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Low Pollinator Sharing Between Coexisting Native and Non-native Plant Pairs: The Effect of Corolla Length and Flower Abundance
Abstract
Understanding the mechanisms by which non-native plants can attract pollinators in their new geographical zones is important because such species infiltrate native communities and can disrupt native ecological interactions. Despite the large number of studies assessing how invasive plants impact plant-pollinator interactions, the specific comparison of pollination interactions between native and non-native plant pairs has received much less attention. Here we focused on four coexisting co-flowering pairs of common native and non-native species, both with abundant flowers but different floral traits, and asked: (1) to what extent native and non-native plants share pollinator species, and whether the non-native plants attract a different set of pollinators, (2) whether the most shared pollinators are the most frequent floral visitors and the most generalized in their interactions, and (3) how much of the variation in the diversity and frequency of pollinator species between native and non-native plant species can be explained by floral trait dissimilarity and flower abundance. Direct pollinator observations revealed that the plant pairs shared a low fraction (0-33%) of insect species, i.e., non-native plants tended to acquire a different set of pollinators than their native counterparts. The most shared pollinators in each plant pair were the most common but not the most generalized species, and non-native species attracted both generalized and specialized pollinators. Corolla length at opening and flower abundance showed to be important in determining the differences in flower visitation rate between natives and non-natives. Our findings support the general pattern that non-native species have no barriers at the pollination stage to integrate into native communities and that they may attract a different assemblage of pollinators relative to those that visit native plants with which they coexist.
... However, none of them has been recorded on giant hogweed. In contrast with some other invasive plants, giant hogweed does not seem to be that important for insects as a source of pollen, and the results copy those of studies in which invasive and native plant species were compared (Seitz et al. 2020;Abdallah et al. 2021;Parra-Tabla and Arceo-Gómez 2021). Most species use large composite flowers of this plant only or predominantly as a source of nectar, and larger and numerous species can be found on composite flowers of this plant in very high numbers. ...
Currently, plant invasions affect native ecosystems across the Earth. Although much attention has already been paid to their effect on local communities, we still lack basic information on the associations between alien and local species. Here, we present the results of our survey of pollinators of the invasive plant Heracleum mantegazzianum (Apiaceae) in central Europe. At 20 sites within the westernmost part of the Czech Republic, which is strongly affected by the invasion of H. mantegazzianum, pollinators on the flowers of H. mantegazzianum were examined and compared to the species composition of pollinators on native vegetation in the surrounding area. While the flowers of H. mantegazzianum were frequently visited by high abundance of insects, the communities of H. mantegazzianum pollinators were relatively species poor, and the proportion of abundances of H. mantegazzianum pollinators was very uneven, with few species of generalist Diptera and the honey bee (Apis mellifera) dominating over all other flower visitors. Significantly larger species of the family Syrphidae visited flowers of giant hogweed than of other plants. Thus, giant hogweed is not a necessary part of flower communities for flower visiting insects, and it should be eradicated because of its negative effects on other plants, landscape and humans. Our results highlight the need for more detailed studies on direct interactions between alien plant species and native pollinator communities as well as indirect interactions between alien plants and native plants through competition for pollinators.
... There are multiple characteristics that confer a given non-native species its invasive capacity, ranging from biotic interactions to many other ecological features (Abdallah et al. 2021;Traveset and Richardson 2021). Among those suggested to play an important role in plant invasiveness are the physiological traits. ...
Islands tend to be more prone to plant invasions than mainland regions, with the Mediterranean ones not being an exception. So far, a large number of studies on comparing leaf morphological and physiological traits between native and non-native plants in Mediterranean environments have been performed, although none of them on Mediterranean islands. To fill this gap, this study focuses on 14 plant species grown in a controlled growth chamber in the absence of stress. The goal was (1) to differentiate leaf morpho-physiological traits between native and non-native plants on a Mediterranean island and (2) to deepen in the underlying causes of the differential photosynthetic traits displayed by non-native species. Results showed that in Mediterranean islands, non-native plant species show on average larger values of net CO2 assimilation, stomatal conductance (gm), photosynthetic nitrogen-use efficiency, among others, and lower leaf mass per area (LMA) and leaf thickness, compared to the native species. Among the assessed traits, this study reports for the first time larger gm, and lower mesophyll conductance limitation in non-native species, which seems to be linked to their lower LMA. These novel traits need to be added to the ‘leaf physiological trait invasive syndrome’. It was also found that on a Mediterranean island, native and non-native species are placed on opposite sides of the leaf economics spectrum, with non-native species being placed on the ‘‘fast-return’’ end. In conclusion, this study demonstrates that non-native species inhabiting a Mediterranean island possess distinct leaf morphological and physiological traits compared to co-occurring native species, at least during the favorable growth season, which increases the chances of a successful invasion.
Understanding the assembly of plant-pollinator communities has become critical to their conservation given the rise of species invasions, extirpations, and species’ range shifts. Over the course of assembly, colonizer establishment produces core interaction patterns, called motifs, which shape the trajectory of assembling network structure. Dynamic assembly models can advance our understanding of this process by linking the transient dynamics of colonizer establishment to long-term network development. In this study, we investigate the role of intra-guild indirect interactions and adaptive foraging in shaping the structure of assembling plant-pollinator networks by developing: 1) an assembly model that includes population dynamics and adaptive foraging, and 2) a motif analysis tracking the intra-guild indirect interactions of colonizing species throughout their establishment. We find that while colonizers leverage indirect competition for shared mutualistic resources to establish, adaptive foraging maintains the persistence of inferior competitors. This produces core motifs in which specialist and generalist species coexist on shared mutualistic resources which leads to the emergence of nested networks. Further, the persistence of specialists develops richer and less connected networks which is consistent with empirical data. Our work contributes new understanding and methods to study the effects of species’ intra-guild indirect interactions on community assembly.
Invasive species are a major threat to Earth's biodiversity, particularly in unique ecosystems such as the Galapagos Islands. Research on the ecology and genetics of these invasive species is essential to understand their interactions with native and endemic flora, and to alleviate the negative effects of these invasions. In one of our studies, it was found that that the most likely origin of the invasive tomatillo in Galapagos is the central region of mainland Ecuador. Hybridization between the invasive and two endemic tomato species was observed. This could imply an imminent fast extinction risk for the endemic tomatoes, as several of the populations reported decades ago couldn't be found anymore. Moreover, genetic hijacking by the invasive species could lead to an even more aggressive invasive tomatillo in the Galapagos Islands.We also study the Guava, which is one of the most aggressive invasive plants in the Galapagos Islands, displacing and outcompeting its endemic relative, the guayabillo, for resources and space. There is also the possibility of hybridization of guava with its endemic relative, which could lead to the fast extinction of the latter. However, this hybridization is probably not occurring, yet guava could still interfere with the successful reproduction of guayabillo, decreasing its populations. The most likely origin of the guava in Galapagos would be the Central Highlands of mainland Ecuador. A better understanding of the interactions between invasive and endemic plants can contribute to the conservation of the endemic species and a better management of invasive species.
Understanding the assembly of plant-pollinator communities has become critical to their conservation given the rise of species invasions, extirpations, and species’ range shifts. Dynamic network analyses can advance such understanding by elucidating the effects of species’ direct and indirect interactions on network assembly. We investigate the role of intra-guild indirect interactions on assembling plant-pollinator networks by developing: 1) an assembly model that includes population dynamics and adaptive foraging, and 2) a motif analysis tracking the effects of colonizing species throughout assembly. Motifs — networks’ building blocks — allow an in-depth analysis of indirect interactions. We found that adaptive foraging reduces the otherwise strong effect of indirect competition for shared mutualistic resources on community assembly. This allows for coexistence between specialist and generalist species within the same guild and the emergence of nested networks. Further, by maintaining the persistence of specialist species, who are most vulnerable to competitive exclusion, adaptive foraging allows for the development of richer and less connected networks which is consistent with empirical pollination networks. Our work contributes new understanding and methods to study the effects of species’ indirect interactions on community assembly.
