Pollen Generalist Research Articles

Structure of a metacommunity of urban bees: Species diversity and spatio-temporal modularity

As the globe gets more urbanised, the question about how natural biodiversity is structured in cities becomes increasingly pertinent. To contribute to an answer, we studied species richness and spatio-temporal structure of bees in a North European metropolitan area. A gradient of 13 sites in the city of Aarhus, Denmark, was censused from April to September 2016. Forty species, i.e. 29 solitary species (40 % of all individuals), ten Bombus species (28 %), and Apis mellifera (32 %), were sampled monthly in pan traps. (i) Information about species traits was extracted from the literature, and trait values were correlated and used to characterize the fauna. Most were soil-nesters, pollen generalists, and common. (ii) Habitat diversity within five concentric circles with trap at the centre and radii from 50 m to 1000 m was related to bee α diversity. The relationship was significant only within 1,000 m for all bees and for bumblebees. Solitary bee diversity was uncorrelated with habitat diversity at all spatial levels. (iii) Spatio-temporal structure was analysed as two networks, one for bees linked to sites, and one for bees linked to months. Link patterns were analysed for levels of nestedness, modularity, and spatio-temporal β diversity. The two networks were weakly and non-significantly nested, but strongly modular, being composed of five and four modules of co-occurring bees, respectively. (iv) Finally, we studied total β diversity, βTOTAL, being the sum of species turnover, βTURN, and species loss/gain or nestedness, βNEST. For both site and season, βTURN was higher than βNEST, and site βTOTAL was higher than season βTOTAL. One reason for this metacommunity structure may be a high spatio-temporal habitat patchiness, sustaining a rich biodiversity. Thus, a few large areas may not compensate for the loss of several small patches. Consequently, establishment of many green, even small habitats is recommended.

The hidden diet – examination of crop content reveals distinct patterns of pollen host use by Central European bees of the genus Hylaeus (Hymenoptera, Colletidae)

Masked or yellow-faced bees of the genus Hylaeus (Colletidae) differ in their mode of pollen transportation from most other bees in that they ingest the pollen directly on the flowers and carry it back to the nest inside the crop located in the anterior half of the metasoma. Due to this hidden mode of pollen transportation, the examination of pollen collected by Hylaeus females requires the dissection of the metasoma. Although this method has never been applied in Europe, the great majority of the Central European Hylaeus species were supposed to be pollen generalists based on observations of flower visits. The microscopical analysis of pollen removed from 30 crops each of 36 Central European Hylaeus species revealed that the proportion of species exhibiting an exclusive or strong preference for pollen from a single plant taxon is much higher than hitherto assumed and that the current assumption of the genus Hylaeus to largely consist of pollen generalists is wrong. Nineteen of the 36 species examined are strictly or largely dependent on a single plant taxon for collecting pollen, such as Apiaceae (n = 11 species), Rosaceae (n = 3), Reseda (Resedaceae) (n = 2), Allium (Amaryllidaceae) (n = 1), Asteraceae (n = 1) and Melilotus (Fabaceae) (n = 1). The 36 Hylaeus species examined collected pollen from the flowers of 31 plant families, of which the Apiaceae and Rosaceae (particularly Potentilla and Rubus) were by far the most important contributing almost 60% to the pollen host spectrum of the entire genus. The comparison between pollen host spectrum and flower visiting records showed that the pollen generalists use the flowers of the Asteraceae as nectar rather than pollen sources, corroborating earlier findings that the digestion of Asteraceae pollen requires physiological adaptations to cope with its unfavourable or protective properties. In summary, the patterns of pollen host use by bees of the genus Hylaeus do not substantially differ from those of other Palaearctic bee taxa despite the masked bees’ unusual habit to ingest the pollen directly on the flowers and to transport it inside their body back to the nest.

