Pollinator competition and the contingency of nectar depletion during an early spring resource pulse

Concerns about competition between pollinators are predicated on the assumption of floral resource limitation. Floral resource limitation, however, is a complex phenomenon involving the interplay of resource production by plants, resource demand by pollinators, and exogenous factors-like weather conditions-that constrain both plants and pollinators. In this study, we examined nectar limitation during the mass flowering of rosaceous fruit trees in early spring. Our study was set in the same region as a previous study that found severe nectar limitation in summer grasslands. We used this seasonal contrast to evaluate two alternative hypotheses concerning the seasonal dynamics of floral resource limitation: either (H1) rates of resource production and consumption are matched through seasonal time to maintain a consistent degree of resource limitation, or (H2) a mismatch of high floral resource production and low pollinator activity in early spring creates a period of relaxed resource limitation that intensifies later in the year. We found generally much lower depletion in our spring study compared to the near 100% depletion found in the summer study, but depletion rates varied markedly through diel time and across sampling days, with afternoon depletion rates sometimes exceeding 80%. In some cases, there were also pronounced differences in depletion rates across simultaneously sampled floral species, indicating different degrees of nectar exploitation. These findings generally support the seasonal mismatch hypothesis (H2) but underscore the complex contingency of nectar depletion. The challenge of future work is to discern how the fluctuation of resource limitation across diel, inter-diel, and seasonal time scales translates into population-level outcomes for pollinators.

Insect root feeders incur negative density-dependent damage across plant species in an alpine meadow

Although herbivores are well known to incur positive density-dependent damage and mortality, thereby likely shaping plant community assembly, the response of belowground root feeders to changes in plant density has seldom been addressed. Locally rare plant species (with lower plant biomass per area) are often smaller with shallower roots than common species (with higher plant biomass per area) in competition-intensive grasslands. Likewise, root feeders are often distributed in the upper soil layers. We hypothesized, therefore, that root feeders would incur negative density (biomass)-dependent damage across plant species. To test this hypothesis, we investigated the diversity and abundance of plant and root feeder species in an alpine meadow and determined the diet of the root feeders using metabarcoding. Across all species, root feeder load decreased with increasing aboveground plant biomass, root biomass, and total plant biomass per area, indicating a negative density dependence of damage across plant species. Aboveground plant biomass per area increased with increasing individual plant biomass and root depth per area across species, suggesting that rare plant species were smaller in size and had shallower root systems compared to common plant species. Both root biomass per area and root feeder biomass per area decreased with soil depth, but the root feeder biomass decreased disproportionately faster compared to root biomass with increasing root depth. Root feeder load decreased with increasing root depth but was not correlated with the feeding preference of root feeder species. Moreover, the prediction derived from a random process incorporating vertical distributions of root biomass and root feeder biomass significantly accounted for interspecific variation in root feeder load. In conclusion, the data indicate that root feeders incur negative density-dependent damage across plant species. On this basis, we suggest that manipulative experiments should be conducted to determine the effect of the negative density-dependent damage on plant community structure and that different types of plant-animal interactions should be concurrently examined to fully understand the effect of plant density on overall herbivore damage across plant species.

Evolutionary plant-pollinator responses to anthropogenic land-use change: impacts on ecosystem services

Agricultural intensification at field and landscape scales, including increased use of agrochemicals and loss of semi-natural habitats, is a major driver of insect declines and other community changes. Efforts to understand and mitigate these effects have traditionally focused on ecological responses. At the same time, adaptations to pesticide use and habitat fragmentation in both insects and flowering plants show the potential for rapid evolution. Yet we lack an understanding of how such evolutionary responses may propagate within and between trophic levels with ensuing consequences for conservation of species and ecological functions in agroecosystems. Here, we review the literature on the consequences of agricultural intensification on plant and animal evolutionary responses and interactions. We present a novel conceptualization of evolutionary change induced by agricultural intensification at field and landscape scales and emphasize direct and indirect effects of rapid evolution on ecosystem services. We exemplify by focusing on economically and ecologically important interactions between plants and pollinators. We showcase available eco-evolutionary theory and plant-pollinator modelling that can improve predictions of how agricultural intensification affects interaction networks, and highlight available genetic and trait-focused methodological approaches. Specifically, we focus on how spatial genetic structure affects the probability of propagated responses, and how the structure of interaction networks modulates effects of evolutionary change in individual species. Thereby, we highlight how combined trait-based eco-evolutionary modelling, functionally explicit quantitative genetics, and genomic analyses may shed light on conditions where evolutionary responses impact important ecosystem services.