This book contains 23 chapters divided into seven parts. Part I reviews the key hypotheses in invasion ecology that invoke biotic interactions to explain aspects of plant invasion dynamics; and reviews models, theories and hypotheses on how invasion performance and impact of introduced species in recipient ecosystems can be conjectured according to biotic interactions between native and non-native species. Part II deals with positive and negative interactions in the soil. Part III discusses mutualistic interactions that promote plant invasions. Part IV describes antagonistic interactions that hinder plant invasions, while part V presents the consequences of plant invasions for biotic interactions among native species. In part VI, novel techniques and experimental approaches in the study of plant invasions are shown. In the last part, biotic interactions and the management of ecosystems invaded by non-native plants are discussed.
Investigating the factors that determine whether interactions are competitive or facilitative is essential to understanding community structure and trait evolution. Co-flowering plants interact indirectly through shared pollinators, and meta-analyses suggest that phylogenetic relatedness and floral trait similarity may predict the outcome of these interactions. In a comparative approach, we manipulated the floral community across five focal species to assess how floral similarity and phylogenetic relatedness affect the outcome of interactions. To assess the extent of pollinator-mediated competition versus facilitation, we compared pollen limitation in five focal species growing with floral neighbors (either congeners or neighbors from a different family) relative to a control (growing alone). We measured floral morphology, color, and nectar traits to calculate multivariate floral similarity between species pairs and inferred a phylogeny to calculate phylogenetic distance. Pollina-tor-mediated interaction values were regressed against floral similarity and phylogenetic distance. We found evidence of pollinator-mediated facilitation in nine of 13 species pairs. Furthermore, floral similarity and phylogenetic distance reduced facilitative interactions, but the latter relationship was not significant when controlling for the identity of the focal species. Our results suggest that facilitative pollinator sharing is more common than reported in the literature, but co-flowering plant species with similar floral traits are less likely to facilitate pollination. A better understanding of the factors that promote facilitation versus competition has important potential applications for managing rare and invasive species.
Pollinator‐mediated processes (biotic filtering, facilitation or competition) are often inferred by patterns of plant reproductive trait diversity (clustering or evenness of reproductive traits within the community). However, one single pattern can be generated by distinct processes, making difficult to predict the main process of community assembly. Incorporating fitness estimates should improve the link between pattern and process.
We investigated patterns of flowering phenology and reproductive traits (floral colour, floral size and anther height) along the season of a pollinator‐depauperated and generalized community. We used data on fitness (pollen receipt and number of pollen tubes) to provide a functional link between trait patterns and assembly mechanisms. We also investigated whether the degree of co‐flowering depended on the floral abundance and pollination functional group (fly‐, bee‐, hummingbird‐pollinated and generalist species) of the plant species.
High floral abundance in the flowering season was associated with low trait diversity in the community. Both features increased fitness at the community level. This indicates that similar species are benefited at periods of high floral abundance, probably due to the joint attraction of generalist pollinators in this pollinator‐depauperated community. In general, rare species flowered more synchronously with the community than abundant ones, although distinct patterns emerged depending on the floral trait and pollination functional group. Furthermore, species highly synchronous and possessing similar floral colour in relation to the community had higher fitness indicating that facilitative mechanisms act favouring flowering synchrony and trait similarity.
Synthesis. Patterns of flowering synchrony and floral trait similarity indicate pollination facilitation in the studied community. Plants benefited from co‐flowering with species possessing similar floral colour via shared pollinator attraction. Thus, we empirically demonstrated some of the predictions of community assembly theory.
The interactions between pairs of native and alien plants via shared use of pollinators have been widely studied. Community level studies however, are necessary in order to fully understand the factors and mechanisms that facilitate successful plant invasion, but these are still scarce. Specifically, few community level studies have considered how differences in invasion level (alien flower abundance), and degree of floral trait similarity between native and invasive species, mediate effects on native plant-pollinator communities. Here, we evaluated the role of alien species on overall plant-floral visitor network structure, and on species-level network parameters, across nine invaded coastal communities distributed along 205 km in Yucatán, México that vary in alien species richness and flower abundance. We further assessed the potential the role of alien plant species on plant-floral visitor network structure and robustness via computational simulation of native and invasive plant extinction scenarios. We did not find significant differences between native and alien species in their functional floral phenotypes or in their visitation rate and pollinator community composition in these invaded sites. Variation in the proportion of alien plant species and flower abundance across sites did not influence plant-pollinator network structure. Species-level network parameters (i.e., normalized degree and nestedness contribution) did not differ between native and alien species. Furthermore, our simulation analyses revealed that alien species are functionally equivalent to native species and contribute equally to network structure and robustness. Overall, our results suggest that high levels of floral trait similarity and pollinator use overlap may help facilitate the integration of alien species into native plant-pollinator networks. As a result, alien species may also play a similar role than that of natives in the structure and stability of native plant and pollinator communities in the studied coastal sand dune ecosystem.
Introduced plants become decoupled from their usual pollinators and by relying on resident pollinator species or other reproductive strategies become established in new areas, spread and, eventually, invade. Here, using breeding system experiments, we studied the reproductive biology of the bird-pollinated South American species Nicotiana glauca across native and non-native areas, both inside and outside the range of flower-visiting birds. In the native range, where the species is visited by hummingbirds, open cross-pollinated flowers set as many seeds and almost as many fruits as open pollination controls, suggesting that pollinators make a major contribution to reproductive success. In South Africa, the flowers were pollinated by sunbirds which, although less efficiently, also contributed to N. glauca reproduction, replacing hummingbirds. In contrast, in Mallorca, where nectar feeding birds are absent, fruit production in open cross-pollinated flowers was near zero, and significantly lower than in open pollination controls, suggesting that reproduction is almost entirely by autonomous self-pollination. Hand-pollination experiments showed that the species is self-compatible throughout its range. Pollinator exclusion experiment showed that N. glauca relies on self-pollination only in pollinator poor areas, where plants have a much higher capacity for autonomous self-pollination than elsewhere. A reduction in anther-stigma distance does not seem to account the higher self-pollination capacity in the non-native environment without pollinators. Despite probable evolutionary adaptation, and flexibility in pollinator association and mating system, seed production in the introduced range was somewhat pollen limited, suggesting that the mismatch between N. glauca and its novel pollination environment might retard invasion.
Plant–animal mutualistic networks sustain terrestrial biodiversity and human food security. Global environmental changes threaten these networks, underscoring the urgency for developing a predictive theory on how networks respond to perturbations. Here, I synthesise theoretical advances towards predicting network structure, dynamics, interaction strengths and responses to perturbations. I find that mathematical models incorporating biological mechanisms of mutualistic interactions provide better predictions of network dynamics. Those mechanisms include trait matching, adaptive foraging, and the dynamic consumption and production of both resources and services provided by mutualisms. Models incorporating species traits better predict the potential structure of networks (fundamental niche), while theory based on the dynamics of species abundances, rewards, foraging preferences and reproductive services can predict the extremely dynamic realised structures of networks, and may successfully predict network responses to perturbations. From a theoretician's standpoint, model development must more realistically represent empirical data on interaction strengths, population dynamics and how these vary with perturbations from global change. From an empiricist's standpoint, theory needs to make specific predictions that can be tested by observation or experiments. Developing models using short‐term empirical data allows models to make longer term predictions of community dynamics. As more longer term data become available, rigorous tests of model predictions will improve.
Corollas (or perianths), considered to contribute to pollinator attraction during anthesis, persist after anthesis in many plants. However, their post-floral function has been little investigated within a cost-benefit framework. We explored the adaptive significance of corolla retention after anthesis for reproduction in Fritillaria delavayi, a perennial herb endemic to the alpine areas of the Hengduan Mountains, southwestern China. We examined whether the persistent corollas enhance reproductive success during seed development. Persistent corollas increased fruit temperature on sunny days, and greatly decreased the intensity of ultraviolet-B/C (UV-B/C) radiation reaching fruits. When corollas were removed immediately after pollination, fecundity and progeny quality were adversely affected. Measurements of flower mass and size showed no further corolla growth during fruiting, and respiration and transpiration tests demonstrated that both respiration rate and transpiration rate of corollas were much lower during fruiting than during flowering, indicating a slight additional resource investment in corolla retention after anthesis. Thus, seed production by F. delavayi may be facilitated by corolla retention during seed development at only a small physiological cost. We conclude that corolla retention may be an adaptive strategy that enhances female reproductive success by having a protective role for ripening seeds in the harsh conditions at high elevation.
