贠建全:广东荷包岛维管束植物名录

以分布于秦岭的金花忍冬（Lonicera chrysantha Turcz.）、忍冬（L.japonica Thunb.）、葱皮忍冬（L.ferdinandii Franch.）和金银忍冬（L.maackii（Rupr.）Maxim.）为对象,通过定位观察、人工授粉实验、人为设置实验斑块的方法对忍冬属4种植物的开花生物学特性、繁育系统、花色变化现象、传粉过程进行了研究。结果表明,4种植物的单花花期、花部特征存在差异。人工授粉实验显示,4种植物均存在一定的花粉限制,自交不亲和。除葱皮忍冬外,其余3种植物随着花色由白变黄,花粉和花蜜报酬减少、雌雄生殖能力逐渐降低;葱皮忍冬花变色后花蜜量变化不显著,且仍保留较强的雌性生殖能力。变色花的保留被认为是植物的一种生殖策略,通过增大植物的花展示来扩大自身的吸引力,以吸引更多远距传粉者访花。人为控制白、黄花不同数量比的实验结果表明,大多数传粉者偏向访问白花（变色前的花）,且白花提供的报酬量和黄花（变色后的花）数量显著影响传粉者的访花频率,即当花蜜量减少或黄花数量增多时,传粉者访花频率随之降低。因此,我们认为忍冬属4种植物的花色变化可能除了增大植物对远距传粉者的吸引力外,对近距传粉者的访花行为也可能具有一定的影响。当传粉者接近植株时,变色后的花可能暗示其花蜜（花粉）报酬已经发生变化,并驱使昆虫离开并飞向同株或异株植物新开放的报酬丰富的白花,这既有利于提高传粉者的觅食效率,又能降低植物同株异花授粉的几率,对忍冬属植物及传粉者都具有重要意义,是植物长期与授粉昆虫相互适应的反映。     

A two-pollinator model for the evolution of floral complexity

For a new, more complex floral form to become established in a population it must overcome the problem of frequency-dependent constancy to successfully attract pollinators. This may be achieved by complex floral forms offering absolute greater rewards than the simpler forms, or by complex flowers offering a higher probability of being rewarding because fewer pollinators are able to visit them. In this paper we examine the effect of three pollinator foraging strategies on the ratio of flights within and between floral morphs and hence on the probability of a new morph establishing in a population without offering a greater reward. We incorporate pollinator behaviour based around observations of two pollinator species systems into three models of competition for pollinators. In the first model the constancy of the pollinator of the new floral morph is a function only of the foraging strategy of the existing pollinator of the original floral morph. In the next model the constancy of the second pollinator is determined by the number of rewarding flowers of each floral morph left by the original pollinator and in the third model it is determined by the ratio of rewarding flowers of each morph left by the original pollinator. The results demonstrate that under conditions of intense competition for pollinators, new, more complex floral forms are indeed able to attract high levels of constant pollinators without offering intrinsically higher rewards. However, for this to occur constancy in one of the pollinators must be a function of the ratio of rewarding to non-rewarding flowers of both floral forms. One prediction from our results is that sympatric speciation of floral complexity based on a higher probability of reward is more likely to occur in flowers offering rewards of pollen rather than nectar. This is because the cost of visiting non-rewarding flowers is usually higher where the reward is pollen rather than nectar. We also predict that complex flowers occurring at low frequency, which offer rewards of nectar, may need intrinsically greater rewards if they are to successfully attract pollinators.     

Nectar robbing in Ipomopsis aggregata : effects on pollinator behavior and plant fitness

Hummingbirds foraging in alpine meadows of central Colorado, United States, face a heterogeneous distribution of nectar rewards. This study investigated how variability in nectar resources caused by nectar-robbing bumblebees affected the foraging behavior of hummingbird pollinators and, subsequently, the reproductive success of a host plant (Ipomopsis aggregata). We presented hummingbirds with experimental arrays of I. aggregata and measured hummingbird foraging behavior as a function of known levels of nectar robbing. Hummingbirds visited significantly fewer plants with heavy nectar robbing (over 80% of available flowers robbed) and visited fewer flowers on those plants. These changes in hummingbird foraging behavior resulted in decreased percent fruit set as well as decreased total seed set in heavily robbed plants. These results indicate that hummingbird avoidance of nectar-robbed plants and flowers reduces plant fitness components. In addition, our results suggest that the mutualisms between pollinators and host plants may be affected by other species, such as nectar robbers. Received: 22 April 1998 / Accepted: 12 May 1998     

