Non-native plants affect generalist pollinator diet overlap and foraging behavior indirectly,via impacts on native plant abundance

Flowering invasive plants can have dramatic effects on the resource landscape available to pollinators. Because many pollinators exhibit behavioral plasticity in response to competitor or resource density, this in turn can result in impacts on ecological processes such as pollination and plant reproduction. We examine how interactions between five common generalist eusocial bees change across an invasion gradient by examining how bee abundance and diet overlap changed with variation in both invasive plant abundance and competitor abundance in a temperate oak-savannah ecosystem. Specifically we focus on the bumblebees Bombus bifarius, B. mixtus, B. melanopygus and B. vosnesenskii, as well as the non-native honeybee Apis mellifera, and their interactions with the native flowering plants Camassia quamash, Camassia liechtlinii, and the invasive shrub Cytisus scoparius. We further examine whether changes in pollinator visits to the invasive and two common native plants can explain changes in diet overlap. Abundance of the invasive plant and other common floral resources had strong impacts on focal bee abundance, with certain species more likely to be present at highly invaded sites. This may be because highly invaded sites tended to be embedded in forested landscapes where those bees are common. Diet overlap was most affected by abundance of a common native plant, rather than the invasive plant, with diet overlap increasing non-linearly with abundance of the native plant. Furthermore, Apis mellifera, did not appear to have direct competitive effects on native bumblebees in this habitat. However, visit patterns suggest that bees most abundant at highly invaded sites may compete for access to native resources. Thus the impacts of this invasive plant on our focal bee species may be primarily indirect, via its’ competitive effects on native plants.     

Analysis of pollinator foraging: tests for non-random behaviour

1. A standardized protocol for analysing the behaviour of pollinators foraging on more than one plant type (species or morph) is needed.
2. A protocol is presented in which the first step is to test whether foraging trips are homogeneous in the frequency of visits to each plant type, or whether there are two or more groups of pollinators with different foraging preferences.
3. Tests for foraging preference and constancy in the sequence of plants visited then should be made separately for each homogeneous group.
4. A hypothetical example is given in which ignoring heterogeneity of preference would lead to the acceptance of a false null hypothesis of random behaviour.
5. A real example is given in which heterogeneity of preference was the only non-random aspect of pollinator behaviour (no significant positive constancy or overall preference), while transfer of powdered dye particles (pollen analogues) was assortative among floral morphs.
6. Heterogeneity of preference is probable, as many factors may cause individual pollinators to forage differently on the same patch of plants, and may be sufficient to produce non-random pollen transfer among plant morphs.     

Trapline foraging by bumble bees: III. Temporal patterns of visitation and foraging success at single plants

We analyzed the temporal structure of visitation by bumble beeworkers to a single Penstemon strictus plant growing in an arrayof conspecifics. When tested against a null distribution usinga randomization model, the observed pattern of arrivals forthe whole group of bees was random, but departures were clusteredin time. Certain individuals visited the plant repeatedly andfrequently throughout the day. These showed significantly regulararrival and departure schedules, which were likely producedby traplining. We explored whether these more frequent and regularforagers gained a higher reward than random or incidental plantvisitors. Using an analytical model, Possingham predicted thata dominating forager that visited a simple, renewing resourcein a regular pattern would garner higher and less variable rewardsthan random visitors. Inspired by these results, but interestedin plant-level visitation, we constructed a simulation modelof resource dynamics for a multiflowered plant with high visitation.The model incorporates the observed visitation schedules ofall bees and independent reward dynamics for each flower onthe plant. We calculated the rewards that observed bees wouldhave collected given a range of resource-renewal parameters.More frequent visitors did not return to the plant when whole-plantresource levels were higher, but these visitors did get greaterrewards. Their increased reward resulted from greater foragingefficiency, primarily through selecting (on average) more rewardingflowers than those selected by less frequent, random visitors.     