Nitrogen addition affects floral and vegetative traits, reproduction, and pollinator performance in Capsicum annuum L

BACKGROUND AND AIMS

It has been demonstrated that nitrogen (N) addition alters flower morphology, floral rewards and pollinator performance. However, little is known about the effects of N addition on plant reproduction, including fruit set and seed set during selfing and outcrossing, floral and vegetative traits, and pollinator performance. We hypothesized that N addition would influence fruit set, seed set in selfed and outcrossed flowers, the relationship between vegetative and flower traits, and pollinator performance.

METHODS

A 2-year pot experiment was conducted in which Capsicum annuum was exposed to three levels of relatively short-term N supply, i.e. 0 g m-2 (no N addition, as a control), 4 g m-2 (4N) and 16 g m-2 (16N), which are equivalent to about 0-, 1- and 4-fold of the peak local N deposition. We measured flower rewards, flower morphology, flowering phenology, as well as pollinator visitation rate, fruit set and seed set by self- and outcross-fertilization of C. annuum.

RESULTS

The four levels of N addition increased plant biomass, biomass allocation to flowers, flower size, stigma-anther separation, nectar production and pollen production, resulting in an increase in pollinator visitation and fruit set. Nevertheless, the control and 16 levels of N addition reduced plant biomass, biomass allocation to flowers, flower size and stigma-anther separation, and nectar and pollen production, and consequently decreased pollinator visitation and fruit set. Exclusion of pollinators and hand-pollination experiments revealed that low levels of N addition were associated with high seed set in outcrossed flowers; however, this trend was reversed in flowers grown in the control and 16N treatments.

CONCLUSION

Our results suggest that an optimal level of 4N can enhance the correlation between flower traits, pollinator performance and plant reproduction. Our findings cast new light on the underlying mechanisms of plant-pollinator interactions and plant adaptation to nitrogen deposition.

Nitrogen Addition Affects Interannual Variation in Seed Production in a Tibetan Perennial Herb

The variability observed in the annual seed production of perennial plants can be seen as an indication of changes in the allocation of resources between growth and reproduction, which can be attributed to fluctuations in the environment. However, a significant knowledge gap exists concerning the impacts of nitrogen addition on the interannual seed production patterns of perennial plants. We hypothesized that the addition of nitrogen would impact the annual variations in the seed production of perennial plants, ultimately affecting their overall reproductive efficiency. A multiyear field experiment was conducted to investigate the effects of varying nitrogen supply levels (e.g., 0, 4, and 8 kg N ha-1 yr-1 of N0, N4, and N8) on vegetative and floral traits, pollinator visitation rates, and seed traits over a period of four consecutive years. The results showed that the N0 treatment exhibited the highest levels of seed production and reproductive efficiency within the initial two years. In contrast, the N4 treatment displayed its highest level of performance in these metrics in the second and third years, whereas the N8 treatment showcased its most favorable outcomes in the third and fourth years. Similar patterns were found in the number of flowers per capitulum and the number of capitula per plant. There exists a positive correlation between aboveground biomass and several factors, including the number of flowers per capitulum, the number of capitula per plant, the volume of nectar per capitulum, and the seed production per plant. A positive correlation was found between pollinator visitation and the number of flowers per capitulum or the number of capitula per plant. This implies that the addition of N affected the maintenance of plant aboveground biomass, flower trait stability, pollinator visitation, and, subsequently, the frequency of seed production and reproductive efficiency. Our results suggest that augmenting the nitrogen content in the soil may have the capacity to modify the inherent variability in seed production that is observed across various years and enhance the effectiveness of reproductive processes. These findings have the potential to enhance our comprehension of the impact of nitrogen addition on the reproductive performance of perennial herbaceous plants and the underlying mechanisms of biodiversity in the context of global environmental changes.