Competition Between Distantly Related Taxa In the Coevolution of Plants and Pollinators

Competition among distantly related plants for pollinators andamong distantly related animals for pollen and nectar playsa potentially important role in the organization of ecologicalcommunities and the coevolution of plant-pollinator relationships.Plants which rely on animals to disperse their pollen potentiallycompete for pollinators by processes similar to interferenceand exploitative competition. Coexisting plant species may evolveto avoid or reduce such competition by character displacementin floral morphology and/or phenology. One important differencebetween competition for pollinators and most other kinds ofcompetition is that pollinator resources are not used up andmade absolutely unavailable to competitors. Consequently, plantspecies can potentially overlap completely in their utilizationof pollinators. The disadvantages of competing apparently aresometimes outweighed by the advantages of sharing pollinators,because distantly related plant species frequently show evolutionaryconvergence in floral morphology, blooming time and nectar rewardsto utilize the same pollinators. Distantly related animal taxa may compete for floral nectarand pollen by both interference and exploitation. The mechanismsof such competition depend primarily on the energetic costsand benefits of foraging and aggression. Exploitative competitionis very important because nectar feeders of small body sizeand low energy requirements can forage economically and reducenectar availability to levels that will not support larger animals.Thus small nectarivores often can exclude larger competitorsfrom flowers to which both taxa have equal access. Plants mayevolve to influence the outcotre of competition among animalvisitors and favor species that provide the best pollinationservices. Thus flowers specialized for pollination by largeanimals often show morphological or phenological specializationswhich make rewards unava'lable to smaller animals. Interferenceis adaptive only when the benefits of exclusive use of a resourceoutweig.i the costs of defending it. Because distantly relatedkinds of flower visitors often differ in body size and energeticrequirements, interference competition among them is probablyrare although it is often important among closely related nectarivores. The community level consequences of competition in the ecologyand evolution of plant-pollinator associations are still poorlyunderstoood. Competition among distantly related pollinatorsfor plant floral rewards appears to play a major role, but competitionamong plants for pollinator services may be only a weak force.Although the basic interaction between plant and pollinatorusually is a mutualistic one, certain species of both plantsand animals parasitize this interaction and compete with themutualists for limited resources. Thus some animals rob nectarand pollen and compete with legitimate pollinators without providingpollination services. Similarly, some plants offer no floralrewards but obtain pollinator services by mimicing rewardingflowers of other species. The effects of these kinds of interactionson the organization of communities of plants and pollinatorsprovide a fertile area for future research.     

Floral Temperature and Optimal Foraging: Is Heat a Feasible Floral Reward for Pollinators?

As well as nutritional rewards, some plants also reward ectothermic pollinators with warmth. Bumble bees have some control over their temperature, but have been shown to forage at warmer flowers when given a choice, suggesting that there is some advantage to them of foraging at warm flowers (such as reducing the energy required to raise their body to flight temperature before leaving the flower). We describe a model that considers how a heat reward affects the foraging behaviour in a thermogenic central-place forager (such as a bumble bee). We show that although the pollinator should spend a longer time on individual flowers if they are warm, the increase in total visit time is likely to be small. The pollinator's net rate of energy gain will be increased by landing on warmer flowers. Therefore, if a plant provides a heat reward, it could reduce the amount of nectar it produces, whilst still providing its pollinator with the same net rate of gain. We suggest how heat rewards may link with plant life history strategies.     