Palaearctic Osmia bees of the subgenera Allosmia and Neosmia (Megachilidae, Osmiini): biology, taxonomy and key to species

Osmia (Allosmia) and O. (Neosmia) are two closely related Palaearctic osmiine bee subgenera (Megachilidae, Megachilinae, Osmiini) comprising eleven and ten species, respectively. Analysis of female pollen loads revealed that the representatives of O. (Allosmia) and O. (Neosmia) are pollen generalists collecting pollen on up to 13 and 17 plant families, respectively. The species of both subgenera exclusively nest in empty snail shells and use chewed leaves as main nest building material, often in combination with fragments of mollusc shells and densely packed small particles, such as small stones, earth crumbs or plant fibers, which are used to barricade the nests. The taxonomic revision of the two subgenera revealed the existence of two undescribed species, O. (Allosmia) gemina spec. nov. from the southern Levant and O. (Neosmia) nigrocalcaribus spec. nov. from the Maghreb. Osmia fossoria Pérez, 1890 syn. nov. and O. decorata Morawitz, 1886 syn. nov. are newly synonymized with O. (Allosmia) lhotelleriei Pérez, 1887 and O. (Allosmia) melanura Morawitz, 1871, respectively, whereas the former synonymization of O. (Neosmia) rosea Friese, 1920 with O. scutispina Gribodo, 1894 is rejected. Osmia (Neosmia) purpurata Ducke, 1899 is regarded as a nomen dubium. Osmia (Allosmia) soror Pérez, 1896 and O. (Neosmia) secunda Peters, 1977 are treated as species rather than as subspecies of O. rufohirta Latreille, 1811 and O. tingitana Benoist, 1969, respectively, due to clear morphological gaps and partly sympatric occurrence. Identification keys for all species of the two subgenera are given.

Pollen specialists are more endangered than non-specialised bees even though they collect pollen on flowers of non-endangered plants

Non-parasitic bees differ in the degree of their taxonomic specialisation for pollen collection to feed their brood. Surveys of published data on pollen specialisation by bees of central Europe showed that about two-thirds of species are pollen generalists (polylectic) while the remainder only collect pollen of one family, genus or species (monolectic and oligolectic). Proportions of monolectic and oligolectic species among bee families and genera in central Europe varies widely, from the entirely pollen-specialised but small family Melittidae, to the mostly polylectic Halictidae and Apidae. The Asteraceae plant family attracts the most monolectic and oligolectic species, followed by Fabaceae, Brassicaceae and Campanulaceae. Several plant families in central Europe host only a single bee species. Of the species included in the Red List of bees of Czechia, oligoleges are proportionally more strongly represented than polyleges. Most of these red-listed oligoleges are associated with specific and regionally endangered habitats, i.e. steppes or wetlands. Most of the bees are more limited by the presence of their habitat or nesting site than by the host plant, although this fact is not simple to evaluate due to the incomplete data on the floral relations and distribution of bee species.

One-year-old flower strips already support a quarter of a city’s bee species

To combat the loss of flower-rich meadows, many cities are supporting greening measures, including the creation of flower strips. To assess the effectiveness of these measures in supporting flower-visiting insects, their faunas need to be compared to the background fauna at various distances from the flower strips. To meet this goal, we quantified the bee faunas of nine 1000 m2-large and newly established flower strips in the city of Munich, all planted with a regional seed mix, and compared them to the fauna recorded between 1997 and 2017 within 500, 1000, and 1500 m from the respective strip. The 68 species recorded during the flower strips’ first season represent 21% of the 324 species recorded for Munich since 1795 and 29% of the 232 species recorded between 1997 and 2017. Non-threatened species are statistically over-represented in the strips, but pollen generalists are not. These findings illustrate the conservation value of urban flower strips for common species that apparently quickly discover this food source. To our knowledge, this is the first quantitative assessment of the speed and distance over which urban flower strips attract wild bees.

More Than Meets the Eye? The Role of Annual Ornamental Flowers in Supporting Pollinators.