Plant population dynamics, pollinator foraging, and the selection of self-fertilization

Many flowering plants rely on pollinators, self-fertilization, or both for reproduction. We model the consequences of these features for plant population dynamics and mating system evolution. Our mating systems-based population dynamics model includes an Allee effect. This often leads to an extinction threshold, defined as a density below which population densities decrease. Reliance on generalist pollinators who primarily visit higher density plant species increases the extinction threshold, whereas autonomous modes of selfing decrease and can eliminate the threshold. Generalist pollinators visiting higher density plant species coupled with autonomous selfing may introduce an effect where populations decreasing in density below the extinction threshold may nonetheless persist through selfing. The extinction threshold and selfing at low density result in populations where individuals adopting a single reproductive strategy exhibit mating systems that depend on population density. The ecological and evolutionary analyses provide a mechanism where prior selfing evolves even though inbreeding depression is greater than one-half. Simultaneous consideration of ecological and evolutionary dynamics confirms unusual features (e.g., evolution into extinction or abrupt increases in population density) implicit in our separate consideration of ecological and evolutionary scenarios. Our analysis has consequences for understanding pollen limitation, reproductive assurance, and the evolution of mating systems.     

Field margins, foraging distances and their impacts on nesting pollinator success

The areas of wild land around the edges of agricultural fields are a vital resource for many species. These include insect pollinators, to whom field margins provide both nest sites and important resources (especially when adjacent crops are not in flower). Nesting pollinators travel relatively short distances from the nest to forage: most species of bee are known to travel less than two kilometres away. In order to ensure that these pollinators have sufficient areas of wild land within reach of their nests, agricultural landscapes need to be designed to accommodate the limited travelling distances of nesting pollinators. We used a spatially-explicit modelling approach to consider whether increasing the width of wild strips of land within the agricultural landscape will enhance the amount of wild resources available to a nesting pollinator, and if it would impact differently on pollinators with differing foraging strategies. This was done both by creating field structures with a randomised geography, and by using landscape data based upon the British agricultural landscape. These models demonstrate that enhancing field margins should lead to an increase in the availability of forage to pollinators that nest within the landscape. With the exception of species that only forage within a very short range of their nest (less than 125 m), a given amount of field margin manipulation should enhance the proportion of land available to a pollinator for foraging regardless of the distance over which it normally travels to find food. A fixed amount of field edge manipulation should therefore be equally beneficial for both longer-distance nesting foragers such as honeybees, and short-distance foragers such as solitary bees.     

Molecular assays of pollen use consistently reflect pollinator visitation patterns in a system of flowering plants

Determining how pollinators visit plants vs. how they carry and transfer pollen is an ongoing project in pollination ecology. The current tools for identifying the pollens that bees carry have different strengths and weaknesses when used for ecological inference. In this study we use three methods to better understand a system of congeneric, coflowering plants in the genus Clarkia and their bee pollinators: observations of plant–pollinator contact in the field, and two different molecular methods to estimate the relative abundance of each Clarkia pollen in samples collected from pollinators. We use these methods to investigate if observations of plant–pollinator contact in the field correspond to the pollen bees carry; if individual bees carry Clarkia pollens in predictable ways, based on previous knowledge of their foraging behaviors; and how the three approaches differ for understanding plant–pollinator interactions. We find that observations of plant–pollinator contact are generally predictive of the pollens that bees carry while foraging, and network topologies using the three different methods are statistically indistinguishable from each other. Results from molecular pollen analysis also show that while bees can carry multiple species of Clarkia at the same time, they often carry one species of pollen. Our work contributes to the growing body of literature aimed at resolving how pollinators use floral resources. We suggest our novel relative amplicon quantification method as another tool in the developing molecular ecology and pollination biology toolbox.     

Yearlong association of insect pollinator,Pseudapis oxybeloides with flowering plants: Planted forest vs. agricultural landscape