Impacts of soil nutrition on floral traits, pollinator attraction, and fitness in cucumbers (Cucumis sativus L.)

Annual plants allocate soil nutrients to floral display and pollinator rewards to ensure pollination success in a single season. Nitrogen and phosphorus are critical soil nutrients whose levels are altered by intensive land use that may affect plants' fitness via pollinator attractiveness through floral display and rewards. In a controlled greenhouse study, we studied in cucumbers (Cucumis sativus) how changes in soil nitrogen and phosphorus influence floral traits, including nectar and pollen reward composition. We evaluated how these traits affect bumble bee (Bombus impatiens, an important cucumber pollinator) visitation and ultimately fruit yield. While increasing nitrogen and phosphorus increased growth and floral display, excess nitrogen created an asymptotic or negative effect, which was mitigated by increasing phosphorus. Male floral traits exhibited higher plasticity in responses to changes in soil nutrients than female flowers. At 4:1 nitrogen:phosphorus ratios, male flowers presented increased nectar volume and pollen number resulting in increased bumble bee visitation. Interestingly, other pollinator rewards remained consistent across all soil treatments: male and female nectar sugar composition, female nectar volume, and pollen protein and lipid concentrations. Therefore, although cucumber pollination success was buffered in conditions of nutrient stress, highly skewed nitrogen:phosphorus soil ratios reduced plant fitness via reduced numbers of flowers and reward quantity, pollinator attraction, and ultimately yield.

Floral economies

Biology and economics are surprisingly similar disciplines. At their core, both fields are the study of competitive interactions for scarce resources and the consequences of those interactions over time. Perhaps the first person to notice this similarity was Charles Darwin, who credited his reading of the influential economist Thomas Robert Malthus with catalysing his understanding of natural selection as the driving force of evolution. While it may not have been recognised at the time, this was not the only area of Darwin's thinking to parallel economic concepts.

Soil moisture affects plant-pollinator interactions in an annual flowering plant

Many environmental factors impact plant and pollinator communities. However, variation in soil moisture and how it mediates the plant-pollinator interactions has yet to be elucidated. We hypothesized that long-term variation in soil moisture can exert a strong selective pressure on the floral and vegetative traits of plants, leading to changes in pollinator visitation. We demonstrated that there are three phenotypic populations of Gentiana aristata in our study alpine region in the Qinghai-Tibetan Plateau that vary in floral colour and other traits. Pink (dry habitat) and blue (intermediate habitat) flower populations are visited primarily by bumblebees, and white (wet habitat) flower populations are visited by flies. These patterns of visitation are driven by vegetative and floral traits and are constant when non-endemic plants are placed in the intermediate habitats. Additionally, the floral communities in different habitats vary, with more insect-pollinated forbs in the dry and intermediate habitats versus the wet habitats. Through a common garden and reciprocal transplant experiment, we demonstrated that plant growth traits, pollinator attractiveness and seed production are highest when the plant population is raised in its endemic habitat. This suggests that these plant populations have evolved to pollinator communities associated with habitat differences. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Rare plant species are at a disadvantage when both herbivory and pollination interactions are considered in an alpine meadow

Rare plant species often suffer less damage than common species because of positive density-dependent herbivory, and it has been suggested that this 'rare species advantage' fosters plant species coexistence. However, it is unknown whether rare species have an advantage when pollination interactions are also considered. We hypothesized that a 'positive density-dependent pollination success' across plant species would result in common plants experiencing higher seed set rates compared to rare species, and that positive density-dependent effects would negate or even override the positive density-dependent damage due to herbivory resulting in higher seed loss rates in common plant species. We tested this hypothesis by concurrently examining a plant-predispersal seed predator system and a plant-pollinator system for 24 Asteraceae species growing in an alpine meadow community (Sichuan Province, China). Having previously reported a positive density-dependent effect on seed loss rates due to seed predators, we here focus on the density-dependent effects on pollination success by investigating pollinator species richness, visitation frequencies and seed set rates for each plant species. We also estimated the seed output rate of each plant species as the product of seed set rate and the rate of surviving seeds (i.e. 1 - the seed loss rate). Consistent with our hypothesis, a positive density-dependent effect was observed for pollinator species richness, visitation frequencies and seed set rates across plant species. Moreover, the positive effect overrode the negative density-dependent effect of herbivores on seed production, such that common species tended to have a higher seed output rate than rare species (i.e. we observed a 'rare species disadvantage'). These results indicate that the low seed output rate of rare species might result from a pollination limitation, and that both mutualistic and antagonistic interactions should be examined simultaneously to fully understand plant species coexistence in local communities.