Ornamental flowers are commonly planted in urban and suburban areas to provide foraging resources for pollinator populations. However, their role in supporting broad pollinator biodiversity is not well established as previous studies have been conducted in urban landscapes with pollinator communities that are distinct from those in natural systems. We observed pollinator visitation patterns to five ornamental annual plant genera and their cultivars over multiple years at two semi-natural sites in Pennsylvania to understand their potential for supporting diverse pollinator communities. There was significant variation in visitor abundance and diversity by season and year for many annual ornamental cultivars. Within some genera, cultivars had similar visitor abundance, diversity, and main visitor taxa, while cultivars in other genera varied greatly in these measures. We observed only polylectic (pollen generalist) bee species visiting annual ornamentals, despite the presence of oligolectic (pollen specialist) bee species in the background population. We conclude that the attractiveness of annual ornamental plants likely depends on both cultivar characteristics and environmental context. While their role in supporting complex pollinator populations is limited both based on the number of and dietary breadth of the species they support, ornamental plants may nonetheless provide long-lasting supplemental foraging resources for the generalist pollinator communities characteristic of urban and suburban environments.

Constrained patterns of pollen use in Nearctic Andrena (Hymenoptera: Andrenidae) compared with their Palaearctic counterparts

Andrena is a large genus of bees primarily distributed across the Holarctic. Despite their abundance in temperate regions, the pollen diets of many Nearctic Andrena remain incompletely resolved. The pollen diets of 50 species of Andrena found in Michigan were characterized using light microscopy. Twenty-four species (48%) were classified as pollen specialists, collecting pollen from one botanical family. The remaining 26 species (52%) were classified as pollen generalists, collecting from many botanical families. Andrena species fell into three broad foraging groups: (1) spring-flying species foraging predominantly from woody plants; (2) spring-flying species specializing on herbaceous ephemerals; and (3) summer-flying species specializing on herbaceous prairie plant species. Species of Nearctic Andrena specialized on pollens from Asteraceae, Geraniaceae, Hydrophyllaceae and Montiaceae or avoided them almost entirely. Botanical families that hosted specialized bees showed a higher variation in utilization by the Andrena community than botanical families without specialists. In contrast, Palaearctic species showed little temporal partitioning and low variation in the utilization of different botanical families. This pattern of pollen use supports previous findings that pollens from certain botanical families cannot be used as a food source without physiological adaptations, but that this phenomenon is more pronounced in the Nearctic Andrena fauna.

Palaearctic Osmia bees of the subgenus Hoplosmia (Megachilidae, Osmiini): biology, taxonomy and key to species.

Hoplosmia, a subgenus of the osmiine bee genus Osmia (Megachilidae), comprises 21 species restricted to the Palaearctic region. Analysis of female pollen loads and field observations indicate that probably all O. (Hoplosmia) species are specialized on Asteraceae except for one pollen generalist species, which exhibits a preference for the pollen of Cistaceae. Among the Asteraceae specialists, differences exist with respect to the three main Asteraceae subfamilies exploited for pollen, with some species exclusively visiting Carduoideae, others exploiting only Asteroideae and Cichorioideae and again others collecting pollen on Asteroideae, Carduoideae and Cichorioideae. All O. (Hoplosmia) species build their brood cells within preexisting cavities: several species exclusively nest in empty snail shells, few species use small cavities in rock and stones and the remaining species colonize linear cavities in dead wood and plant stems or nest in abandoned burrows of other bees and wasps. Chewed leaves serve as material to construct brood cell partitions and nest plug except for two species, which use mud as nest building material. The taxonomic revision of O. (Hoplosmia) revealed the existence of an undescribed species, O. centaureae spec. nov., which occurs in a small area that ranges from the Dead Sea over the Jordan Valley to northernmost Israel. Due to clear morphological gaps and widely disjunct distribution with the nominotypical subspecies, O. pinguis carbo (Zanden 1974) is elevated to species rank. Based on morphology and biology, three species groups are recognized within Hoplosmia. Identification keys for all O. (Hoplosmia) species are given including the hitherto unknown male or female sex of three species.

Palaearctic Protosmia bees of the subgenus Chelostomopsis(Megachilidae, Osmiini): biology, taxonomy and key to species.