The yearlong association of a native bee, Pseudapis oxybeloides (Halictidae: Hymenoptera) was studied with 72 plant species in a sub-tropical planted forest and some adjacent agricultural landscapes at Multan, Pakistan. The study resulted in 66 interactions of P. oxybeloides with only 24 plant species in 15 families while other 48 plant species were not visited by this bee. The maximum abundance of P. oxybeloides (7–9 individuals) was recorded on Achyranthes aspera and Launaea procumbens followed by Ageratum conyzoides, Trianthema portulacastrum and Cleome viscosa (5–6 individuals). Majority (19) of plant species were visited by only 1–4 individuals. The bee activity was started in the month of March which attained its peak in May followed by a gradual decline until September. No bees were observed during the months of January and February. There was a significant positive relationship between bee abundance and number of flowering plant species. Bee abundance had a strong positive relationship with temperature while it had a strong negative relationship with relative humidity (%). Floral abundance increased with the number of flowering plant species while it was not influenced by floral span of plant species. Besides giving the floral host plants of P. oxybeloides, the current study also gives a better understanding of its seasonality along with its relationships with different biotic and abiotic factors under local conditions. These findings can help in maintaining and managing P. oxybeloides population particularly and other native bees in general at local scale.     

Impact of insect pollinator group and floral display size on outcrossing rate

Despite the strong influence of pollination ecology on the evolution of selfing, we have little information on how distinct groups of insect pollinators influence outcrossing rate. However, differences in behavior between pollinator groups could easily influence how each group affects outcrossing rate. We examined the influence of distinct insect pollinator groups on outcrossing rate in the rocky mountain columbine, Aquilegia coerulea. The impact of population size, plant density, size of floral display, and herkogamy (spatial separation between anthers and stigmas) on outcrossing rate was also considered as these variables were previously found to affect outcrossing rate in some plant species. We quantified correlations between all independent variables and used simple and two-factor regressions to determine direct and indirect impact of each independent variable on outcrossing rate. Outcrossing rate increased significantly with hawkmoth abundance but not with the abundance of any of the other groups of floral visitors, which included bumblebees, solitary bees, syrphid flies, and muscidae. Outcrossing rate was also significantly affected by floral display size and together, hawkmoth abundance and floral display size explained 87% of the variation in outcrossing rate. None of the other independent variables directly affected the outcrossing rate. This is the first report of a significant impact of pollinator type on outcrossing rate. Hawkmoths did not visit fewer flowers per plant relative to other pollinator groups but preferred visiting female-phase flowers first on a plant. Both the behavior of pollinators and floral display size affected outcrossing rate via their impact on the level of geitonogamous (among flower) selfing. Given that geitonogamous selfing is never advantageous, the variation in outcrossing rate and maintenance of mixed mating systems in populations of A. coerulea may not require an adaptive explanation.     

Variations of behavioural patterns during development in the water stick insect

Variations of associations between different behavioural patterns were analysed during post-embryonic development in the water stick insect,Ranatra linearis L. (Heteroptera: Nepidae). Behavioural patterns recorded were: quiescence, swimming, crawling, leg waving, body swaying, respiratory ascents breathing, respiratory descents, grooming and feeding activities (predatory attempts, foreleg movements, ingestion). A factorial analysis of correspondance stressed the contrast between the organization of larval behaviour and that of adult behaviour. No behavioural patterns were dropped from the repertoire during post-embryonic development ofRanatra linearis and no new patterns developed in adults outside the reproductive period. Larval behaviour was characterized by more frequent respiratory activities and grooming and adult behaviour was characterized by more frequent locomotory activities. At all larval stages grooming was associated with respiratory activities and crawling, but not with feeding activities.     

克隆生长对被子植物传粉过程的影响

克隆植物与其传粉者的相互作用是植物繁殖生态学的重要研究领域之一。植物克隆生长与有性繁殖通常相伴进行, 往往产生较大的花展示与复杂的克隆空间结构, 通过传粉过程对有性繁殖过程产生影响, 共同决定植物的适合度。本文回顾了克隆生长对被子植物传粉过程影响的国内外研究进展, 从植物克隆大小、花资源空间配置、克隆构型与种群遗传结构四个方面讨论了克隆生长对传粉过程的影响及其生态学与进化生物学意义。早期研究预期, 随着克隆增大, 同株异花授粉水平增加, 因而通过增大自交率或花粉阻塞效应降低植物的适合度。但是, 后来的一些模拟与野外实验研究发现, 传粉者在同一克隆内访问的花数量并不会随克隆增大而一直增加, 访花行为也主要发生在分株内; 而且分子标记的自交率组分分析也表明自交主要发生在分株内。另一方面, 人工模型模拟以及传粉者访问行为研究表明, 当花朵数量相同时, 与所有花集中生长在同一分株上相比, 将花朵分散在多个分株上的克隆生长方式不会增加, 反而降低了同株异花授粉的发生水平。如果花序内花雌雄同熟, 花朵同时提供与接收花粉, 克隆生长会使植物接收到更高比例的异交花粉, 在提高后代质量的同时不增加同株异花授粉概率。这是从传粉生物学角度对植物克隆生长习性进化的一个全新的解释。今后, 克隆植物传粉生物学研究需要针对传粉者与克隆生长之间的相互作用建立理论模型, 探究克隆大小、克隆构型、花资源空间配置模式对传粉者访问频率和行为、花粉散布、交配格局的影响。同时, 需要在自然种群中, 尤其是克隆与非克隆的近缘类群、同一物种克隆与非克隆种群开展比较研究, 利用更高效的分子标记来研究克隆生长的生态与进化意义。     