Induced Floral and Extrafloral Nectar Production Affect Ant‐pollinator Interactions and Plant Fitness

Thousands of plant species throughout tropical and temperate zones secrete extrafloral nectar to attract ants, whose presence provides an indirect defense against herbivores. Extrafloral nectaries are located close to flowers and may modify competition between ants and pollinators. Here, we used Lima bean (Phaseolus lunatus L.) to study the plants interaction between ants and flower visitors and its consequences for plant fitness. To test these objectives, we carried out two field experiments in which we manipulated the presence of ants and nectar production via induction with jasmonic acid (JA). We then measured floral and extrafloral nectar production, the number of patrolling ants and flower visitors as well as specific plant fitness traits. Lima bean plants under JA induction produced more nectar in both extrafloral nectaries and flowers, attracted more ants and produced more flowers and seeds than non‐induced plants. Despite an increase in floral nectar in JA plants, application of this hormone had no significant effects on flower visitor attraction. Finally, ant presence did not result in a decrease in the number of visits, but our results suggest that ants could negatively affect pollination efficiency. In particular, JA‐induced plants without ants produced a greater number of seeds compared with the JA‐treated plants with ants.     

Variable responses of hawkmoths to nectar-depleted plants in two native Petunia axillaris (Solanaceae) populations

Pollination success of deceptive orchids is affected by the density and distribution of nectar providing plant species and overall plant density. Here we extended the framework of how plant density can affect pollination to examine how it may promote the success of plant intraspecific cheaters. We compared hawkmoth behaviour in two native populations of Petunia axillaris, where we simultaneously offered rewarding and manually depleted P. axillaris. We asked whether pollinator foraging strategies change as a function of plant density and whether such changes may differentially affect nectarless plants. We observed the first choice and number of flowers visited by pollinators and found that in the dense population, pollinators visited more flowers on rewarding plants than on nectar-depleted plants. In the sparse population, such discrimination was absent. As we found no differences in nectar volume between plants of the two populations, the observed differences in plant density may be temporal. We reason that if differences were more permanent, an equivalent of the remote habitat hypothesis prevails: in a sparse population, cheating plants benefit from the absence of inter- and intraspecific competitors because pollinators tend to visit all potential resources. In a denser population, a pollinator’s optimal foraging strategy involves more selectivity. This would cause between-plant competition for pollinators in a pollinator-limited context, which applies to most hawkmoth-pollinated systems. We propose that nectar-provisioning of plants can be density-dependant, with cheaters able to persist in low density areas.     

Departure rules used by bees foraging for nectar: A field test

Summary Departure rules used by solitary long-tongued bees (Anthophora spp. andEucera spp.) collecting nectar from flowers ofAnchusa strigosa (Boraginaceae) were studied. The amount of nectar a bee receives from an individual flower was estimated by measuring the time elapsed since the previous bee visit to that flower. Measurements of nectar accumulation in experimentally emptied flowers indicated that this time interval is an accurate predictor of nectar volumes in flowers. We found that nectar rewards influence the probability of departure from individual plants, as well as distances of movements within plants. The probability of departure from individual plants was negatively related to the amount of reward received at the two lastvisited flowers. This result indicates that the bees used a probabllistic departure rule, rather than a simple threshold departure rule, and that rewards from both the current and the previously visited flower were important in determining departure points. Distances of inter-flower movements within plants were negatively related to the amount of reward received at the current flower. The overall results suggest that the pollinators ofA. strigosa make two types of departure decisions-departures from the whole plant and departures from the neighbourhood of individual flowers-and that they use different departure rules for each scale. Factors influencing the decision-making processes of the observed foraging behaviour are discussed.     

Inter- and intra-floral heterogeneity of nectar production in Helleborus foetidus L. (Ranunculaceae)

Three southern Spanish populations of Helleborusfoetidus L. (Ranunculaceae) were sampled for nectar content in the absence of nectarivorous flower visitors. Nectar volume was measured in individual nectaries of flowers at the same stage in the anthesis cycle. Total nectar content per flower was extremely variable between plants within populations and between flowers within plants, but much less so between populations. Average sugar content per flower was roughly similar in the three populations sampled. High variances in nectar abundance occurred also among nectaries within the same flower. Heterogeneity in pollinator rewards thus occurs simultaneously at several spatial scales, namely between nectaries, between flowers and between plants. This implies a strong component of uncertainty to foraging pollinators, which may therefore tend to avoid Helleborus flowers.     