Changes in Community Composition Induced by Experimental Warming in an Alpine Meadow: Beyond Plant Functional Type

Climate warming exerts profound effects on plant community composition. However, responses to climate warming are often reported at the community and functional type levels, but not at the species level. To test whether warming-induced changes are consistent among community, functional type, and species levels, we examined the warming-induced changes at different levels in an alpine meadow from 2015 to 2018. The warming was achieved by deploying six (open top) chambers [including three non-warmed chambers and three warmed chambers; 15 × 15 × 2.5 m (height) for each] that resulted in a small increase in mean annual temperature (0.3–0.5°C, varying with years) with a higher increase during the non-growing season (0.4–0.6°C) than in the growing season (0.03–0.47°C). The results show that warming increased plant aboveground biomass but did not change species richness, or Shannon diversity and evenness at the community level. At the functional type level, warming increased the relative abundance of grasses from 3 to 16%, but decreased the relative abundance of forbs from 89 to 79%; relative abundances of sedges and legumes were unchanged. However, for a given functional type, warming could result in contrasting effects on the relative abundance among species, e.g., the abundances of the forb species Geranium pylzowianum, Potentilla anserine, Euphrasia pectinate, and the sedge species Carex atrofusca increased in the warmed (compared to the non-warmed) chambers. More importantly, the difference in species identity between warmed and non-warmed chambers revealed warming-induced species loss. Specifically, four forb species were lost in both types of chambers, one additional forb species (Angelica apaensis) was lost in the non-warmed chambers, and two additional species (one forb species Saussurea stella and one sedge species Blysmus sinocompressus) were lost in the warmed chambers. Consequently, changes at the species level could not be deduced from the results at the community or functional type levels. These data indicate that species-level responses to climate changes must be more intensively studied. This work also highlights the importance of examining species identity (and not only species number) to study changes of community composition in response to climate warming.

Plant volatiles mediate evolutionary interactions between plants and tephritid flies and are evolutionarily more labile than non-volatile defenses

Studies show that plant defenses influence the host-use of herbivores and tend to be evolutionarily more labile than herbivore traits (e.g. feeding preferences). However, all previous studies have focused exclusively on non-volatile plant defenses thereby overlooking the roles of plant volatiles. We hypothesized that volatiles are equally important determinants of herbivore host-use and are evolutionarily more labile than herbivore traits. To test these hypotheses, the following experiments were conducted. We identified the volatiles and non-volatiles of 17 Asteraceae species and measured their relative contents. We also used a highly resolved bipartite trophic network of the 17 host species and 20 herbivorous (pre-dispersal seed predator) tephritid fly species to determine the evolutionary interactions between plants and herbivores. The chemical data showed that interspecific similarity in volatiles-but not non-volatiles and phylogenetic distance-significantly accounted for the herbivore community across the plant species; this implies that plant volatiles-but not non-volatile compounds and species identity-dictate plant-tephritid fly interactions. Moreover, we observed phylogenetic signal for non-volatiles but not for volatiles; therefore closely related herbivores do not necessarily use closely related host species with similar non-volatiles, but do tend to attack plants producing similar volatiles. Thus, plant volatiles are evolutionarily more labile than non-volatiles and herbivore traits associate with host use. These results show that the interactions between plants and herbivores are evolutionary asymmetric, shed light on the role of plant volatiles in plant-herbivore interactions, and highlight the need to include data for both volatiles and non-volatiles when investigating plant-animal interactions.