Diversity and abundance patterns of phytophagous insect communities on alien and native host plants in the Brassicaceae

The herbivore load (abundance and species richness of herbivores) on alien plants is supposed to be one of the keys to understand the invasiveness of species. We investigate the phytophagous insect communities on cabbage plants (Brassicaceae) in Europe. We compare the communities of endophagous and ectophagous insects as well as of Coleoptera and Lepidoptera on native and alien cabbage plant species. Contrary to many other reports, we found no differences in the herbivore load between native and alien hosts. The majority of insect species attacked alien as well as native hosts. Across insect species, there was no difference in the patterns of host range on native and on alien hosts. Likewise the similarity of insect communities across pairs of host species was not different between natives and aliens. We conclude that the general similarity in the community patterns between native and alien cabbage plant species are due to the chemical characteristics of this plant family. All cabbage plants share glucosinolates. This may facilitate host switches from natives to aliens. Hence the presence of native congeners may influence invasiveness of alien plants.     

The influence of climatic seasonality on the diversity of different tropical pollinator groups

Tropical South America is rich in different groups of pollinators, but the biotic and abiotic factors determining the geographical distribution of their species richness are poorly understood. We analyzed the species richness of three groups of pollinators (bees and wasps, butterflies, hummingbirds) in six tropical forests in the Bolivian lowlands along a gradient of climatic seasonality and precipitation ranging from 410 mm to 6250 mm. At each site, we sampled the three pollinator groups and their food plants twice for 16 days in both the dry and rainy seasons. The richness of the pollinator groups was related to climatic factors by linear regressions. Differences in species numbers between pollinator groups were analyzed by Wilcoxon tests for matched pairs and the proportion in species numbers between pollinator groups by correlation analyses. Species richness of hummingbirds was most closely correlated to the continuous availability of food, that of bees and wasps to the number of food plant species and flowers, and that of butterflies to air temperature. Only the species number of butterflies differed significantly between seasons. We were not able to find shifts in the proportion of species numbers of the different groups of pollinators along the study gradient. Thus, we conclude that the diversity of pollinator guilds is determined by group-specific factors and that the constant proportions in species numbers of the different pollinator groups constitute a general pattern.     

Wound induced defences in plants and their consequences for patterns of insect grazing

Summary Three scales of wound-induced chemical responses in plants are identified: (1) highly localised chemical changes associated with disruption of cell compartmentation; (2) changes induced in cells surrounding the damaged area, forming a kind of halo around the damage, and (3) more widely-dispersed changes which may affect an entire organ, branch or plant. A brief review of the literature reveals that such chemical responses are very widespread in plants, and many of the substances formed are known to affect adversely the growth, development, or reproduction of insects. It is argued that wound-induced changes in plant chemistry represent for insects a powerful selective pressure for the dispersal of grazing. Levels and patterns of invertebrate grazing in a range of herbaceous and deciduous woody plants sampled at the end of the growing seasons were examined. Leaves of many species exhibited a strikingly evident over-dispersion of grazing initiations, and in some cases the arrangement of holes appeared close to regularity. The pattern of damage between leaves was, in most cases, heavily biased towards a large proportion of leaves receiving a low level of grazing. These highly dispersed patterns of grazing damage are consistent with the hypothesis that wound-induced responses play an important role in determining patterns of insect feeding. They have important implications for the expected levels of insect exploitation of host plants and for the advantages to the plant of distributing grazing damage evenly through the canopy.     