Chelostomopsis represents a subgenus of the osmiine bee genus Protosmia (Megachilidae) containing three Palaearctic and one Nearctic species. Analysis of female pollen loads and literature data indicate that all Chelostomopsis species are broad pollen generalists exploiting the flowers of numerous plant families, such as Fabaceae, Asteraceae, Lamiaceae, Cistaceae and Brassicaceae. Preexisting linear cavities in dead wood or pine cones serve as nesting sites and pure resin is used to construct cell partitions and nest plug. The taxonomic revision of the Palaearctic P. (Chelostomopsis) species revealed the existence of a still undescribed species from the Levant, Protosmia (Chelostomopsis) angustimandibulae spec. nov. Keys for the identification of the three Palaearctic P. (Chelostomopsis) species are given.

Palaearctic Hoplitis bees of the subgenera Formicapis and Tkalcua (Megachilidae, Osmiini): biology, taxonomy and key to species.

Formicapis and Tkalcua are related subgenera of the osmiine bee genus Hoplitis (Megachilidae) containing four and three species, respectively. H. (Formicapis) species inhabit open forests and windfalls mainly in the boreal zone with one species distributed across the Holarctic and three species occurring in eastern Asia. H. (Tkalcua) species live in deserts and semideserts from the Canary Island and northern Africa eastwards to the Arabian Peninsula. Analysis of female pollen loads, field observations and literature data reveal that H. (Formicapis) species have a distinct or even exclusive preference for the flowers of herbaceous Rosaceae as pollen hosts, while H. (Tkalcua) species appear to be pollen generalists. Nesting sites of H. (Formicapis) are insect burrows in dead wood, while the only H. (Tkalcua) species, whose nesting behaviour is known, builds its brood cells inside empty snail shells. Species of both subgenera construct brood cell partitions and nest plug with masticated leaves, which, however, are mixed with sand grains in H. (Tkalcua) zandeni (Teunissen & van Achterberg 1992). The taxonomic revision of H. (Formicapis) and H. (Tkalcua) revealed the existence of two undescribed species, H. (Formicapis) coreensis spec. nov. from South Korea and H. (Tkalcua) emiratensis spec. nov. from the United Arab Emirates. Hoplitis excisa (Morawitz 1880) is newly assigned to H. (Formicapis), whereas H. latifemoralis (Wu 1985), formerly considered to belong to H. (Formicapis), is removed from this subgenus due to morphological and biological differences and newly assigned to Hoplitis subgenus incertum. Identification keys for all H. (Formicapis) and H. (Tkalcua) species are given.

Nest architecture and use of floral oil in the oil-collecting South African solitary bee Rediviva intermixta (Cockerell) (Hymenoptera: Apoidea: Melittidae)

Nest architecture, use of floral oil for brood cell lining and pollen collecting are described for the first time for the genus Rediviva, using the South African endemic oil-collecting solitary bee species Rediviva intermixta. The nest consists of a dead-end vertical tunnel with a single brood cell located at the end of each of several horizontally branching lateral tunnels. Brood cells are lined with a thin layer of waxy material, presumably derived from chemically modified floral oil. Rediviva intermixta is a pollen generalist but relies on a small number of host plant species for oil-collecting. Brood cells are provisioned with pollen from at least six plant families, but with a preference for non-oil-producing Scrophulariaceae. The nesting biology and Dufour’s gland size of the species are discussed and compared with the closely related genera Melitta and Redivivoides (non-oil-collecting) and Macropis (oil-collecting). The differences between Macropis and Rediviva suggest that oil-collecting in the two genera evolved independently.

Life span in the wild: the role of activity and climate in natural populations of bees

Summary Animal life span is constrained by ecology and physiology. The latter has been studied under controlled conditions, but little is known about determinants of life span under natural conditions. We studied the relationships between length of adult life, magnitude of foraging activity, and environmental abiotic conditions in two bee species: a pollen specialist Andrena vaga (Andrenidae) and a pollen generalist Anthophora plumipes (Apidae). Our research indicates that life span is driven both directly by climate and indirectly through climate‐dependent activity patterns. We found a negative relationship between proportion of active days and length of life; in contrast, a high activity rate within the active days had no negative effect on longevity. Individuals active during warm and/or wet days lived longer, with precipitation being a more important determinant of life span than temperature. Timing of the first appearance at the site was also an important predictor of bee life span. Individuals that first appeared closer to the end of season (critical time horizon) lived a shorter time than individuals observed earlier. The first observed active day in the season was also correlated with seasonal temperature and precipitation. We demonstrate that life span and activity patterns of wild populations of insects are regulated by a tractable interplay of ecological (mostly climatic) variables that were previously studied only in isolation or in vitro.

