Robber-like pollinators: overwintered queen bumblebees foraging on Corydalis ambigua

ABSTRACT.

• 1 The behaviour of nectar-collecting Bombus hypocrita sapporensis Cockerell queens was observed on a population of a spring ephemeral plant Corydalis ambigua Cham, et Schlecht.
• 2 Daily patterns of activity and behaviour changed with the progress of flowering. Activity peaked shortly before sunset early in the flowering season but approximately at noon towards the end of flowering. In the peak flowering period the queens tended to visit nearby plants and to change direction often, whereas early or late in the flowering period they flew further between visits and were less likely to change direction.
• 3 Each plant was visited 0 to 24 times (mean 9.4 ±SD 5.2) by the queens during the whole flowering season.
• 4 The queens collected nectar, rarely through the front of the flowers but mostly through the spurs perforated by themselves or predecessors. At the beginning of the flowering season the illegitimate foragers often visited the front of the flowers before moving to the spurs; later, most queens quickly learned to land directly on the spurs.
• 5 Even the 59.7% of plants that were visited only by illegitimate foragers set seeds. Close observation confirmed that the illegitimate foragers opened the inner petals enclosing anthers and stigma frequently when visiting the front of the flowers before robbing, or occasionally when walking about on the flowers or collecting nectar through the perforated spurs.

     

Floral display size in comfrey, Symphytum officinale L. (Boraginaceae): relationships with visitation by three bumblebee species and subsequent seed set

The fecundity of insect-pollinated plants may not be linearly related to the number of flowers produced, since floral display will influence pollinator foraging patterns. We may expect more visits to plants with more flowers, but do these large plants receive more or fewer visits per flower than small plants? Do all pollinator species respond in the same way? We would also expect foragers to move less between plants when the number of flowers per plant are large, which may reduce cross-pollination compared to plants with few flowers. We examine the relationships between numbers of inflorescence per plant, bumblebee foraging behaviour and seed set in comfrey, Symphytum officinale, a self-incompatible perennial herb. Bumblebee species differed in their response to the size of floral display. More individuals of Bombus pratorum and the nectar-robbing B.?terrestris were attracted to plants with larger floral displays, but B. pascuorum exhibited no increase in recruitment according to display size. Once attracted, all bee species visited more inflorescences per plant on plants with more inflorescences. Overall the visitation rate per inflorescence and seed set per flower was independent of the number of inflorescences per plant. Variation in seed set was not explained by the numbers of bumblebees attracted or by the number of inflorescences they visited for any bee species. However, the mean seed set per flower (1.18) was far below the maximum possible (4 per flower). We suggest that in this system seed set is not limited by pollination but by other factors, possibly nutritional resources.     

Effectiveness of buzz pollination in Pedicularis chamissonis: significance of multiple visits by bumblebees

Pollination success of plants is highly susceptible to the frequency of visits and foraging behavior of pollinators. Pollination of the nectarless flowers of Pedicularis species depends on bumblebee workers collecting pollen by vibrating the anthers (buzz pollination). However, little is known about the efficiency of the pollination system. Foraging behavior, pollen removal from anthers and pollen deposition on stigmas of P. chamissonis were studied to assess the effectiveness of buzz pollination in an alpine snowbed population of northern Japan. Although bumblebees tended to visit most of the flowers open at a given time within inflorescences during a single visit, pollen removal rate at the first visit was about 20%, and buzzing period decreased with increasing number of previous visits, resulting in a decreasing proportion of pollen removed per visit as the number of visits increased. These trends enable plants to provide pollen for more pollinators. The number of pollen grains deposited on stigmas was not saturated during the first visit and increased with additional visits. Irrespective of weak self-compatibility, evidence of interference between self and outcross pollen was lacking for seed production. Therefore, buzz pollination in P. chamissonis acts as a mechanism that improves the chance of cross-pollination upon multiple visits if pollinator visitation is frequent.     