Withania somniferous (L.) Dunal (Family: Solanaceae, commonly known as Ashwagandha, English name: Winter cherry) is an important perennial plant species with immense therapeutic uses in traditional as well as modern system of medicine (Datta et al 2010). Due to restorative property of roots, the species is also known as 'Winter cherry' (Tripathi et al 1996; Andallu & Radhika 2000, Winters 2006). The Indian Himalayan region (BHR), one of the richest reservoirs of biological diversity in the world, is undergoing irrational extraction of wild, medicinal herbs, thus endangering many of its high value gene stock. Withonia somnifera L. (Dunal) is a member of solanaceae, also known for thousands of years by Ayurvedic practitioners. Withania somnifera root contains flavonoids, alkaloids, steroid and many active functional ingredients (Kumar et al 2015). Withania sominifera having small white flowers mainly in rainy and winter seasons that can be develop into fruit during the winter seasons. Plants products can be obtained from the roots, leaves, and branches, by using many different biological techniques. Withania which is also known as Ashwagandha having effective Volume: 1; Issue: 7; December-2015. ISSN: 2454-5422 Naveen Gaurav et al 2015 165 property can also used in blends and supplements which are designed to show many multiple effects. It is described as an herbal tonic and health food in Vedas and considered as 'Indian Ginseng' in traditional Indian system of medicine (Singh et al 2001).
Through competition for pollinators, invasive plants may suppress native flora. Community-level studies provide an integrative assessment of invasion impacts and insights into factors that influence the vulnerability of different native species. We investigated effects of the non-native herb Lythrum salicaria on pollination of native species in 14 fens of the eastern United States. We compared visitors per flower for 122 native plant species in invaded and uninvaded fens and incorporated a landscape-scale experiment, removing L. salicaria flowers from three of the invaded fens. Total flower densities were more than three times higher in invaded than uninvaded or removal sites when L. salicaria was blooming. Despite an increase in number of visitors with number of flowers per area, visitors per native flower declined with increasing numbers of flowers. Therefore, L. salicaria invasion depressed visitation to native flowers. In removal sites, visitation to native flowers was similar to uninvaded sites, confirming the observational results and also suggesting that invasion had not generated a persistent buildup of visitor populations. To study species-level impacts, we examined effects of invasion on visitors per flower for the 36 plant species flowering in both invaded and uninvaded fens. On average, the effect of invasion represented about a 20% reduction in visits per flower. We measured the influence of plant traits on vulnerability to L. salicaria invasion using meta-analysis. Bilaterally symmetrical flowers experienced stronger impacts on visitation, and similarity in flower color to L. salicaria weakly intensified competition with the invader for visitors. Finally, we assessed the reproductive consequences of competition with the invader in a dominant flowering shrub, Dasiphora fruticosa. Despite the negative effect of invasion on pollinator visitation in this species, pollen limitation of seed production was not stronger in invaded than uninvaded sites, suggesting little impact of competition for pollinators on its population demography. Negative effects on pollination of native plants by this copiously-flowering invader appeared to be mediated by increases in total flower density that were not matched by increases in pollinator density. The strength of impact was modulated across native species by their floral traits and reproductive ecology. This article is protected by copyright. All rights reserved.
Background and aims:
Colour is one of the main floral traits used by pollinators to locate flowers. Although pollinators show innate colour preferences, the view that the colour of a flower may be considered an important predictor of its main pollinators is highly controversial because flower choice is highly context-dependent, and initial innate preferences may be overridden by subsequent associative learning. Our objective is to establish whether there is a relationship between flower colour and pollinator composition in natural communities.
Methods:
We measured the flower reflectance spectrum and pollinator composition in four plant communities (85 plant species represented by 109 populations, and 32 305 plant-pollinator interactions in total). Pollinators were divided into six taxonomic groups: bees, ants, wasps, coleopterans, dipterans and lepidopterans.
Key results:
We found consistent associations between pollinator groups and certain colours. These associations matched innate preferences experimentally established for several pollinators and predictions of the pollination syndrome theory. However, flowers with similar colours did not attract similar pollinator assemblages.
Conclusions:
The explanation for this paradoxical result is that most flower species are pollination generalists. We conclude that although pollinator colour preferences seem to condition plant-pollinator interactions, the selective force behind these preferences has not been strong enough to mediate the appearance and maintenance of tight colour-based plant-pollinator associations.
Large floral displays favour pollinator attraction and the import and export of pollen. However, large floral displays also have negative effects, such as increased geitonogamy, pollen discounting and nectar/pollen robber attraction. The size of the floral display can be measured at different scales (e.g. the flower, inflorescence or entire plant) and variations in one of these scales may affect the behaviour of flower visitors in different ways. Moreover, the fragmentation of natural forests may affect flower visitation rates and flower visitor behaviour. In the present study, video recordings of the inflorescences of a tree species (Tabebuia aurea) from the tropical savannah of central Brazil were used to examine the effect of floral display size at the inflorescence and tree scales on the visitation rate of pollinators and nectar robbers to the inflorescence, the number of flowers approached per visit, the number of visits per flower of potential pollinators and nectar robbers, and the interaction of these variables with the degree of landscape disturbance. Nectar production was quantified with respect to flower age. Although large bees are responsible for most of the pollination, a great diversity of flower insects visit the inflorescences of T. aurea. Other bee and hummingbird species are highly active nectar robbers. Increases in inflorescence size increase the visitation rate of pollinators to inflorescences, whereas increases in the number of inflorescences on the tree decrease visitation rates to inflorescences and flowers. This effect has been strongly correlated with urban environments in which trees with the largest floral displays are observed. Pollinating bees (and nectar robbers) visit few flowers per inflorescence and concentrate visits to a fraction of available flowers, generating an overdispersed distribution of the number of visits per inflorescence and per flower. This behaviour reflects preferential visits to young flowers (including flower buds) with a greater nectar supply.
Background and aims:
Brassicaceae is one of the most diversified families in the angiosperms. However, most species from this family exhibit a very similar floral bauplan. In this study, we explore the Brassicaceae floral morphospace, examining how corolla shape variation (an estimation of developmental robustness), integration and disparity vary among phylogenetically related species. Our aim is to check whether these floral attributes have evolved in this family despite its apparent morphological conservation, and to test the role of pollinators in driving this evolution.
Methods:
Using geometric morphometric tools, we calculated the phenotypic variation, disparity and integration of the corolla shape of 111 Brassicaceae taxa. We subsequently inferred the phylogenetic relationships of these taxa and explored the evolutionary lability of corolla shape. Finally, we sampled the pollinator assemblages of every taxon included in this study, and determined their pollination niches using a modularity algorithm. We explore the relationship between pollination niche and the attributes of corolla shape.
Key results:
Phylogenetic signal was weak for all corolla shape attributes. All taxa had generalized pollination systems. Nevertheless, they belong to different pollination niches. There were significant differences in corolla shape among pollination niches even after controlling for the phylogenetic relationship of the plant taxa. Corolla shape variation and disparity was significantly higher in those taxa visited mostly by nocturnal moths, indicating that this pollination niche is associated with a lack of developmental robustness. Corolla integration was higher in those taxa visited mostly by hovering long-tongued flies and long-tongued large bees.
Conclusions:
Corolla variation, integration and disparity were evolutionarily labile and evolved very recently in the evolutionary history of the Brassicaceae. These floral attributes were strongly related to the pollination niche. Even in a plant clade having a very generalized pollination system and exhibiting a conserved floral bauplan, pollinators can drive the evolution of important developmental attributes of corolla shape.
