Pollination ecology of a plant in its native and introduced areas

Exotic and native plants play equally important roles in supporting and structuring plant-hummingbird networks within urban green spaces

Background

Urban gardens, despite their transformed nature, serve as invaluable microcosms for a quantitative examination of floral resource provision to urban pollinators, considering the plant's origin. Thus, knowledge has increased, emphasizing the importance of these green areas for hosting and conserving pollinator communities. However, there is a significant knowledge gap concerning the changing availability of these native and exotic floral resources over time and their impact on structuring interaction networks with specific pollinators.

Methods

Over a year-long period, monthly surveys were conducted to record both native and exotic plant species visited by hummingbirds in an urban garden at Tlaxcala, Mexico. Flower visits were recorded, and the total flowers on each plant visited were tallied. Additionally, all observed hummingbirds were recorded during the transect walks, regardless of plant visits, to determine hummingbird abundance. The interactions were summarized using matrices, and network descriptors like connectance, specializacion, nestedness, and modularity were computed. Plant and hummingbird species in the core and periphery of the network were also identified. Lastly, simulations were performed to assess the network's resilience to the extinction of highly connected native and exotic plant species, including those previously situated in the network's core.

Results

We recorded 4,674 interactions between 28 plant species, and eight hummingbird species. The majority of plants showed an ornithophilic syndrome, with 20 species considered exotic. Despite asynchronous flowering, there was overlap observed across different plant species throughout the year. Exotic plants like Jacaranda mimosifolia and Nicotiana glauca produced more flowers annually than native species. The abundance of hummingbirds varied throughout the study, with Saucerottia berillyna being the most abundant species. The plant-hummingbird network displayed high connectance, indicating generalization in their interaction. Significant nestedness was observed, mainly influenced by exotic plant species. The core of the network was enriched with exotic plants, while Basilinna leucotis and Cynanthus latirostris played central roles among hummingbirds. Network resilience to species extinction remained generally high.

Conclusions

Our findings provide valuable insights into the dynamics and structure of plant-hummingbird interactions in urban gardens, emphasizing the influence of exotic plant species and the network's resilience to perturbations. Understanding and managing the impact of exotic plants on such networks is crucial for the conservation and sustainable functioning of urban ecosystems.

Advancing the missed mutualist hypothesis, the under-appreciated twin of the enemy release hypothesis

Introduced species often benefit from escaping their enemies when they are transported to a new range, an idea commonly expressed as the enemy release hypothesis. However, species might shed mutualists as well as enemies when they colonize a new range. Loss of mutualists might reduce the success of introduced populations, or even cause failure to establish. We provide the first quantitative synthesis testing this natural but often overlooked parallel of the enemy release hypothesis, which is known as the missed mutualist hypothesis. Meta-analysis showed that plants interact with 1.9 times more mutualist species, and have 2.3 times more interactions with mutualists per unit time in their native range than in their introduced range. Species may mitigate the negative effects of missed mutualists. For instance, selection arising from missed mutualists could cause introduced species to evolve either to facilitate interactions with a new suite of species or to exist without mutualisms. Just as enemy release can allow introduced populations to redirect energy from defence to growth, potentially evolving increased competitive ability, species that shift to strategies without mutualists may be able to reallocate energy from mutualism toward increased competitive ability or seed production. The missed mutualist hypothesis advances understanding of the selective forces and filters that act on plant species in the early stages of introduction and establishment and thus could inform the management of introduced species.

Reproductive Biology of Rheum webbianum Royle, a Vulnerable Medicinal Herb From Alpines of North-Western Himalaya

Information on reproductive biology and pollination ecology studies of threatened plants are essential to develop strategies for their sustainable utilization and effective conservation. As such, these studies were conducted on Rheum webbianum, a high-value "vulnerable" medicinal herb of the north-western Himalaya. This species presents a unique mode of reproductive behavior through the involvement of different floral events, including the movement of reproductive organs. The plants survive extremely cold conditions through underground perennating rhizomes that sprout into juvenile shoots with the onset of the favorable climatic conditions. The peduncle arises from the axils of the radical leaves, bearing a globular collection of densely arranged hermaphrodite flowers with temporally separated male and female phases; the male phase precedes the female phase (protandry). Anther dehiscence and stigma receptivity is post-anthesis. Anthers dehisce longitudinally along margins, liberating a large mass of spherical and tricolpate pollen with spinulose exine. Pollen viability decreased to < 10% on day 9. Pistil is tristylous, with each style terminating into a fan-shaped stigma lobe. The pollen receptive surface of each stigmatic lobe remains incurved at an angle of 360° and shows upward movement after anthesis, forming a funnel-like structure at an angle of 180° with respect to the ovary. Pollination syndrome is ambophilous. Spontaneous autogamy or geitonogamy to a certain extent is achieved in this species due to the arrangement of flowers in the inflorescence and overlapping of male and female reproductive phases among them. Incurved stigmatic lobes and outward movement of stamens too facilitate outcrossing. Pollen/ovule ratio estimates, results of pollination experiments on breeding behavior, outcrossing, and self-compatibility indices demonstrated that plants are self-compatible and cross-fertile.