Effects of floral herbivory on foraging behaviour of bumblebees and female reproductive success in Pedicularis gruina (Orobanchaceae)

The effects of floral herbivores on floral traits may result in alterations in pollinator foraging behaviour and subsequently influence plant reproductive success. Fed-upon plants may have evolved mechanisms to compensate for herbivore-related decreased fecundity. We conducted a series of field experiments to determine the relative contribution of floral herbivores and pollinators to female reproductive success in an alpine herb, Pedicularis gruina, in two natural populations over two consecutive years. Experimental manipulations included bagging, hand supplemental, geitonogamous pollination, and simulated floral herbivory. Bumblebees not only avoided damaged flowers and plants but also decreased successive visits of flowers in damaged plants, and the latter may reduce the level of geitonogamy. Although seed set per fruit within damaged plants was higher than that in intact plants, total seed number in damaged plants was less than that in intact plants, since floral herbivory-mediated pollinator limitation led to a sharp reduction of fruit set. Overall, the results suggest that resource reallocation within inflorescences of damaged plants may partially compensate for a reduction in seed production. Additionally, a novel finding was the decrease in successive within-plant bumblebee visits following floral herbivory. This may increase seed quantity and quality of P. gruina since self-compatible species exhibit inbreeding depression. The patterns of compensation of herbivory and its consequences reported in this study give an insight into the combined effects of interactions between floral herbivory and pollination on plant reproductive fitness.     

Pollination Biology of the Exotic Rattleweed Crotalaria retusa L. (Fabaceae) in NE Brazil1

The rattleweed Crotalaria retusa was introduced in Brazil from Africa, and combines a series of characters that have ensured its establishment in NE Brazil. We focused on its reproductive biology and pollinator behavior to explain its reproductive success. We performed manual pollination and germination experiments, and monitored the behavior of C. retusa's main pollinators in monospecific plots, and in mixed plots where C. retusa occurred together with two congeners, Crotalaria pallida and Crotalaria lanceolata. Crotalaria retusa is self‐compatible and capable of automatic selfing. Inbreeding depression was expressed at the level of percent seed germination, but not seed set. Few insects visited the inflorescences. Legitimate pollinators were two large carpenter bees, Xylocopa frontalis and Xylocopa grisescens which, together, accounted for more than 90 percent of the visits. The former foraged on C. retusa exclusively and has low pollen spread potential. The latter flew longer distances between plants and visited fewer flowers per inflorescence, potentially increasing the extent of pollen carryover, but at the risk of increasing heterospecific pollen transfer, because it visits other Crotalaria species during the same foraging bout. The different foraging strategies, allied to morphological disadvantages represented by pollen overlap on X. grisescens' body, may partially explain the much lower seed germination observed in C. pallida and C. lanceolata. These results are consistent with the hypothesis that a reduction in flower constancy may significantly depress viable seed set by increasing the chances of self‐pollination.     

Pollinator visitation in populations of tristylous Eichhornia paniculata in northeastern Brazil

Summary The frequencies of floral morphs in populations of tristylous Eichhornia paniculata often deviate from the theoretical expectation of equality. This variation is associated with the breakdown of tristyly and the evolution of self-fertilization. Differences in morph frequencies could result from selection pressures due to variable levels of insect visitation to populations and contrasting foraging behavior among the floral morphs. We estimated pollinator densities in 16 populations and quantified visitation sequences to morphs in five populations of E. paniculata in northeastern Brazil. Foraging behavior among floral morphs was measured as the frequency of visits to morphs relative to their frequency in the population (preference) and number of flights between inflorescences of the same versus different morphs (constancy). Pollinator density (number/m²/minute) was not correlated with population size, plant density or morph diversity. Pollinator densities varied most among populations of less than 200 plants. Whether pollinators discriminated among the morphs, depended on whether they primarily collected nectar or pollen. In four populations, nectar-feeding bees (Ancyloscelis and Florilegus spp.) and butterflies showed no consistent preference or constancy among the morphs. In contrast, pollen-collecting bees (Trigona sp.) visited a lower proportion of longstyled inflorescences than expected and tended to visit more mid-and short-styled inflorescences in succession, once they were encountered. Pollinator constancy for morphs did not result from differences in inflorescence production or spatial patchiness among the morphs. Although non-random pollinator visitation to morphs in heterostylous populations could potentially affect mating and hence morph frequencies, the observed visitation patterns in this study do not provide evidence that pollinators play a major role in influencing floral morph frequencies.     