Aim To investigate the role of alien plants in mutualistic plant–hummingbird
networks, assessing the importance of species traits, floral abundance and insularity
on alien plant integration.
Location Mainland and insular Americas.
Methods We used species-level network indices to assess the role of alien
plants in 21 quantitative plant–hummingbird networks where alien plants
occur. We then evaluated whether plant traits, including previous adaptations
to bird pollination, and insularity predict these network roles. Additionally, for
a subset of networks for which floral abundance data were available, we tested
whether this relates to network roles. Finally, we tested the association between
hummingbird traits and the probability of interaction with alien plants across
the networks.
Results Within the 21 networks, we identified 32 alien plant species and 352
native plant species. On average, alien plant species attracted more hummingbird
species (i.e. aliens had a higher degree) and had a higher proportion of
interactions across their hummingbird visitors than native plants (i.e. aliens
had a higher species strength). At the same time, an average alien plant was visited
more exclusively by certain hummingbird species (i.e. had a higher level of
complementary specialization). Large alien plants and those occurring on
islands had more evenly distributed interactions, thereby acting as connectors.
Other evaluated plant traits and floral abundance were unimportant predictors
of network roles. Short-billed hummingbirds had higher probability of including
alien plants in their interactions than long-billed species.
Main conclusions Once incorporated into plant–hummingbird networks, alien
plants appear strongly integrated and, thus, may have a large influence on network dynamics. Plant
traits and floral abundance were generally poor predictors of how well alien
species are integrated. Short-billed hummingbirds, often characterized as functionally
generalized pollinators, facilitate the integration of alien plants. Our
results show that plant–hummingbird networks are open for invasion.
Biological invasions are a threat to the maintenance of ecological processes, including pollination. Plant-flower visitor networks are traditionally used as a surrogated for pollination at the community level, despite they do not represent the pollination process, which takes place at the stigma of plants where pollen grains are deposited. Here we investigated whether the invasion of the alien plant Impatiens glandulifera (Balsaminaceae) affects pollen transfer at the community level. We asked whether more alien pollen is deposited on the stigmas of plants on invaded sites, whether deposition is affected by stigma type (dry, semidry and wet) and whether the invasion of I. glandulifera changes the structure of the resulting pollen transfer networks. We sampled stigmas of plants on 10 sites invaded by I. glandulifera (hereafter, balsam) and 10 non-invaded control sites. All 20 networks had interactions with balsam pollen, although significantly more balsam pollen was found on plants with dry stigmas in invaded areas. Balsam pollen deposition was restricted to a small subset of plant species, which is surprising because pollinators are known to carry high loads of balsam pollen. Balsam invasion did not affect the loading of native pollen, nor did it affect pollen transfer network properties; networks were modular and poorly nested, both of which are likely to be related to the specificity of pollen transfer interactions. Our results indicate that pollination networks become more specialized when moving from the flower visitation to the level of pollen transfer networks. Therefore, caution is needed when inferring pollination from patterns of insect visitation or insect pollen loads as the relationship between these and pollen deposition is not straightforward.
Oceanic archipelagos are threatened by the introduction of alien species which can severely disrupt the structure, function and stability of native communities. Here we investigated the pollination interactions in the two most disturbed Galápagos Islands, comparing the three main habitats and the two seasons, and assessing the impacts of alien plant invasions on network structure. We found that the pollination network structure was rather consistent between the two islands, but differed across habitats and seasons. Overall, the arid zone had the largest networks and highest species generalization levels whereas either the transition between habitats or the humid habitat showed lower values. Our data suggest that alien plants integrate easily into the communities, but with low impact on overall network structure, except for an increase in network selectiveness. The humid zone showed the highest nestedness and the lowest modularity, which might be explained by the low species diversity and the higher incidence of alien plants in this habitat. Both pollinators and plants were also more generalized in the hot season, when networks showed to be more nested. Alien species (both plants and pollinators) represented a high fraction (c. 56%) of the total number of interactions in the networks. It is thus likely that, in spite of the overall weak effect we found of alien plant invasion on pollination network structure, these introduced species influence the reproductive success of native ones, and by doing so, they affect the functioning of the community. This certainly deserves further investigation.
Published by Oxford University Press on behalf of the Annals of Botany Company.
Generalized pollination systems and autonomous self-fertilization are traits that have been linked with plant invasiveness. However, whether specialized pollination requirements pose a significant barrier to plant invasions is not yet clear. Likewise, the contribution of pollinators to the fecundity of facultatively self-pollinating invasive plant species is poorly understood. We addressed these issues using the self-compatible and autonomously self-pollinating Lilium formosanum, which also has large, showy flowers that are adapted for pollination by hawk moths. We investigated the pollination of this lily-which is indigenous to Taiwan-in KwaZulu-Natal, South Africa, where it is invasive. The long-tongued hawk moth Agrius convolvuli was identified as the primary pollinator on the basis of field observations, pollen load analysis, presence of lepidopteran scales on stigmas, and higher seed production in emasculated flowers exposed at night than in those exposed during the day. Remarkably, this moth is native to much of the Old World, including Taiwan and South Africa. Autonomous self-pollination resulted in seed production, but at a reduced level relative to the seed production of open-and hand-pollinated flowers, which was significant in one out of two populations examined. Thus, pollinators potentially contribute to invasion by increasing seed production and genetic variability through cross-pollination, although contributions of pollinators to seed set versus that of autonomous self-pollination may vary between populations. We conclude that specialized pollination requirements do not present a barrier to invasions when plants are specialized to pollinators or pollinator functional groups with very wide distributions.
There is increasing world-wide concern about the impact of the introduction of exotic species on ecological communities. Since many exotic plants depend on native pollinators to successfully establish, it is of paramount importance that we understand precisely how exotic species integrate into existing plant–pollinator communities.
In this manuscript, we have studied a global data base of empirical pollination networks to determine whether community, network, species or interaction characteristics can help identify invaded communities.
We found that a limited number of community and network properties showed significant differences across the empirical data sets – namely networks with exotic plants present are characterized by greater total, plant and pollinator richness, as well as higher values of relative nestedness.
We also observed significant differences in terms of the pollinators that interact with the exotic plants. In particular, we found that specialist pollinators that are also weak contributors to community nestedness are far more likely to interact with exotic plants than would be expected by chance alone.
Synthesis. By virtue of their interactions, it appears that exotic plants may provide a key service to a community's specialist pollinators as well as fill otherwise vacant ‘coevolutionary niches’.
Although the behavioral plasticity of flower traits has received considerable attention, its adaptive value is not thoroughly understood. We experimentally examined flower opening/closure in field bindweed (Convolvulus arvensis), a species with short flower persistence (1 day) in which adaptive responses to pollination were not previously expected. In line with the pollination hypothesis we suggested that flower exposure in this species should respond to pollination. More specifically, we predicted that flower closure in the
pollinated flowers would be quicker than that in unpollinated flowers. As predicted, the unpollinated flowers were open for a longer time than the pollinated flowers. There was no difference, however, between the self- and cross-pollinated flowers in terms of flower longevity. There was an inverse relationship between flower longevity and fertility, which also suggests that pollination leads to reduced flower longevity. Collectively, our results suggest that field bindweed flowers respond to pollination adaptively, because prolonged flower
longevity may increase the likelihood of successful pollination and flower closure reduces energy spent on flower maintenance and/or intraspecific competition with genetically familiar flowers. The behavioral plasticity of flower closure does not seem to be restricted only to plants with high flower longevity.