紫花苜蓿的传粉昆虫种类及其访花行为

紫花苜蓿(Medicago sativa)属典型的异花授粉植物,其种子生产主要依靠蜜蜂等传粉昆虫.本文对河西地区张掖实验站紫花苜蓿传粉昆虫种类及其访花行为进行了调查.结果表明:苜蓿访花昆虫共计22种,分别属于4个目,11个科,根据弹花效率初步确定鳞地蜂、黑颚条蜂、净切叶蜂、细切叶蜂和紫木蜂为河西地区主要传粉昆虫;主要传粉昆虫的日活动规律出现单峰型和双峰型2种,净切叶蜂、细切叶蜂和紫木蜂属于前者,只在11:30-15:30出现一个活动高峰,而鳞地蜂与黑颚条蜂则为双峰型,在9:30-11:30和16:30-18:30分别出现访花高峰,不同的传粉昆虫之间存在互补关系;主要传粉昆虫的弹花频率和小花停留时间存在显著差异,且访花行为与野生蜂体型紧密相关.     

Movement patterns of a clear-wing hawkmoth,Hemaris fuciformis,foraging at red catchfly,Viscaria vulgaris

Summary The bee hawkmoth, Hemaris fuciformis, tended to fly in the same direction between successive visits to inflorescences of Viscaria vulgaris. Upon leaving an inflorescence it did not fly to the first inflorescence in its path, but to the second. At inflorescences, the number of animals that started probing decreased from the bottom to the top. These movement patterns probably serve to reduce the risk of revisiting flowers. The movements of the moths at inflorescences with 2 flowers per tier were investigated and related to the nectar distribution. The nectar is distributed according to a bonanza-blank pattern, flowers being either empty or full, probably because previous visitors have emptied some, but not all, flowers. It is shown that the two flowers of a tier more often than random belong to the category: blank-bonanza. Thus, if a blank is found, the animal should move sideways to the second flower at the same tier in order to find a bonanza. The flowers of the inflorescences tend to be arranged in vertical rows, and it is shown that two flowers, one above the other, more often than random belong to the two categories: blank-blank or bonanza-bonanza. Thus, if a bonanza is found, the animal should move up to the flower above, in order to find another bonanza. Rules of movement at inflorescences and a rule of departure are proposed based on the probability distribution of the nectar. The observed movements agree quite well with the predicted ones.     

Voracity of the main stored-product insect pests on wheat grain

The results of an experimental study of voracity of adults and larvae of seven main stored-product insect pests are described. One adult of Sitophilus oryzae, S. granarius, Rhizopertha dominica, Tribolium castaneum, T. confusum, and Oryzaephilus surinamensis consumed 0.38, 0.48, 0.89, 0.15, 0.16, and 0.11 mg of wheat grain per day, respectively. During their development, larvae of these insects consumed 7.60, 13.77, 7.92, 3.32, 3.60, and 2.32 mg of wheat grain, respectively. One larva of Sitotroga cerealella consumed 17.46 mg of wheat grain. The coefficients of harmfulness of these species are calculated, and a method for estimating the “total infestation density” of grain is proposed.     

Linking traits of foraging animals to spatial patterns of plants: social and solitary ants generate opposing patterns of surviving seeds

Foraging traits of seed predators are expected to impact the spatial structure of plant populations, community dynamics and diversity. Yet, many of the key mechanisms governing distance- or density-dependent seed predation are poorly understood. We designed an extensive set of field experiments to test how seed predation by two harvester ant species interact with seed dispersal in shaping the spatial patterns of surviving seeds. We show that the Janzen–Connell establishment pattern can be generated by central-place foragers even if their focal point is located away from the seed source. Furthermore, we found that differences in the social behaviour of seed predators influence their sensitivity to seed density gradients and yield opposing spatial patterns of surviving seeds. Our results support the predictions of a recent theoretical framework that unifies apparently opposing plant establishment patterns, and suggest that differences in foraging traits among seed predators can drive divergent pathways of plant community dynamics.