THE GENETIC CONSEQUENCES OF WORKER ANT POLLINATION IN A SELF-COMPATIBLE,CLONAL ORCHID

The self-compatible orchid Microtis parviflora is pollinated by the flightless worker caste of the ant Iridomyrmex gracilis. The orchid is clonal and forms small patches, usually less than 1 m², of disconnected individual ramets. Ant pollinators visited and revisited a limited proportion of available inflorescences, and 40% of all flower visits occurred within plants promoting self-pollination. Pollen labels indicated that self-pollination accounted for 51% of the pollen transfers, although pollen carryover extended beyond 16 flowers on 2 or 3 inflorescences. The distribution of ant movements between plants was leptokurtic with a mean of 12.4 ± 14.9 cm and a maximum of 89 cm, but a high proportion of movements were within clones accentuating the level of self-pollination. However, some pollen transfers between inflorescences of unlike genotypes contributed to a low incidence (max = 8%) of outcrossing. In 12 patches examined by electrophoresis, the density varied from 11 to 61 inflorescences per m² and a maximum of only 4 genotypes were detected. Electrophoretic analysis revealed populations were highly inbred: only 23% (N = 17) of the loci were polymorphic and the mean gene diversity h, was 2.7%. Heterozygotes were observed in only one population given a mean fixation index F, of 0.982. These results reflect the combined effects of restricted ant foraging and clonality. Nevertheless, while ant foraging was restricted, some outcrossing occurred and in the absence of clonality it is likely that ant foraging would have yielded a mixed mating system similar to those reported for a wide array of insect pollinators. Given the ability of ants to generate pollen flow, the reasons for the rarity of ant pollination appear to lie elsewhere.     

两种生境下野慈姑繁殖差异及其机制

植物的生长环境不但可直接作用于其可利用资源的多寡及其繁殖分配,而且可通过影响植物所在的群落以及传粉者的组成或行为而间接的导致繁殖差异。然而,直接与间接作用的方向或强度很少被同时关注,从而限制了从机制上理解环境对植物繁殖的影响。选取野慈姑(Sagittaria trifolia L.)为研究材料,将相同基因型组成的植株分别种植在光照差异明显的两个同质园(林荫区与日照区),于盛花期对野慈姑的开花数量、昆虫访花进行了观察,确定其主要访花昆虫、记录访花行为以及野慈姑的繁殖产出水平。研究表明,日照区野慈姑每天开放的花朵和植株数量显著高于林荫区。两个区域内野慈姑的主要访花者有四大类:蚜蝇类、蜂类、蝶类和蝇类;其中林荫区的主要传粉昆虫为蚜蝇类,而日照区为蜂类。日照区昆虫单位时间内的访花频率、昆虫每回合访问的雄花数、总花数和花序数均显著高于林荫区。日照区野慈姑的座果率显著高于林荫区,而单果种子数量与种子面积与林荫区相当。总体而言,日照区野慈姑的繁殖产出达到了林荫区的三倍以上,这是环境的直接作用与环境介导的传粉作用在同一方向上叠加的结果,且前者占主导地位。在关注植物、物理环境和生物因子三者之间互作的前提下,量化每个繁殖阶段的水平,特别是传粉昆虫在不同生境下的表现,阐述了环境异质性导致植物繁殖差异的根本原因。     