Invasiveness of non-native species often depends on acquired interactions with either native or naturalized species. A natural colonizer, the autogamous, invasive orchid Spathoglottis plicata has acquired at least three interspecific interactions in Puerto Rico: a mycorrhizal fungus essential for seed germination and early development; a native, orchid-specialist weevil, Stethobaris polita, which eats perianth parts and oviposits in developing fruits; and ants, primarily invasive Solenopsis invicta, that forage at extrafloral nectaries. We tested in field experiments and from observational data whether weevils affect reproductive success in the orchid; and whether this interaction is density-dependent. We also examined the effectiveness of extrafloral nectaries in attracting ants that ward off weevils. Only at small spatial scales were weevil abundance and flower damage correlated with flower densities. Plants protected from weevils had less floral damage and higher fruit set than those accessible to weevils. The more abundant ants were on inflorescences, the less accessible fruits were to weevils, resulting in reduced fruit loss from larval infections. Ants did not exclude weevils, but they affected weevil activity. Native herbivores generally provide some biotic resistance to plant invasions yet Spathoglottis plicata remains an aggressive colonizer despite the acquisition of a herbivore/seed predator partly because invasive ants attracted to extrafloral nectaries inhibited weevil behavior. Thus, the invasion of one species facilitates the success of another as in invasional meltdowns. For invasive plant species of disturbed habitats, having ant-tended extrafloral nectaries and producing copious quantities of seed, biotic resistance to plant invasions can be minimal.
The Solanaceae is a plant family of great economic importance. Despite a wealth of phylogenetic work on individual clades and a deep knowledge of particular cultivated species such as tomato and potato, a robust evolutionary framework with a dated molecular phylogeny for the family is still lacking. Here we investigate molecular divergence times for Solanaceae using a densely-sampled species-level phylogeny. We also review the fossil record of the family to derive robust calibration points, and estimate a chronogram using an uncorrelated relaxed molecular clock.
Our densely-sampled phylogeny shows strong support for all previously identified clades of Solanaceae and strongly supported relationships between the major clades, particularly within Solanum. The Tomato clade is shown to be sister to section Petota, and the Regmandra clade is the first branching member of the Potato clade. The minimum age estimates for major splits within the family provided here correspond well with results from previous studies, indicating splits between tomato and potato around 8 Million years ago (Ma) with a 95% highest posterior density (HPD) 7-10 Ma, Solanum and Capsicum c. 19 Ma (95% HPD 17-21), and Solanum and Nicotiana c. 24 Ma (95% HPD 23-26).
Our large time-calibrated phylogeny provides a significant step towards completing a fully sampled species-level phylogeny for Solanaceae, and provides age estimates for the whole family. The chronogram now includes 40% of known species and all but two monotypic genera, and is one of the best sampled angiosperm family phylogenies both in terms of taxon sampling and resolution published thus far. The increased resolution in the chronogram combined with the large increase in species sampling will provide much needed data for the examination of many biological questions using Solanaceae as a model system.
The yellow horned-poppy Glaucium flavum Crantz shows a final dark germination which is of characteristically 'mediterranean' type (maximal response at the temperature range 5-15 °C), though a considerable broadening is brought about, both by a red light pulse and a stratification treatment. Seeds imbibed in darkness at 25 °C for even a few hours are induced to develop a secondary dormancy (thermodormancy) which can be released by light and stratification. The well known time dependence of light sensitivity and the gradually imposed induction of light indifference at supraoptimal temperatures have also been shown. Seeds imbibed under regimes simulating those met naturally in Greece during November or April, do not germinate when illuminated with white light (ζ = 1.26). Full manifestation of germination occurs either in complete darkness or under various, red-enriched light conditions (ζ higher than 2.07). A partial promotion is observed with very low fluence rates of white light (in the order 10-3 10-4 of daylight). The existence of a surface-avoiding seedling emergence mechanism based on lightinhibited seed germination was verified in a pot experiment under natural conditions, with seeds buried to various depths. Only those seeds buried at 0.5 cm germinate optimally and readily after the onset of the rainy season (November-December) although those at 1 and 2 cm also germinate to a considerable extent.
Facilitation occurs when an increase in the density of one species causes an increase in the population growth rate or the density of a second species. In plants, ample evidence demonstrates that one species can facilitate another by ameliorating abiotic conditions, but the hypothesis that pollination facilitation – in which the presence of one flowering species increases pollinator visits to a second species – can also occur remains controversial. To identify the necessary conditions for pollination facilitation to occur, we constructed population models of two plant species that share the same pollinator and compete for establishment sites, and we assumed that heterospecific pollen can interfere with successful seed set. We found that facilitation for pollination occurs only when the pollinator visitation rate is an initially accelerating function of the combined numbers of flowering plants of both species in a patch. The presence of a second species can allow populations of a focal species either to persist for a longer amount of time before going extinct (“weak facilitation”) or to persist indefinitely at a stable equilibrium density (“strong facilitation”). When only a single plant of either species can occupy a site, the plant species with the higher initial density can experience strong facilitation but will eventually out-compete the other species. However, when site occupancy was not exclusive, strong facilitation sometimes led to coexistence of the two species. Increasing the extent of pollen carryover increased the range of initial population densities leading to strong facilitation. In light of our theoretical results, we discuss the apparent rarity of pollination facilitation in nature.
Invasive species are frequently regarded as superlative competitors that can vegetatively crowd out natives, but little is known about whether invasives can compete for pollination services with native plants. We hypothesized that, when the showy invasive species Lythrum salicaria (purple loosestrife) was present, pollinator visitation and seed set would be reduced in a native congener, L. alatum (winged loosestrife). To test this hypothesis, we constructed mixed and monospecific plots of the two species. Over two years of study, we found that L. salicaria significantly reduced both pollinator visitation and seed set in L. alatum. Furthermore, pollinators moved frequently between the two plant species, which may cause heterospecific pollen transfer. Thus, reductions in both pollen quantity and pollen quality may reduce L. alatum seed set. If similar patterns occur in the field, invasive plants may be an even greater threat to natives than previously thought.
Not all floral visitors attracted to flowers are pollinators. Instead, some visitors circumvent the floral opening, usually removing nectar without contacting the anthers and/or stigma. Here we review the evolutionary ecology of nectar robbing from both the plant and animal perspective. Effects of robbing on female and male components of plant reproduction range from negative to positive. Their underlying mechanisms are diverse, including direct effects and indirect effects mediated through changes in pollination. We detail how plants may be able to deter robbers through morphological and chemical traits. For the evolutionary ecology of robbing to move beyond a phytocentric perspective, studies must also address the causes of robbing and the consequences for both robbers and pollinators. We use an energetics approach to evaluate these causes and consequences. Finally, we highlight unanswered questions in need of further research.
We compared the native bees visiting the flowers of three species of invasive plants, saltcedar (Tamarix spp.) and white and yellow sweet clover (Melilotus albus, M. officinalis), with those visiting seven concurrently blooming native plant species in mid-summer at three sites in Capitol Reef National Park, Utah. Overall, as many total species of bees visited the flowers of the three invasive plant species as visited the seven natives. On average, invasive species were visited by twice as many bee species as were natives. With a single exception, visitors of invasives were generalist bees, rather than specialists. Colletes petalostemonis, the only native legume specialist recorded, was an abundant forager on the flowers of both species of Melilotus, demonstrating that at least some specialist bees will move to invasive plants that are closely related to their usual hosts. Species abundant on the flowers of invasives tended to collect both pollen and nectar, suggesting that bees are using pollen of Tamarix and Melilotus to provision their offspring. We argue that invasives with entomophilous flowers are unlikely to either facilitate the reproduction of uncommon native plants or consistently compete with them for pollinators. Rather, they are likely, over time, to selectively increase the carrying capacity and population size of native bees, specifically generalists, and specialists of closely related plant species.
Major anthocyanins, ascorbic acid content, total phenolic content, and the radical scavenging activity against ABTS and DPPH radicals in petals of orange Nasturtium flowers (Tropaeolum majus), were investigated. Anthocyanin (ACN) content in the petals was 72 mg/100 g FW and pelargonidin 3-sophoroside represented 91% of the total ACN content. The ascorbic acid content was 71.5 mg/100 g and the total phenolic content as determined by the Folin–Ciocalteau method was 406 mg GAE/100 g FW. The radical scavenging activities against ABTS and DPPH radicals were 458 and 91.87 μm trolox eq/g FW, respectively. The excellent free radical scavenging activities along with high phenolic and ascorbic acid content of Nasturtium flowers suggest that they could be source of natural pigments and antioxidants for applications in functional foods.