Plant evolution can mediate negative effects from honey bees on wild pollinators

Pollinators are introduced to agroecosystems to provide pollination services. Introductions of managed pollinators often promote ecosystem services, but it remains largely unknown whether they also affect evolutionary mutualisms between wild pollinators and plants.Here, we developed a model to assess effects of managed honey bees on mutualisms between plants and wild pollinators. Our model tracked how interactions among wild pollinators and honey bees affected pollinator and plant populations.We show that when managed honey bees have a competitive advantage over wild pollinators, or a greater carrying capacity, the honey bees displace the wild pollinator. This leads to reduced plant density because plants benefit less by visits from honey bees than wild pollinators that coevolved with the plants.As wild pollinators are displaced, plants evolve by increasing investment in traits that are attractive for honey bees but not wild pollinators. This evolutionary switch promotes wild pollinator displacement. However, higher mutualism investment costs by the plant to the honey bee can promote pollinator coexistence.Our results show plant evolution can promote displacement of wild pollinators by managed honey bees, while limited plant evolution may lead to pollinator coexistence. More broadly, effects of honey bees on wild pollinators in agroecosystems, and effects on ecosystem services, may depend on the capacity of plant populations to evolve.

Pollination ecology in China from 1977 to 2017

China is one of most biodiverse countries in the world, containing at least 10% of all angiosperm species. Therefore, we should anticipate a diverse, pollinator fauna. China also has a long history of applied ethnobiology, including a sustainable agriculture based on apiculture and plant-pollinator interactions. However, the science of pollination ecology is a far younger sub-discipline in China, compared to in the West. Chinese studies in pollination ecology began in the 1970s. For this review, we compiled a complete reference database (>600 publications) of pollination studies in China. Using this database, we identified and analyzed gaps and limitations in research on the pollination systems of native and naturalized species. Specifically, we asked the following questions: 1) What do we know about the pollination systems of native, Chinese species? 2) How does Chinese pollination ecology compare with the development of pollination research abroad and which aspects of research should be pursued by Chinese anthecologists in the near future? 3) What research on pollination in China will advance our understanding and contribute to our ongoing analyses of endemism and conservation? Subsequently, we segregated and identified prospective lines of future research that are unique to China and can only be done in China. This requires discussing priorities within a systematic approach.

Domestic honeybees affect the performance of pre-dispersal seed predators in an alpine meadow

Flowering plants interact simultaneously with mutualistic pollinators and antagonistic herbivores such that plant-mediated interactions between pollinators and herbivores must exist. Although the effects of herbivores on pollinator behavior have been investigated extensively, the effect of pollinators on herbivore performance has seldom been explored. We hypothesized that insect pollinators could improve the survival and growth of pre-dispersal seed predators by increasing seed production. We tested this hypothesis along three transects radiating from well-established apiaries in an alpine meadow by supplementing pollination in sites close to and distant from apiaries and subsequently examining seed production of the dominant nectariferous plant species Saussurea nigrescens (Asteraceae) and the performance of three dominant pre-dispersal seed predators (tephritid fly species). Pollen supplementation (1) significantly increased seed set and mass of developed seed per capitulum (i.e., flowerhead) in the distant but not the close sites, (2) did not change the survival and growth rates of the smaller-bodied species (Tephritis femoralis and Campiglossa nigricauda) at either site, but (3) improved the performance of the larger-bodied seed predator (Terellia megalopyge) at distant sites but not close sites. In addition, the larger-bodied tephritid fly showed higher infestation rates and relative abundance in the close sites than in the distant sites, whereas the smaller-bodied species had lower relative abundances in the close sites and similar infestation rates in both site types. These observations demonstrate contrasting effects of plant mutualists on the performance of antagonists with potential consequences for population sizes of insect herbivores.

The worldwide importance of honey bees as pollinators in natural habitats

The western honey bee (Apis mellifera) is the most frequent floral visitor of crops worldwide, but quantitative knowledge of its role as a pollinator outside of managed habitats is largely lacking. Here we use a global dataset of 80 published plant-pollinator interaction networks as well as pollinator effectiveness measures from 34 plant species to assess the importance of A. mellifera in natural habitats. Apis mellifera is the most frequent floral visitor in natural habitats worldwide, averaging 13% of floral visits across all networks (range 0-85%), with 5% of plant species recorded as being exclusively visited by A. mellifera For 33% of the networks and 49% of plant species, however, A. mellifera visitation was never observed, illustrating that many flowering plant taxa and assemblages remain dependent on non-A. mellifera visitors for pollination. Apis mellifera visitation was higher in warmer, less variable climates and on mainland rather than island sites, but did not differ between its native and introduced ranges. With respect to single-visit pollination effectiveness, A. mellifera did not differ from the average non-A. mellifera floral visitor, though it was generally less effective than the most effective non-A. mellifera visitor. Our results argue for a deeper understanding of how A. mellifera, and potential future changes in its range and abundance, shape the ecology, evolution, and conservation of plants, pollinators, and their interactions in natural habitats.