Pollination and fruit‐set of Grevillea robusta in western Kenya

The floral visitors of silky oak, Grevillea robusta A. Cunn. ex R.Br., their foraging behaviour and their effects on fruit‐set were studied at Malava, western Kenya. Grevillea robusta is a popular tree for farm plantings in the eastern and central African highlands. Yield of seed has been disappointingly low in some areas and a lack of appropriate pollinators has been suggested as a possible cause. Investigations involved the monitoring of visitors on active inflorescences, assessment of the rewards available to potential pollinators, and exclusion experiments to establish the effects of various visitors on fruit‐set. The flowers are visited mainly by birds and insects. The likely pollinators of G. robusta are sunbirds (Nectarinia amethystina, N. cyanolaema, N. olivacea, N. superba and N. venusta) and white‐eyes (Zosterops kikuyuensis and Z. senegalensis). Very little aggressive behaviour between birds was recorded. No nocturnal pollinators were observed. Nectar was the major floral reward for pollinators, but is likely depleted by ants and honey bees, the foraging behaviour of which confirmed them to be nectar‐robbers. These insects hardly ever touched stigmas during their visits. Eighty‐nine per cent of bird visits were in the morning (07.00–10.00 hours) when nectar volume was highest. Inflorescences bagged to exclude birds set no fruits, and unmanipulated flowers and flowers bagged with self‐pollen set no fruits, indicating a self‐incompatibility mechanism. Control cross‐pollinated flowers displayed greatly increased fruit‐set (25.1%) compared with natural open‐pollination (0.9%). All these findings confirm the importance of cross‐pollen transfer to flowers and the necessity of pollinators for fruit‐set. Effective seed production requires activity of pollinators for self‐pollen removal and cross‐pollen deposition. Seed production stands for G. robusta should be established where flowering is prolific and bird pollinators are abundant.     

Seasonal variation in honeyeater foraging behaviour,infloresence abundance and fruit set in Banksia spinulosa (Proteaceae)

Seasonal variation in the foraging behaviour of honeyeaters and the production of fruit were examined in relation to the flowering intensity of Banksia spinulosa over two flowering seasons. The abundance of inflorescences was greater in the mid than in the early and late periods of the flowering season. In the mid period, many plants were blooming and each plant had many flowering inflorescences. Inflorescences received most visits by honeyeaters in the early flowering period; the visitation rate declining as flowering progressed. Eastern spinebills were the most common floral visitors at all times during the season. The number of foraging probes made at inflorescences by eastern spinebills did not differ throughout the season. Foraging movements between inflorescences on the same plant were more frequent in the mid period than in the early and late periods. Long distance movements between plants (more than 10 m apart) were promoted by aggressive interactions between honeyeaters. Inflorescences flowering in the late period were less likely to develop follicles because there were fewer visits by birds and/or because resources had been allocated to inflorescences pollinated earlier in the season. The number of follicles produced per infructescence did not differ between flowering periods. Overall, the number of inflorescences produced per plant, the number of visits received per inflorescence and the proportion of inflorescences that developed follicles were greater in 1987 than in 1988.     

Field metabolic rates of phytophagous bats: do pollination strategies of plants make life of nectar-feeders spin faster?

Recently, it was argued that extrinsic factors, such as high foraging costs, lead to elevated field metabolic rates (FMR). We tested this suggestion by comparing the FMR of nectar-feeding and fruit-eating bats. We hypothesized that the foraging effort per energy reward is higher for nectar-feeding mammals than for fruit-eating mammals, since energy rewards at flowering plants are smaller than those at fruiting plants. Using the doubly labelled water method, we measured the FMR of nectar-feeding Glossophaga commissarisi and fruit-eating Carollia brevicauda, which coexisted in the same rainforest habitat and shared the same daytime roosts. Mass-specific FMR of G. commissarisi exceeded that of C. brevicauda by a factor of almost two: 5.3±0.6 kJ g⁻¹ day⁻¹ for G. commissarisi and 2.8±0.4 kJ g⁻¹ day⁻¹ for C. brevicauda. Since nectar-feeding bats imbibe nectar droplets of only 193 J energy content during each flower visit, a G. commissarisi bat has to perform several 100 flower visits per night to meet its energy requirement. The fruit-eating C. brevicauda, on the other hand, needs to harvest only 3–12 Piper infructescenses per night, as the energy reward per Piper equals ca. 6–30 kJ. We argue that the flowering and fruiting plants exert different selective forces on the foraging behaviour and energetics of pollinators and the seed dispersers, respectively. A comparison between nectar-feeding and non-nectar-feeding species in various vertebrate taxa demonstrates that pollinators have elevated FMRs.     