Selection on signal–reward correlation: limits and opportunities to the evolution of deceit in Turnera ulmifolia L

Because pollinators are unable to directly assess the amount of rewards offered by flowers, they rely on the information provided by advertising floral traits. Thus, having a lower intra-individual correlation between signal and reward (signal accuracy) than other plants in the population provides the opportunity to reduce investment in rewards and cheat pollinators. However, pollinators' cognitive capacities can impose a limit to the evolution of this plant cheating strategy if they can punish those plants with low signal accuracy. In this study, we examined the opportunity for cheating in the perennial weed Turnera ulmifolia L. evaluating the selective value of signal accuracy, floral display and reward production in a natural population. We found that plant reproductive success was positively related to signal accuracy and floral display, but not to nectar production. The intensity of selection on floral display was more than three times higher than on signal accuracy. The pattern of selection indicated that pollinators can select for signal accuracy provided by plants and suggests that learning abilities of pollinators can limit the evolution of deceptive strategies in T. ulmifolia.     

Nectar ecology of Datura ferox (Solanaceae): an invasive weed with nocturnal flowers in agro-ecosystems from central Argentina

Plant–pollinator interactions provide highly important ecological functions, and are influenced by floral nectar characteristics. The night blooming Datura ferox is an excellent model to test general hypotheses on the relationship between nectar traits (e.g., nectar secretion patterns, nectar chemical composition), pollinators and reproductive success for invasive, weedy species in highly modified ecosystems as crop fields. We hypothesized an adjustment between nectar composition and secretion dynamics through flower anthesis and the activity and requirements of nocturnal pollinators. Nectar chemical analyses showed low quantities of amino acids and lipids, phenolics, and alkaloids were not detected. D. ferox showed sucrose-dominant nectar with comparable amount of hexoses. Sugar proportions did not vary between populations or during flowering season. Most nectar is secreted before flower opening. Nectar resorption was detected at the end of anthesis. Experimentally drained flowers of both populations increased nectar production up to 50 % in the total amount of sugar per flower compared to control flowers. Nectar standing crop was relatively constant during the flowering season, but differences were detected between populations. Nectar traits of D. ferox would be favoring cross-pollination and maintaining seed production of this weed, since recently open flowers display a higher amount of nectar and they can renew nectar after a pollinator visit or reabsorb it at the end of anthesis. This nectar source may be important for native pollinators considering that human-induced forest fragmentation is related with the impoverishment of native flora from agro-ecosystems.     

ECOLOGICAL COSTS OF PLANT RESISTANCE TO HERBIVORES IN THE CURRENCY OF POLLINATION

In this paper, we examine how ecological costs of resistance might be manifested through plant relationships with pollinators. If defensive compounds are incorporated into floral structures or if they are sufficiently costly that fewer rewards are offered to pollinators, pollinators may discriminate against more defended plants. Here we consider whether directional selection for increased resistance to herbivores could be constrained by opposing selection through pollinator discrimination against more defended plants. We used artificial selection to create two populations of Brassica rapa plants that had high and low myrosinase concentrations and, consequently, high and low resistance to flea beetle herbivores. We measured changes in floral characters of plants in both damaged and undamaged states from these populations with different resistances to flea beetle attack. We also measured pollinator visitation to plants, including numbers of pollinators and measures of visit quality (numbers of flowers visited and time spent per flower). Damage from herbivores resulted in reduced petal size, as did selection for high resistance to herbivores later in the plant lifetime. In addition, floral display (number of open flowers) was also altered by an interaction between these two effects. Changes in floral traits translated into overall greater use of low-resistance, undamaged plants based on total amount of time pollinators spent foraging on plants. Total numbers of pollinators attracted to plants did not differ among treatments; however, pollinators spent significantly more time per flower on plants from the low-resistance population and tended to visit more flowers on these plants as well. Previous work by other investigators on the same pollinator taxa has shown that longer visit times are associated with greater male and female plant fitness. Because initial numbers of pollinators did not differ between selection regimes, palatability and/or amount of rewards offered by high- and low-resistance populations are likely to be responsible for these patterns. During periods of pollinator limitation, less defended plants may have a selective advantage and pollinator preferences may mediate directional selection imposed by herbivores. In addition, if pollinator preferences limit seed set in highly defended plants, then lower seed set previously attributed to allocation costs of defense may also reflect greater pollinator limitation in these plants relative to less defended plants.     