1. Invasive alien plant species pose a severe threat to native plant communities world-wide, especially on islands. While many studies focus on the direct impact of alien plants on native systems, indirect effects of plant invaders on co-flowering natives, for example through competition for pollination services, are less well studied and the results are variable.
2. We used six temporally and taxonomically highly resolved plant–pollinator networks from the island of Mahé, Seychelles, to investigate the indirect impact of invasive alien plant species on remnant native plant communities mediated by shared pollinators. We employed fully quantitative network parameters and information on plant reproductive success, and pollinator diversity and behaviour, to detect changes in plant–pollinator networks along an invasion gradient.
3. The number of visits to and fruit set of native plants did not change with invasion intensity. Weighted plant linkage and interaction evenness, however, was lower at invaded sites than at less invaded sites. These patterns were primarily driven by shifts in interactions of the most common pollinator, the introduced honey bee Apis mellifera, while weak interactions and strong native interactions remained unchanged.
4.Synthesis. The implications of these findings are twofold: first, quantitative network parameters are important tools for detecting underlying biological patterns. Secondly, alien plants and pollinators may play a greater role in shaping network structure at high than low levels of invasion. We could not show, however, whether the presence of invasive plants result in a simplification of plant–pollinator networks that is detrimental to native plants and pollinators alike.
In a field experiment with 30 locally occurring old-field plant species grown in a common garden, we found that non-native plants suffer levels of attack (leaf herbivory) equal to or greater than levels suffered by congeneric native plants. This phylogenetically controlled analysis is in striking contrast to the recent findings from surveys of exotic organisms, and suggests that even if ‘enemy release’ does accompany the invasion process, this may not be an important mechanism of invasion, particularly for plants with close relatives in the recipient flora.
It is clear that the majority of flowering plants are pollinated by insects and other animals, with a minority utilising abiotic pollen vectors, mainly wind. However there is no accurate published calculation of the proportion of the ca 352 000 species of angiosperms that interact with pollinators. Widely cited figures range from 67% to 96% but these have not been based on firm data. We estimated the number and proportion of flowering plants that are pollinated by animals using published and unpublished community-level surveys of plant pollination systems that recorded whether each species present was pollinated by animals or wind. The proportion of animal-pollinated species rises from a mean of 78% in temperate-zone communities to 94% in tropical communities. By correcting for the latitudinal diversity trend in flowering plants, we estimate the global number and proportion of animal pollinated angiosperms as 308 006, which is 87.5% of the estimated species-level diversity of flowering plants. Given current concerns about the decline in pollinators and the possible resulting impacts on both natural communities and agricultural crops, such estimates are vital to both ecologists and policy makers. Further research is required to assess in detail the absolute dependency of these plants on their pollinators, and how this varies with latitude and community type, but there is no doubt that plant–pollinator interactions play a significant role in maintaining the functional integrity of most terrestrial ecosystems.
The effects of invasive species on native species comprise important conservation issues. Determining the mechanisms by which invasives exclude natives is indispensable to efficiently control their impact, but most invasives remain poorly studied. The purpose of this study was to elucidate potentially important but neglected mechanisms, reproductive interference, in wild Taraxacum systems, in which invasive Taraxacum
officinale has displaced its native congener T. japonicum in Japan. Hand-pollination of mixed pollen grains significantly reduced the native seed-set compared to conspecific-only pollination. Moreover, natives with a high ratio of invasive pollen on their stigmas suffered severe seed-set reduction, and the proportion of invasive pollen on native stigmas increased as frequencies of the alien neighbor increased. These results, combined with those of previous studies, revealed that depositing invasive pollen on native stigmas contributes to the observed alien-frequency-dependent reduction of native seed-set, and strongly suggest that reproductive interference was the primary cause of displacement in the Taraxacum systems.
The disruption of mutualisms by invasive species has consequences for biodiversity loss and ecosystem function. Although invasive plant effects on the pollination of individual native species has been the subject of much study, their impacts on entire plant‐pollinator communities are less understood. Community‐level studies on plant invasion have mainly focused on two fronts: understanding the mechanisms that mediate their integration; and their effects on plant‐pollinator network structure. Here we briefly review current knowledge and propose a more unified framework for evaluating invasive species integration and their effects on plant‐pollinator communities. We further outline gaps in our understanding and propose ways to advance knowledge in this field. Specifically, modeling approaches have so far yielded important predictions regarding the outcome and drivers of invasive species effects on plant communities. However, experimental studies that test these predictions in the field are lacking. We further emphasize the need to understand the link between invasive plant effects on pollination network structure and their consequences for native plant population dynamics (population growth). Integrating demographic studies with those on pollination networks is thus key in order to achieve a more predictive understanding of pollinator‐mediated effects of invasive species on the persistence of native plant biodiversity.
Species' floral traits and flowering times are known to be the major drivers of pollinator‐mediated plant–plant interactions in diverse co‐flowering communities. However, their simultaneous role in mediating plant community assembly and plant–pollinator interactions is still poorly understood. Since not all species flower at the same time, inference of facilitative and competitive interactions based on floral trait distribution patterns should account for fine phenological structure (intensity of flowering overlap) within co‐flowering communities. Such an approach may also help reveal the simultaneous action of competitive and facilitative interactions in structuring co‐flowering communities.
Here we used modularity within a co‐flowering network context, as a novel approach to detect convergent and/or over‐dispersed patterns in floral trait distribution and pollinator sharing. Specifically, we evaluate differences in floral trait and pollinator distribution patterns within (high temporal flowering overlap) and among co‐flowering modules (low temporal flowering overlap). We further evaluate the consistency of observed floral trait and pollinator sharing distribution patterns across space (three geographical regions) and time (dry and rainy seasons).
We found that floral trait similarity was significantly higher in plant species within co‐flowering modules than in species among them. This suggests pollinator facilitation may lead to floral trait convergence, but only within co‐flowering modules. However, our results also revealed seasonal and spatial shifts in the underlying interactions (facilitation or competition) driving co‐flowering assembly, suggesting that the prevalent dominant interactions are not static.
Synthesis . Overall, we provide strong evidence showing that the use of flowering time and floral trait distribution alone may be insufficient to fully uncover the role of pollinator‐mediated interactions in community assembly. Integrating this information along with patterns of pollinator sharing will greatly help reveal the simultaneous action of facilitative and competitive pollinator‐mediated interactions in co‐flowering communities. The spatial and temporal variation in flowering and trait distribution patterns observed further emphasize the importance of adopting a more dynamic view of community assembly processes.
Non-native plant species with a taxonomic affinity to co-occurring natives may perform best as invaders into plant-pollinator networks because their similarity in floral morphologies with natives enabled them to co-opt their pollinators. However, non-native species may impact the reproduction of natives by altering their pollination only if they share pollinators and if natives depend on pollinators for seed-set, premises that are often assumed but seldom tested. The non-native invasive species Taraxacum officinale (Asteraceae) co-exist and co-flower with other asteraceous species in the high elevation of the central Chilean Andes. In this study we determined the dependence on pollinators for seed-set in five co-occurring and co-flowering native asteraceous species and assessed the similarity in insect pollinators between the non-native and this five native species that differ in their floral similarity with the former species. Pollinator exclusion experiments were performed to determine the dependence of pollinators for seed-set in the native species. Characteristics such as capitula size and colour were measured to quantitatively assess the similarity in the floral morphology between T. officinale and the native species. The identity and visitation rates of insect pollinators that visit T. officinale and the five native species were determined. Native species were highly dependent on the transport of pollen by insects to produce seeds. T. officinale shared a moderate to high proportion (34–94%) of insect pollinators with the five asteraceous native species studied, but the level of overlap in pollinators was not related with the floral similarity between the non-native and the native species. The presence of the non-native species T. officinale has the potential to alter the pollinator services to co-flowering native species in the Andes of central Chile, with no higher impacts on those species with the most similar flowers.