Impacts of plant invasions in native plant-pollinator networks

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.

Stigmatic limitations on reproductive success in a paleotropical tree: causes and consequences

Success in reproduction is subject to the successful initiation as well as successful completion of a chain of consecutive events starting from flower formation and ending with viable seed production. A pivotal role in this chain is played by the stigma which is the seat of pollen recognition and initiation of pollen-pistil interaction. An interesting feature of the family Bignoniaceae is the presence of thigmosensitive stigmas which open, close and re-open in response to touch. Kigelia pinnata bears a touch sensitive stigma and is a low fruit setter in Jammu and Kashmir (India). One possible reason might be pollen limitation coupled with reported self-incompatibility. However, not much is known about the mechanism of self-incompatibility in K. pinnata or of the role of its thigmotropic stigma. Carefully designed manual pollination experiments along with critical field observations revealed naturally deposited pollen load to be too low to cause permanent closure of stigma lobes. A strong relationship exists between the threshold pollen load on stigma, its permanent closure and fertilization. Of the various pollination treatments undertaken, fruits were formed only in open and manual cross pollinations when ∼9200 pollen of legitimate type is deposited on the stigma. Thigmosensitivity further limited the opportunity for the deposition of optimum pollen loads. Although, frequented by as many as seven different visitors, an average of 8 bats per night are available for as many as 30 trees which reflects a baseline deficiency of effective pollinators. This limitation in pollen and pollinator availability affects pollination success of this species and ends up with low fruit set. Fluorescence microscopy reveals successful germination and tube growth of pollen grains of both self and cross type but fruit-set is 100 % in cross-pollinated pistils only. Despite slower rates of ovule penetration and evidence of delayed fertilization, absolutely no fruit initiation occurs in self-pollinated pistils. This strongly points towards self incompatibility being late acting.

Mating system, population growth, and management scenario for Kalanchoe pinnata in an invaded seasonally dry tropical forest

Ecological invasions are a major issue worldwide, where successful invasion depends on traits that facilitate dispersion, establishment, and population growth. The nonnative succulent plant Kalanchoe pinnata, reported as invasive in some countries, is widespread in remnants of seasonally dry tropical forest on a volcanic outcrop with high conservation value in east‐central Mexico where we assessed its mating system and demographic growth and identified management strategies. To understand its local mating system, we conducted hand‐pollination treatments, germination, and survival experiments. Based on the experimental data, we constructed a life‐stage population matrix, identified the key traits for population growth, weighted the contributions of vegetative and sexual reproduction, and evaluated management scenarios. Hand‐pollination treatments had slight effects on fruit and seed setting, as well as on germination. With natural pollination treatment, the successful germination of seeds from only 2/39 fruit suggests occasional effective natural cross‐pollination. The ratios of the metrics for self‐ and cross‐pollinated flowers suggest that K. pinnata is partially self‐compatible. Most of the pollinated flowers developed into fruit, but the seed germination and seedling survival rates were low. Thus, vegetative propagation and juvenile survival are the main drivers of population growth. Simulations of a virtual K. pinnata population suggest that an intense and sustained weeding campaign will reduce the population within at least 10 years. Synthesis and applications. The study population is partially self‐compatible, but sexual reproduction by K. pinnata is limited at the study site, and population growth is supported by vegetative propagation and juvenile survival. Demographic modeling provides key insights and realistic forecasts on invasion process and therefore is useful to design management strategies.

Direct and Indirect Influence of Non-Native Neighbours on Pollination and Fruit Production of a Native Plant

Background

Entomophilous non-native plants can directly affect the pollination and reproductive success of native plant species and also indirectly, by altering the composition and abundance of floral resources in the invaded community. Separating direct from indirect effects is critical for understanding the mechanisms underlying the impacts of non-native species on recipient communities.

Objectives

Our aims are: (a) to explore both the direct effect of the non-native Hedysarum coronarium and its indirect effect, mediated by the alteration of floral diversity, on the pollinator visitation rate and fructification of the native Leopoldia comosa and (b) to distinguish whether the effects of the non-native species were due to its floral display or to its vegetative interactions.

Methods

We conducted field observations within a flower removal experimental setup (i.e. non-native species present, absent and with its inflorescences removed) at the neighbourhood scale.

Results

Our study illustrates the complexity of mechanisms involved in the impacts of non-native species on native species. Overall, Hedysarum increased pollinator visitation rates to Leopoldia target plants as a result of direct and indirect effects acting in the same direction. Due to its floral display, Hedysarum exerted a direct magnet effect attracting visits to native target plants, especially those made by the honeybee. Indirectly, Hedysarum also increased the visitation rate of native target plants. Due to the competition for resources mediated by its vegetative parts, it decreased floral diversity in the neighbourhoods, which was negatively related to the visitation rate to native target plants. Hedysarum overall also increased the fructification of Leopoldia target plants, even though such an increase was the result of other indirect effects compensating for the observed negative indirect effect mediated by the decrease of floral diversity.