Models of optimal foraging and resource partitioning: deep corollas for long tongues

We model the optimal foraging strategies for 2 nectarivore species,differing in the length of their proboscis, that exploit thenectar provided by 2 types of flowers, differing in the depthsof their corollas. When like flowers appear in clumps, nectarivoresmust decide whether to forage at a patch of deep or shallowflowers. If nectarivores forage optimally, at least one flowertype will be used by a single nectarivore species. Long-tonguedforagers will normally visit deep flowers and short-tonguedforagers shallow flowers, although extreme asymmetries in metaboliccosts may lead to the opposite arrangement. When deep and shallowflowers are randomly interspersed, nectarivores must decide,on encounter with a flower, whether to collect its nectar orcontinue searching. At low nectarivore densities, the optimalstrategy involves exploiting every encountered flower; however,as nectarivore densities increase and resources become scarce,long-tongued individuals should start concentrating on deepflowers and short-tongued individuals on shallow flowers. Therefore,regardless of the spatial distribution of flowers, corolla depthcan determine which nectarivore species exploit the nectar fromeach flower type in a given community. It follows that corollaelongation can evolve as a means to keep nectar thieves at bayif short-tongued visitors are less efficient pollinators thanlong-tongued visitors.     

Competition versus facilitation: conspecific effects on pollinator visitation and seed set in the iris Lapeirousia oreogena

Ecological interactions between conspecific plants can range from facilitative to competitive depending on the spacing and abundance of individuals. Competitive interactions are expected when plants flower en‐masse and availability of pollinators is limited. We tested this prediction using Lapeirousia oreogena, a mass‐flowering South African iris that is pollinated by a single species of long‐proboscid fly. Controlled hand‐pollination experiments showed that L. oreogena is self‐compatible, but reliant on pollinator visits for seed set. Seed production per flower showed a significant negative relationship with patch size (and the correlated measure of number of individuals per patch), but was not affected by flower density or distance to neighbouring patches. There was a tendency for fly abundance to increase with patch size, but the rate of visits to individual flowers by flies was not affected by patch size. Seed set of hand‐pollinated flowers did not differ for plants in and out of dense patches, indicating that the large differences in seed set among patches were likely to reflect pollinator visits, rather than the genetic or physiological capacity of plants to produce seeds. The reduced fecundity of L. oreogena in large patches with a greater numbers of flowers is consistent with the idea that plants with highly specialized pollination systems can experience intra‐specific competition for pollination.     

Changes in Florets’ Vertical Direction within Inflorescence Affects Pollinator Behavior,and Fitness in <i>Trifolium repens</i>

Ecological interactions between flowers and pollinators greatly affect the reproductive success. To facilitate these interactions, many flowers are known to display their attractive qualities, such as scent emission, flower rewards and floral vertical direction, in a rhythmic fashion. However, less is known about how plants regulate the relationship between these flower traits to adapt to pollinator visiting behavior and increase reproduction success. Here we investigated the adaptive significance of the flower bending from erect to downward in Trifolium repens. We observed the flowering dynamic characteristics (changes of vertical direction of florets, flowering number, pollen grain numbers, pollen viability and stigma receptivity over time after blossom) and the factors affecting the rate of flower bending in T. repens. Then we altered the vertical direction of florets in inflorescence of different types (upright and downward), and compared the pollinator behaviors and female reproductive success. Our results showed that florets opened sequentially in inflorescence, and then bend downwards slowly after flowering. The bending speed of florets was mainly influenced by pollination, and bending angle increased with the prolongation of flowering time, while the pollen germination rate, stigma receptivity and nectar secretion has a rhythm of “low-high-low” during the whole period with the time going. The visiting frequency of all the four species of pollinators on upward flowers was significantly higher than that of downward flowers, and they especially prefer to visit flowers with a bending angle of 30°–60°, when the flowers was exactly of the highest flower rewards (nectar secretion and number of pollen grains), stigma receptivity and pollen germination rate. The seed set ratio and fruit set ratio of upward flowers were significantly higher than downward flowers, but significantly lower than unmanipulated flowers. Our results indicated that the T. repens could increase female and male fitness by accurate pollination. The most suitable flower angle saves pollinators’ visiting energy and enables them to obtain the highest nectar rewards. This coordination between plants and pollinators maximizes the interests of them, which is a crucial factor in initiating specialized plant-pollinator relationships.     