Experimental warming and precipitation interactively modulate the mortality rate and timing of spring emergence of a gallmaking Tephritid fly

Global climate change is mostly characterized by temperature increase and fluctuating precipitation events, which may affect the spring phenology and mortality rate of insects. However, the interaction effect of temperature and precipitation on species performance has rarely been examined. Here we studied the response of the gall-making Tephritid fly Urophora stylata (Diptera: Tephritidae) to artificial warming, changes in precipitation, and the presence of galls. Our results revealed a significant interaction effect of warming, precipitation, and galls on the life-history traits of the focal species. Specifically, when the galls were intact, warming had no effect on the phenology and increased the mortality of the fly under decreased precipitation, but it significantly advanced the timing of adult emergence and had no effect on the mortality under increased precipitation. When galls were removed, warming significantly advanced the timing of emergence and increased fly mortality, but precipitation showed no effect on the phenology and mortality. In addition, gall removal significantly increased adult fresh mass for both females and males. Our results indicate that the effect of elevated temperature on the performance of species may depend on other environmental conditions, such as variations in precipitation, and species traits like the formation of galls.

Artificial asymmetric warming reduces nectar yield in a Tibetan alpine species of Asteraceae

BACKGROUND AND AIMS

Asymmetric warming is one of the distinguishing features of global climate change, in which winter and night-time temperatures are predicted to increase more than summer and diurnal temperatures. Winter warming weakens vernalization and hence decreases the potential to flower for some perennial herbs, and night warming can reduce carbohydrate concentrations in storage organs. This study therefore hypothesized that asymmetric warming should act to reduce flower number and nectar production per flower in a perennial herb, Saussurea nigrescens, a key nectar plant for pollinators in Tibetan alpine meadows.

METHODS

A long-term (6 years) warming experiment was conducted using open-top chambers placed in a natural meadow and manipulated to achieve asymmetric increases in temperature, as follows: a mean annual increase of 0·7 and 2·7 °C during the growing and non-growing seasons, respectively, combined with an increase of 1·6 and 2·8 °C in the daytime and night-time, respectively, from June to August. Measurements were taken of nectar volume and concentration (sucrose content), and also of leaf non-structural carbohydrate content and plant morphology.

KEY RESULTS

Six years of experimental warming resulted in reductions in nectar volume per floret (64·7 % of control), floret number per capitulum (8·7 %) and capitulum number per plant (32·5 %), whereas nectar concentration remained unchanged. Depletion of leaf non-structural carbohydrates was significantly higher in the warmed than in the ambient condition. Overall plant density was also reduced by warming, which, when combined with reductions in flower development and nectar volumes, led to a reduction of ∼90 % in nectar production per unit area.

CONCLUSIONS

The negative effect of asymmetric warming on nectar yields in S. nigrescens may be explained by a concomitant depletion of leaf non-structural carbohydrates. The results thus highlight a novel aspect of how climate change might affect plant-pollinator interactions and plant reproduction via induction of allocation shifts for plants growing in communities subject to asymmetric warming.

Bee nutrition and floral resource restoration

Bee-population declines are linked to nutritional shortages caused by land-use intensification, which reduces diversity and abundance of host-plant species. Bees require nectar and pollen floral resources that provide necessary carbohydrates, proteins, lipids, and micronutrients for survival, reproduction, and resilience to stress. However, nectar and pollen nutritional quality varies widely among host-plant species, which in turn influences how bees forage to obtain their nutritionally appropriate diets. Unfortunately, we know little about the nutritional requirements of different bee species. Research must be conducted on bee species nutritional needs and host-plant species resource quality to develop diverse and nutritionally balanced plant communities. Restoring appropriate suites of plant species to landscapes can support diverse bee species populations and their associated pollination ecosystem services.