Numerous plant species have been introduced to non-native environments across the globe. Daucus carota was introduced to North America from Europe. Its extensive presence on Nantucket, an island off the coast of Massachusetts, has been documented at least since the nineteenth century. To address whether the presence of D. carota affects the pollination of native species on Nantucket, we sought to answer three questions: 1) Does the presence of D. carota increase pollinator visits and diversity on Sericocarpus asteroides? 2) Does the removal of D. carota restore pollinator visits and diversities to those found in plots with only S. asteroides? and 3) Is there a relationship between the amount of heterospecific pollen and distance to the nearest D. carota population? To answer these questions, we determined that both species attract the same pollinators by first observing pollinator visits on D. carota, then performing the same observational study on S. asteroides. We recorded pollinator visitors in S. asteroides populations that were in allopatry and sympatry with D. carota. We followed this study with a manipulative experiment within sympatric plots in which we removed D. carota inflorescences and again recorded pollinator visits. In addition, we assayed S. asteroides inflorescences from a range of distances from D. carota for the presence of heterospecific pollen. Both species were generalist pollinated, and S. asteroides received more visits in the presence of D. carota. After the removal of D. carota inflorescences, the number and diversity of visitors declined. Finally, whereas populations of S. asteroides close to D. carota receive more overall pollen, there was no increase in heterospecific pollen.
The introduction of an alien plant is widely assumed to have negative consequences for the pollinator-mediated fitness of nearby natives. Indeed, a number of studies, including a highly cited meta-analysis, have concluded that the trend for such interactions is competitive. Here we provide evidence that publication bias and study design have obscured our ability to assess the pollinator-mediated impacts of alien plants. In a meta-analysis of 76 studies, we demonstrate that alien/native status does not predict the outcome of pollinator-mediated interactions among plants. Moreover, we found no evidence that similarity in floral traits or phylogenetic distance between species pairs influences the outcome of pollinator-mediated interactions. Instead, we report that aspects of study design, such as distance between the control and nearest neighbour, and/or the arrangement of study plants better predict the impact of a neighbour than does alien/native status. Our study sheds new light on the role that publication bias and experimental design play in the evaluation of key patterns in ecology. We conclude that, due to the absence of clear, generalisable pollinator-mediated impacts of alien species, management schemes should base decisions on community-wide assessments of the impacts of individual alien plant species, and not solely on alien/native status itself.
Invasive non‐native plants form interactions with native species and have the potential to cause direct and indirect impacts on those species, as well as the functioning of invaded ecosystems.
Many entomophilous invasive plants form interactions with resident pollinators; sometimes, these interactions are necessary for the reproductive success of the invader. However, the direct role native pollinators play in plant invasion is not well understood and varies according to invasive plant traits, including breeding system and pollination syndrome.
The majority of studies addressing impacts on plant–pollinator mutualisms have focussed on the indirect impacts of plant invasion for native plant pollination. Fewer studies have focussed on the direct effects of invasive plants on native flower visitors.
Impacts of invasive plants on native pollinators can occur at a range of scales: from the individual flower visitors (in terms of nutrition, health and fitness), to populations (size, density and growth rates), communities (richness, diversity and composition) and community‐level interactions (insect–flower interaction networks). Most research to date has focussed on community‐level impacts, with almost nothing known about the effects of invaders on native flower visitor individuals or populations.
Invasive plant traits, including reward quantity and quality, spatial and temporal availability and accessibility, modulate effects on native flower visitors, and thus, different plant species have different impacts. Similarly, flower visitors do not all respond in the same way to invasive plants. Thus, generalizations are difficult to make, but understanding impacts at the individual and population level for different visitor taxa is key to explaining community‐level impacts.
There have been varied approaches to determining impacts, with most studies attempting to compare invaded vs. non‐invaded habitats. The pros and cons of different approaches are discussed.
Since it is impractical to study every invasive plant in every ecological context in which it occurs, we recommend a better understanding of relevant individual‐level traits to predict direct interactions between invasive plants and native pollinators.
A Lay Summary is available for this article.
Determining whether and how to manage an introduced species requires basic ecological and biological knowledge. If a decision is made to actively manage an invader, doing so efficiently and effectively is critical. Basic biological or ecological information can be key in designing effective and cost-efficient management approaches. We used a greenhouse experiment and observational field studies of naturalized populations to study the modes of pollination, fecundity in the field, seed ecology, population dynamics, and demography of the introduced and toxic weed, black henbane. We showed that henbane was able to self-pollinate. Furthermore, outcrossing did not increase number or size of seeds. Plants in the sampled populations produced an estimated average of 25,300 ± 4,004 seeds by the middle of the growing season. We found no difference in the viability of field-collected seeds that were 1 to 4 yr old. The number of flowering plants in a population was best predicted by the number of rosettes at that location in July of the previous year. The probability of rosettes surviving over the winter to reach reproductive maturity increased with precipitation and growing degree days. Total population sizes fluctuated dramatically between years. Henbane populations are ephemeral, but with large seed banks, outbreaks are possible if conditions are right. Given that this weed is toxic to livestock and humans, it is important to identify infestations and manage populations. Nomenclature: Black henbane, Hyoscyamus niger L. HSYNI.
Petals of the alpine, arctic perennial herb Ranunculus glacialis are retained also during seed maturation, as opposed to most species where the petals wither after they have contributed to insect attraction during anthesis. To assess the adaptive significance of perianth retention after anthesis for annual reproduction, we experimentally removed perianths of R. glacialis and explored its impact on achene surface temperature, the growth rate of achenes, carbon allocation, and seed production. Perianth removal immediately after anthesis decreased achene surface temperature, decelerated the growth of achenes and reduced seed set, compared to plants with intact perianth. Measurement of mass allocation showed no further perianth growth during seed maturation, and a 13C labelling experiment demonstrated that photosynthate allocation to perianths during seed maturation was much smaller than developing achenes. Thus, annual seed production of R. glacialis might be accelerated by perianth retention during seed maturation, while the cost of perianth retention is small compared to that of seed development. In alpine and arctic environments, cold temperatures limit the growth rate of achenes. Hence, the heating of developing achenes by perianth retention might be an adaptive trait that enhances female reproductive success in this arctic, alpine species.
Mutualisms structure ecosystems and mediate their functioning. They also enhance invasions of many alien species. Invasions disrupt native mutu-alisms, often leading to population declines, reduced biodiversity, and al-tered ecosystem functioning. Focusing on three main types of mutualisms (pollination, seed dispersal, and plant-microbial symbioses) and drawing on examples from different ecosystems and from species-and community-level studies, we review the key mechanisms whereby such positive interactions mediate invasions and are in turn influenced by invasions. High interac-tion generalization is "the norm" in most systems, allowing alien species to infiltrate recipient communities. We identify traits that influence invasive-ness (e.g., selfing capacity in plants, animal behavioral traits) or invasibility (e.g., partner choice in mycorrhizas/rhizobia) through mutualistic interac-tions. Mutualistic disruptions due to invasions are pervasive, and subsequent cascading effects are also widespread. Ecological networks provide a useful framework for predicting tipping points for community collapse in response to invasions and other synergistic drivers of global change. Facilitation: an interaction in which the presence of one species benefits the performance of a second, neighboring species
It is widely assumed that floral diversification occurs by adaptive shifts between pollination niches. In contrast to specialized flowers, identifying pollination niches of generalist flowers is a challenge. Consequently, how generalist pollination niches evolve is largely unknown.
We apply tools from network theory and comparative methods to investigate the evolution of pollination niches among generalist species belonging to the genus Erysimum . These species have similar flowers.
We found that the studied species may be grouped in several multidimensional niches separated not by a shift of pollinators, but instead by quantitative variation in the relative abundance of pollinator functional groups. These pollination niches did not vary in generalization degree; we did not find any evolutionary trend toward specialization within the studied clade. Furthermore, the evolution of pollination niche fitted to a Brownian motion model without phylogenetic signal, and was characterized by frequent events of niche convergences and divergences.