Spatial distribution of nectar production in a natural Echium vulgare population: Implications for pollinator behaviour

The distribution of trait values in many populations is not homogenous but creates a mosaic of patches. This may lead to differences in selection on the patch level compared to selection on the population level. As an example we investigated the spatial distribution of nectar production and its effects on pollinator behaviour in a natural population of Echium vulgare. Nectar production per flower, number of flowers and total nectar production showed a hierarchy and spatial aggregation as expressed by Gini coefficients and significant Moran's I values. Plants in patches of high nectar production received significantly more pollinator visits and had a significant emanating effect on pollinator visits of neighbouring plants. The same was true for plants in patches with high number of flowers. To disentangle these effects a path analysis was applied, which suggested that the direct effect of nectar production per flower although present, seems to be small compared to the effect of the number of flowers. Nectar production per flower affected pollinator visits mainly indirectly by way of total nectar production, which includes the effect of number of flowers. Assuming a minor pollinator-mediated selection for number of flowers, pollinator-mediated selection for total nectar production equals that for nectar production per flower. If so, the observed spatial structure of nectar production and its emanating effect on pollinator behaviour is of importance for natural selection. Plants of low nectar production occurring close to patches of plants with high nectar production benefited from the enhanced pollinator service of their neighbours while saving costs of increased nectar production. Consequently, plants with low nectar production may have a selective advantage at patch level while plants with high nectar production may have a selective advantage at population level. Results presented stress the importance of small-scale patterns for ecological relationships and evolutionary change.     

Nectar Sugar Production across Floral Phases in the Gynodioecious Protandrous Plant Geranium sylvaticum

Many zoophilous plants attract their pollinators by offering nectar as a reward. In gynodioecious plants (i.e. populations are composed of female and hermaphrodite individuals) nectar production has been repeatedly reported to be larger in hermaphrodite compared to female flowers even though nectar production across the different floral phases in dichogamous plants (i.e. plants with time separation of pollen dispersal and stigma receptivity) has rarely been examined. In this study, sugar production in nectar standing crop and secretion rate were investigated in Geranium sylvaticum, a gynodioecious plant species with protandry (i.e. with hermaphrodite flowers releasing their pollen before the stigma is receptive). We found that flowers from hermaphrodites produced more nectar than female flowers in terms of total nectar sugar content. In addition, differences in nectar production among floral phases were found in hermaphrodite flowers but not in female flowers. In hermaphrodite flowers, maximum sugar content coincided with pollen presentation and declined slightly towards the female phase, indicating nectar reabsorption, whereas in female flowers sugar content did not differ between the floral phases. These differences in floral reward are discussed in relation to visitation patterns by pollinators and seed production in this species.     

Density and seed set in a self-compatible forb, Penstemon digitalis (Plantaginaceae), with multiple pollinators

Seed production may be limited because flowers do not get enough suitable pollen or because plants lack the resources to make seeds. We used replicated plantings to test factors that influence effects of bumblebee behavior on pollen limitation, as measured by the difference in seed set between hand- and naturally pollinated flowers, of Penstemon digitalis in patches of four to 41 flowering individuals. Seed set per flower was 376% higher in the largest as compared with the smallest Penstemon patches. This positive density dependence reflects activity of long-tongued bees, which (1) have higher effective density as patch size increases, (2) visit greater proportions of plants as patch size increases, and (3) visit smaller proportions of flowers per visited plant as patch size increases. Our results suggest that economics of flight and maneuverability of large, long-tongued bumblebees lead them to transfer more pollen between than within Penstemon plants in large patches. Density of smaller, short-tongued bumblebees was not positively associated with Penstemon seed set, but these bees may be important pollinators at low plant densities. Our experimental system indicates a clear positive relationship between activity of effective pollinators and seed set in a species capable of pollinating itself.     