Influence of resource topography on pollinator flight directionality of two species of bees

Analyses of the search patterns of ApisMellifera workers foraging on artificial inflorescences and Bombus Pennsylvanicusqueens foraging on inflorescences of red clover are used to identify a general rule specifying intrapatch search patterns. Bees land, move about the inflorescences in search of nectar, and come to a last-faced position from which takeoff occurs. Last-faced directions are generated by a forward-moving tendency while probing artificial inflorescences or real florets. This tendency is modulated by restricted bee locomotion, caused by the size and shape of the visited inflorescence and the spatial distribution of florets within such inflorescences. The result is that bees tend to circle inflorescences. The process is terminated with an undefined stopping rule, whereupon bees usually depart in the direction they last face. The last-faced direction is a by-product of intrafloral search and becomes less associated with arrival directions as intrafloral search continues. Pollinator flight directionality is usually represented as a frequency distribution of angular changes in direction which typically has a mean near 0° and a variance that increases with increases in energy gain. We show that these characteristics are artifacts of pooling mirror-image movement data sets;a mean of 0° is the result of canceling the means of the left- and right-hand turns. The putative increased variance associated with energy gain is the result of adding the variances associated with left- and right-handed turns. These computational artifacts may be avoided by changing the sign of the left-handed turns prior to pooling with the right-handed turns (or vice versa). Pooling bee visits based only on the number of florets probed can destroy information relevant to studies examining behavioral mechanisms of flight directionality. Analytical problems encountered when turns exceed 360° are also addressed.     

Effects of experimental manipulation of inflorescence size on pollination and pre-dispersal seed predation in the hummingbird-pollinated plant Ipomopsis aggregata

Large floral displays should theoretically provide advantages to plants through increased pollinator visitation and resulting fruit and seed set. However empirical tests of the response of pollinators to floral display size have been limited by a lack of direct experimentation, and the results of such studies have been equivocal. In addition, other selective agents such as pre-dispersal seed predators might modulate effects of floral display on pollination. By artificially altering flower number, we examined the direct effects of floral display in the monocarpic herb, Ipomopsis aggregata (Polemoniaceae), on visitation rates by broad-tailed and rufous hummingbird pollinators, as well destruction of fruits by a pre-dispersal seed predator (Hylemya: Anthomyiidae). In addition, we quantified the ultimate effects of flower number on female reproductive success. Plants with larger floral displays were most likely to be visited first in any given foraging bout (P < 0.01). As expected, plants with more flowers received more total flower visits. However, we found no gain in the proportion of flowers visited for many- versus few-flowered plants, or the total number of approaches/hour. In fact, a significantly greater percentage of flowers were visited on few-flowered plants. Plants did not compensate for our reduction in flowers by increasing investment in the number or proportion of flowers that set fruit, the number of seeds/fruit, or seed weight. Pre-dispersal seed predation was greater for many- than for few-flowered plants (P < 0.001), but this did not offset the potential fitness gains of producing large displays. Our data support the hypothesis that large floral displays function primarily in long-distance attraction of pollinators, and enhance maternal success. Received: 21 March 1996 / Accepted: 24 October 1996     

The movement patterns of carpenter beesXylocopa micans and bumblebeesBombus pennsylvanicus onPontederia cordata inflorescences

The movement patterns of carpenter bees (Xylocopa micans) and bumblebees (Bombus pennsylvanicus) foraging for nectar on vertical inflorescences ofPontederia cordata were studied near Miami, Florida. The floral biology ofP. cordata is unique in several ways: (a) many short-lived flowers per inflorescence, (b) constant nectar production throughout the life span of each flower, and (c) abscence of vertical patterning of nectar and age of flowers. Inflorescences ranged between 3.5 and 15.8 cm long and had between 9 and 55 open flowers. Both carpenter bees and bumblebees arrived mostly on the bottom third of the inflorescence and left after visiting flowers on the top third of the inflorescence. The departure position from the inflorescence was higher up than observed in studies of other insect pollinators foraging on other speces of plants. This pattern of departure probably occurs in the absence of a vertical gradient of nectar or floral morphology.     