We presume that the evolution of Erysimum pollination niches has occurred mostly by recurrent shifts between slightly different generalized pollinator assemblages varying spatially as a mosaic and without any change in specialization degree. Most changes in pollination niches do not prompt floral divergence, a reason why adaptation to pollinators is uncommon in generalist plants.
Biotic interactions do not occur in isolation but are imbedded in a network of species interactions. Network analysis facilitates the compilation and understanding of the complexity found in natural ecosystems and is a powerful tool to reveal information on the degree of specialization of the interacting partners and their niches. The indices measuring these properties are based on qualitative or quantitative observations of interactions between partners from different trophic levels, which informs about the structure of network patterns, but not about the underlying mechanisms. Functional traits may control the interaction strength between partners and also the (micro‐) structure of networks. Here, we ask whether flower visitors specialize on certain plant traits and how this trait specialization contributes to niche partitioning and interaction partner diversity.
We introduce two novel statistical approaches suited to evaluate the dimension of the realized niche and to analyse which traits determine niches. As basis for our analysis, we measured 10 quantitative flower traits and evaluated whether 31 arthropod taxa i visited flowers displaying only subsets of the available trait characteristics, indicating a specialization on these traits by narrow trait‐widths 〈 S i 〉. The product of 10 trait‐ and species‐specific trait‐widths 〈 S i 〉 was defined as trait‐volume V i (expansion of a n ‐dimensional hypervolume) occupied by each taxon i . These indices are applicable beyond flower–visitor interactions to quantify realized niches based on various biotic and abiotic factors.
Each flower visitor species showed some degree of specialization to a unique set of flower traits (realized niche). Overall, our data suggested a hierarchical sequence of flower traits influencing the flower visitors' behaviour and thus network structure: flowering phenology was found to have the strongest effect, followed by flower height, nectar‐tube depth and floral reflectance. Less important were pollen‐mass/flower, sugar/flower, anther position, phylogeny, display size and abundance.
The species‐specific specialization on traits suggests that plant communities with more diverse floral niches may sustain a larger number of flower visitors with non‐redundant fundamental niches. Our study and statistical approach provide a basis for a better understanding of how plant traits shape interactions between flowers and their visitors and thus network structure.
The possession of certain floral traits can determine which insects visit a plant species. If two species possess similar floral traits that determine shared flower visitors, floral traits can be said to mediate plant–plant interactions. Such indirect interactions are important for understanding fundamental relationships of plant communities, such as competition and facilitation but thus far have only been tested within a native community context.
We test whether floral‐trait similarity can be used to predict interactions between an invasive plant and co‐occurring native species in South Africa's Cape Floristic Region. We surveyed flower visitation at invaded and uninvaded plots across four sites and correlated floral‐trait similarity between invasive and native species with both invasion impact on native flower visitation and flower visitor overlap of natives and the invasive species.
Similarity of all traits (categorical and continuous) and categorical traits alone explained invasion impact (flower visitor overlap) between the native and invasive species. The majority of flower visitor overlap was attributed to the native honeybee Apis mellifera subsp. capensis .
This study is the first to show that floral traits can be used to predict novel plant–plant interactions, even amongst ecologically generalized flower visitors and plants and to predict potential impacts of an invasive species on native flowering communities. However, floral traits were not useful for predicting changes in visitation to plant species.
Synthesis . Results advance our understanding of the role of plant traits in ecological communities and reveal that they are important in mediating not only plant–pollinator interactions but also plant–plant interactions. Our findings also shed light on invasive–native plant interactions via pollinators and have the potential to predict certain invasion impacts.
The seeming ubiquity of spatio-temporal variation in pollination regime suggests that flowers ought to be adapted to a wide range of pollinators, yet many comparative biologists perceive that in groups with complex flowers there is considerable specialization onto pollination syndromes. Statistical documentation of such syndromes has been presented for very few groups of flowers. Accordingly, we measured, for 49 species of Penstemon and close relatives, both the morphology of the flowers and visitation by pollinators. We describe the mechanics of pollination for representative species. Ordinations show a distinct difference between hummingbird-pollinated species and hymenopteran-pollinated species. Flower color is particularly good at separating hummingbird- from hymenopteran-flowers. Other characters are also correlated with this dichotomy. Within the hymenopteran-pollinated species, there are additional relationships between floral morphology and the size of the principal pollinators. Flowers frequented by large bees, such as Xylocopa, have large open vestibules and relatively short floral tubes. Flowers frequented by smaller bees, such as Osmia, have long narrow floral tubes. Unlike nectar-collecting bees, pollen-collecting bees tend to be attracted to flowers of the hummingbird syndrome. The overarching pattern was that syndrome characterizations were successful at predicting pollination by hummingbirds versus Hymenoptera, two types of animals that are profoundly different, but less successful at predicting visitation by one kind of bee versus another.
Besides competition for abiotic resources, an increasing number of studies show evidence of the effects of invasive species
on the pollination success and reproductive output of indigenous species. We studied the effect of the invasive Impatiens glandulifera Royle on the process of reproduction in the indigenous Lythrum salicaria L. and Alisma plantago-aquatica L. and the naturalized Oenothera biennis L. The latter three species (target species) were transplanted into pots and placed in invaded and non-invaded areas. During
flowering season of each of these species, we measured species composition and abundance of pollinators, pollinator behaviour,
pollen deposition and female reproductive output of the target species. Competitive effects were found for L. salicaria, in which fewer pollinator species and number of foraging individuals were observed, and also, lower pollen deposition and
seed set were measured in these invaded populations. In contrast, the reproductive success of A. plantago-aquatica and O. biennis was not affected by the presence of I. glandulifera. Our data indicate that when invasive and indigenous species show a large overlap in pollinator community, which is the case
for I. glandulifera and L. salicaria, competition between these species can occur. When both species have a different pollinator community, pollination success
and reproductive output is not affected, even when the indigenous populations are densely and abundantly invaded.
http://onlinelibrary.wiley.com/doi/10.1111/j.1442-1984.2009.00258.x/abstract
We examined the floral display size and potential pollinators of female and male Rhus trichocarpa in northern Kyoto, Japan, in June 2005. The entomophilous pollination system comprised 431 insects and one spider belonging to 124 species. Most pollinators were non-social bees and occasional Diptera and Coleoptera, whereas eusocial bees were not dominant in the pollination system. Male-biased frequencies were observed in the eusocial bees and in some of the non-social bees, probably because they are sensitive to the larger male floral rewards. A pollinator introduction experiment confirmed that male-biased, unbiased and infrequent non-social bees can pollinate R. trichocarpa, indicating that non-social bees can contribute to fruit set. The results suggest that there are likely to be two different functional groups of generalist pollinators: (i) the majority of the pollinators may contribute to reproductive success through unbiased and occasional visits; and (ii) a minority group of eusocial bees may contribute to reproductive success through male-biased visits.
Invasive plant species are often considered as potential competitors of native species due to their usually greater capacity for col-onization and expansion, but we still have scarce information on whether invasives can also compete for pollination services with natives. In the present study, we hypothesized that the showy flowers of the highly invasive Carpobrotu spp. can compete with native species (Cistus monspeliensis, Cistus salviifolius, Anthyllis cytisoides and Lotus cytisoides) with which it shares habitat and flowering time, influencing pollinator visitation. To test this, we censused insects visiting the flowers of native species in the field and recorded the number of flowers visited in adjacent areas with and without the presence of Carpobrotus. We also assessed the presence of exotic pollen on stigmas of native species and evaluated its effect on reproduction. We detected potential competition for pollinators only in one native species (L. cytisoides), a facilitative effect in two other species (C. salviifolius and A. cytisoides), and a neutral effect in a fourth one (C. monspeliensis). Moreover, such effects appear not to be consistent in time. The presence of Carpobrotus pollen on native stigmas was almost negligible, and hand-pollination experiments showed that such exotic pollen does not interfere signifi-cantly with native pollen, not affecting seed set. Our results indicate that the role of the invasive Carpobrotus in promoting or con-straining the natural pollination dynamics is likely to have species specific effects on the native flora.