Pollinator response to within-patch spatial context determines reproductive output of a giant rosette plant

The reproductive output of many plants depends on the interaction between plant spatial pattern and pollinator behaviour. Pollinators tend to concentrate their efforts on patches of flowers offering higher rewards. The spatial relationship of an individual plant to those around it (its spatial context) is also important for its reproductive output. This study examines the effect of patch size and spatial context on the reproductive output of Puya hamata, a hummingbird-pollinated, semelparous, giant rosette plant in the Andes. Hummingbird behavioural response to Puya patch size and the effect of plant density on flowering plant size were assessed. The reproductive output (flower, fruit and seed production, seed viability, germination rates) of plants in relation to patch size and spatial context was determined. Isolated Puya inflorescences were visited by a higher diversity of mostly trap-lining hummingbirds, while plants in aggregations were almost exclusively visited by one territorial species. Spatial context did not affect Puya size at flowering, or the numbers of flowers, fruits and seeds produced. However, with respect to seed viability and germination rates, reproductive output was highest in isolated plants, and plants on the edges of patches, but lowest in plants at the centre of large patches. The effect of spatial context on reproductive output in P. hamata depends on several key conditions being met: the patches are normally made up of closely related plants and the pollinators, by switching to territoriality, restrict gene flow into patches and lower the effective reproductive output of the plants within those patches. Other plant–pollinator relationships meeting these conditions are likely to produce similar outcomes. Such situations are worthy of attention, not just for their ecological interest, but also for their implications for the management of species and genetic diversity.     

Foraging strategies of insects for gathering nectar and pollen, and implications for plant ecology and evolution

The majority of species of flowering plants rely on pollination by insects, so that their reproductive success and in part their population structure are determined by insect behaviour. The foraging behaviour of insect pollinators is flexible and complex, because efficient collection of nectar or pollen is no simple matter. Each flower provides a variable but generally small reward that is often hidden, flowers are patchily distributed in time and space, and are erratically depleted of rewards by other foragers. Insects that specialise in visiting flowers have evolved an array of foraging strategies that act to improve their efficiency, which in turn determine the reproductive success of the plants that they visit. This review attempts a synthesis of the recent literature on selectivity in pollinator foraging behaviour, in terms of the species, patch and individual flowers that they choose to visit.

The variable nature of floral resources necessitate foraging behaviour based upon flexible learning, so that foragers can respond to the pattern of rewards that they encounter. Fidelity to particular species allows foragers to learn appropriate handling skills and so reduce handling times, but may also be favoured by use of a search image to detect flowers. The rewards received are also used to determine the spatial patterns of searches; distance and direction of flights are adjusted so that foragers tend to remain within rewarding patches and depart swiftly from unrewarding ones. The distribution of foragers among patchy resources generally conforms to the expectations of two simple optimal foraging models, the ideal free distribution and the marginal value theorem.

Insects are able to learn to discriminate among flowers of their preferred species on the basis of subtle differences in floral morphology. They may discriminate upon the basis of flower size, age, sex or symmetry and so choose the more rewarding flowers. Some insects are also able to distinguish and reject depleted flowers on the basis of ephemeral odours left by previous visitors. These odours have recently been implicated as a mechanism involved in interspecific interactions between foragers.

From the point of view of a plant reliant upon insect pollination, the behaviour of its pollinators (and hence its reproductive success) is likely to vary according to the rewards offered, the size and complexity of floral displays used to advertise their location, the distribution of conspecific and of rewards offered by other plant species, and the abundance and behaviour of other flower visitors.