Argentine ants displace floral arthropods in a biodiversity hotspot

Argentine ant (Linepithema humile (Mayr)) invasions are often associated with the displacement of ground‐dwelling arthropods. Argentine ant invasions can also exert other effects on the community through interactions with plants and their associated arthropods. For example, carbohydrate resources (e.g. floral or extrafloral nectar) may influence foraging behaviour and interactions among ants and other arthropods. In South Africa's Cape Floristic Region, Argentine ants and some native ant species are attracted to the floral nectar of Leucospermum conocarpodendron Rourke (Proteaceae), a native tree that also has extrafloral nectaries (EFNs). Despite having relatively low abundance in pitfall traps, Argentine ants visited inflorescences more frequently and in higher abundance than the most frequently observed native ants, Camponotus spp., though neither native nor Argentine ant floral foraging was influenced by the EFNs. Non‐metric multidimensional scaling revealed significant dissimilarity in arthropod communities on inflorescences with Argentine ants compared to inflorescences with native or no ants, with Coleoptera, Diptera, Hymenoptera, Arachnida, Orthoptera, and Blattaria all being underrepresented in inflorescences with Argentine ants compared to ant‐excluded inflorescences. Native honeybees (Apis mellifera capensis Eschscholtz) spent 75% less time foraging on inflorescences with Argentine ants than on inflorescences without ants. Neither Argentine ant nor native ant visits to inflorescences had a detectable effect on seed set of Le. conocarpodendron. However, a pollen supplementation experiment revealed that like many other proteas, Le. conocarpodendron is not pollen‐limited. Flower predation was negatively associated with increased ant visit frequency to the inflorescences, but did not differ among inflorescences visited by native and Argentine ants. Displacement of arthropods appears to be a consistent consequence of Argentine ant invasions. The displacement of floral arthropods by Argentine ants may have far‐reaching consequences for this biodiversity hotspot and other regions that are rich in insect‐pollinated plants.     

Size-dependent pollination efficiency in Anchusa officinalis (Boraginaceae): causes and consequences

Summary Bumblebees foraging on the self-incompatible Anchusa officinalis fly between near neighbour plants and between near neighbour inflorescences within plants. Although many-flowered plants attracted most bumblebees these plants received fewer visits on a per flower basis than smaller plants, and each bumblebee visited a smaller proportion of the flowers. The calculated effective visitation rate per flower was highest on plants of an intermediate size. If pollen-carryover was assumed to be limited the most efficient plant was predicted to be smaller since the proportion of fertilized flowers per bumblebee visit is expected to decrease further on the largest plants in relation to the total flower number. These predictions were tested by measuring fruit-set in the field. The percentage fruit-set decreased with plant size at all sizes that were investigated. That the most efficient plant was small indicates that pollen-carryover was indeed limited. However, the low percentage fruit-set associated with large size did not present a serious problem since the total estimated seed production per plant still increased with size. Selection favoring smaller plants may be low or absent in Anchusa.     

Oviposition choices by a pre-dispersal seed predator (Hylemya sp.)

Summary Although the importance of pollinators has most often been examined in the evolution of floral characters, seed predators may also play a role in shaping floral evolution. In this study, I examined the role of interplant distance, plant size, and flower morphology on Ipomopsis aggregatás (Polemoniaceae) attractiveness to a pre-dispersal seed predator, Hylemya sp. (Anthomyiidae) and to hummingbird pollinators. The attractiveness of I. aggregata individuals to Hylemya was nonlinearly related to interplant distance in experimental arrays. Clumped and highly dispersed plants were preyed upon more frequently than those at intermediate distances. I found no relationship between interplant distance and visitation rates by hummingbird pollinators in these experimental arrays. However, in natural populations studied, clumped plants were more frequently approached by hummingbirds than those growing more widely dispersed. Display size was unrelated to visitation by Hylemya on inflorescences I clipped and maintained as large, small and control. Display size was also unrelated to the total number of visits by hummingbird pollinators to each of these experimental plants, however large display plants were more likely to be visited first in any given visitation sequence. Of various morphological measurements, corolla length showed the strongest positive correlation with Hylemya egg presence. To the extent that plant spacing and morphology is correlated with pollinator visits and ultimate seed set, Hylemya could be choosing flowers optimally, and playing a role in the evolution of floral traits.     

