in reproductive success.     

Pollination niche overlap between a parasitic plant and its host

Niche theory predicts that species which share resources should evolve strategies to minimise competition for those resources, or the less competitive species would be extirpated. Some plant species are constrained to co-occur, for example parasitic plants and their hosts, and may overlap in their pollination niche if they flower at the same time and attract the same pollinators. Using field observations and experiments between 1996 and 2006, we tested a series of hypotheses regarding pollination niche overlap between a specialist parasitic plant Orobanche elatior (Orobanchaceae) and its host Centaurea scabiosa (Asteraceae). These species flower more or less at the same time, with some year-to-year variation. The host is pollinated by a diverse range of insects, which vary in their effectiveness, whilst the parasite is pollinated by a single species of bumblebee, Bombus pascuorum, which is also an effective pollinator of the host plant. The two species therefore have partially overlapping pollination niches. These niches are not finely subdivided by differential pollen placement, or by diurnal segregation of the niches. We therefore found no evidence of character displacement within the pollination niches of these species, possibly because pollinators are not a limiting resource for these plants. Direct observation of pollinator movements, coupled with experimental manipulations of host plant inflorescence density, showed that Bombus pascuorum only rarely moves between inflorescences of the host and the parasite and therefore the presence of one plant is unlikely to be facilitating pollination in the other. This is the first detailed examination of pollination niche overlap in a plant parasite system and we suggest avenues for future research in relation to pollination and other shared interactions between parasitic plants and their hosts.     

Pollinator individual‐based networks reveal the specialized plant–pollinator mutualism in two biodiverse communities

Generalization of pollination systems is widely accepted by ecologists in the studies of plant–pollinator interaction networks at the community level, but the degree of generalization of pollination networks remains largely unknown at the individual pollinator level. Using potential legitimate pollinators that were constantly visiting flowers in two alpine meadow communities, we analyzed the differences in the pollination network structure between the pollinator individual level and species level. The results showed that compared to the pollinator species‐based networks, the linkage density, interaction diversity, interaction evenness, the average plant linkage level, and interaction diversity increased, but connectance, degree of nestedness, the average of pollinator linkage level, and interaction diversity decreased in the pollinator individual‐based networks, indicating that pollinator individuals had a narrower food niche than their counterpart species. Pollination networks at the pollinator individual level were more specialized at the network level (H′₂) and the plant species node level (d′) than at the pollinator species‐level networks, reducing the chance of underestimating levels of specialization in pollination systems. The results emphasize that research into pollinator individual‐based pollination networks will improve our understanding of the pollination networks at the pollinator species level and the coevolution of flowering plants and pollinators.     

Relative importance of diurnal and nocturnal pollinators for reproduction in the early spring flowering shrub Stachyurus praecox (Stachyuraceae)

Generalized pollination systems may be favored in early spring flowering plants, as during this period pollinator activity is unpredictable. Many previous studies have concentrated on the importance of diurnal visitors in pollination, and consequently, information on the contribution of nocturnal visitors to pollination in early spring is limited. This study was conducted to evaluate the relative importance of diurnal and nocturnal pollinators in the early spring flowering dioecious shrub Stachyurus praecox (Stachyuraceae), in two temperate forests in central Japan. Visitors to the female and male flowers were observed during day and night, and their relative contributions to seed set were compared. The pollinator observations revealed that the diurnal and nocturnal insects visited both male and female flowers, and that the main flower visitors were diurnal small bees and flies as well as nocturnal settling moths. The diurnal and nocturnal flower visitors also acted as pollinators, as the pollen grains of S. praecox were attached to the insects collected from the female flowers. Pollination experiments demonstrated that the contributions of diurnal pollinators to the seed set were higher than those of the nocturnal pollinators. The results of this study indicate that S. praecox has a generalized pollination system, comprising both diurnal insects and nocturnal settling moths. Although the roles of diurnal insects are more important in the pollination of S. praecox, nocturnal settling moths may have a complementary role in early spring.