AN EXPERIMENTAL STUDY OF THE FLORAL ECOLOGY OF DODECATHEON MEADIA

An established colony of Dodecatheon meadia on glacial drift in southeastern Wisconsin was observed for its insect pollination interrelationships. Twelve randomly distributed plants from which insects were excluded failed to produce seed, while plants exposed to insect visits set abundant seed. Female solitary bees of 2 species, viz., Augochloropsis metallica fulgida (Smith) and Lasioglossum forbesii (Robertson), and queens and workers of 7 species of Bombus were observed collecting pollen from the pendant anther cone by rapid wing vibration, while hanging inverted from the cone tip. This behavior—cinematographically recorded—was found fundamentally identical to that of females of the same species of solitary bees and of queens and workers of 5 species of Bombus on Solanum dulcamara, and of females of Lasioglossum forbesii (Robertson) and workers of 2 species of Bombus on Lycopersicum esculentum Mill. No other insects were found to pollinate these morphologically similar flowers in this manner, and crepuscular pollinators were not observed. Experimental modifications of form, function, and position of Dodecatheon flowers were offered to the pollinators, and behavioral changes of the insects were noted. Constituents of insect pollen loads were examined, and relationships of the pollinators to the flora of the region were noted. The detailed ecological study of insect pollen vector behavior as an aid in discovering biosystematic relationships of plant populations is suggested.     

Does pollination limit tolerance to browsing in <Emphasis Type="Italic">Ipomopsis aggregata</Emphasis>?

Ungulate browsing of flowering stalks of the semelparous herb Ipomopsis aggregata leads to regrowth of lateral inflorescences, a response that has been reported to yield overcompensation in some cases (browsed plants with higher reproductive success than unbrowsed), but undercompensation in others. Little is known about the mechanisms that cause such variable tolerance to herbivory. We explored one possible mechanism—variation in effects of browsing on pollination—by clipping I. aggregata inflorescences to mimic browsing, observing subsequent visits by pollinators and nectar-robbers, and adding pollen by hand to flowers of some clipped and unclipped plants. Clipping reduced floral display size and increased inflorescence branching, but neither hummingbirds, the primary pollinators, nor nectar-robbing bumblebees showed any preference for unclipped versus clipped plants. Clipping delayed flowering; this shift in phenology caused clipped plants to miss the peak of hummingbird activity and to have lower per-flower visitation rates than unclipped controls in one year, but to have greater overlap with birds and higher visitation rates in the subsequent year. In three sites and 2 years, clipped plants exposed to natural pollination suffered extreme undercompensation, producing on average only 16% as many seeds as unclipped controls. This was not directly attributable to clipping effects on pollination, however, because clipped plants were unable to increase fecundity when provided with supplemental pollen by hand. Taken altogether, our results suggest that compensation was constrained less by indirect effects of browsing on pollination than by its direct impacts on resource availability and hence on the ability of plants to regrow lost inflorescence tissue and to fill seeds. Exploring the physiological and developmental processes involved in regrowth of inflorescences and provisioning of seeds is a promising future direction for research designed to understand variation in browsing tolerance.     

Fruit-set and seed weight variation inAnthyllis vulneraria subsp.vulgaris (Fabaceae)

Fruit-set and seed weight variation was studied in a population ofAnthyllis vulneraria subsp.vulgaris (Fabaceae) in northwestern Spain. The plants produce several shoots, each bearing two to four inflorescences that open one at a time from bottom to top. Fruit-set, seed-pod weight and seed weight were found to be significantly higher in proximal inflorescences than in distal inflorescences of the same shoot; mean seed weight was up to three times higher in proximal than in distal inflorescences. By contrast, none of the three variables varied significantly among plants or among shoots of the same plant. Similarly, none of the three variables differed significantly between early- and late-flowering plants, or between plants monitored in 1993 and in 1994. These results are compatible with the view that shoots function as semiautonomous units as regards resource allocation, and that within the shoot resources are preferentially allocated to proximal (= early-opening) inflorescences. In the plants studied, the ratio of seed-pod weight to seed weight was fairly constant, suggesting that the pod is important for seed success.