Pollen mixing in pollen generalist solitary bees: a possible strategy to complement or mitigate unfavourable pollen properties?

Generalist herbivorous insects, which feed on plant tissue that is nutritionally heterogeneous or varies in its content of secondary metabolites, often benefit from dietary mixing through more balanced nutrient intake or reduced exposure to harmful secondary metabolites. Pollen is similarly heterogeneous as other plant tissue in its content of primary and secondary metabolites, suggesting that providing their offspring with mixed pollen diets might be a promising strategy for pollen generalist bees to complement nutrient imbalances or to mitigate harmful secondary metabolites of unfavourable pollen. In the present study, we compared larval performance of the pollen generalist solitary bee species Osmia cornuta (Megachilidae) on five experimental pollen diets that consisted of different proportions of unfavourable pollen diet of Ranunculus acris (Ranunculaceae) and favourable pollen diet of Sinapis arvensis (Brassicaceae). In addition, we microscopically analysed the pollen contained in the scopal brushes of field-collected females of O.cornuta and three closely related species to elucidate to what degree these pollen generalist bees mix pollen of different hosts in their brood cells. In striking contrast to a pure Ranunculus pollen diet, which had a lethal effect on most developing larvae of O.cornuta, larval survival, larval development time and adult body mass of both males and females remained nearly unaffected by the admixture of up to 50% of Ranunculus pollen diet to the larval food. Between 42% and 66% of all female scopal pollen loads analysed contained mixtures of pollen from two to six plant families, indicating that pollen mixing is a common behaviour in O.cornuta and the three related bee species. The present study provides the first evidence that the larvae of pollen generalist bees can benefit from the nutrient content of unfavourable pollen without being negatively affected by its unfavourable chemical properties if such pollen is mixed with favourable pollen. We conclude that the widespread pollen mixing by females of pollen generalist bees should also be considered as a possible strategy to exploit flowers with unfavourable pollen and to optimize larval food quality.

Phylogeny and floral hosts of a predominantly pollen generalist group of mason bees (Megachilidae: Osmiini)

Within the genus Osmia, the three subgenera Osmia, Monosmia, and Orientosmia form a closely-related group of predominantly pollen generalist (‘polylectic’) mason bees. Despite the great scientific and economic interest in several species of this clade, which are promoted commercially for orchard pollination, their phylogenetic relationships remain poorly understood. We inferred the phylogeny of 21 Osmia species belonging to this clade by applying Bayesian and maximum likelihood methods based on five genes and morphology. Because our results revealed paraphyly of the largest subgenus (Osmia s.s.), we synonymized Monosmia and Orientosmia under Osmia s.s. Microscopical analysis of female pollen loads revealed that five species are specialized (‘oligolectic’) on Fabaceae or Boraginaceae, whereas the remaining species are polylectic, harvesting pollen from up to 19 plant families. Polylecty appears to be the ancestral state, with oligolectic lineages having evolved twice independently. Among the polylectic species, several intriguing patterns of host plant use were found, suggesting that host plant choice of these bees is constrained to different degrees and governed by flower morphology, pollen chemistry or nectar availability, thus supporting previous findings on predominantly oligolectic clades of bees. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 78–91.

Maintenance of richness despite reduced abundance of desert bees (Hymenoptera: Apiformes) to persistent grazing

Abstract Anthropogenic changes to ecosystems typically result in decreased species richness and abundance relative to that of adjacent semi‐pristine or pristine areas. Yet, there is debate about the generality of this result for bees given that most studies have included extremely disturbed areas and are from a limited set of biogeographical areas. Repeat sampling of an unusually specialised, species‐rich bee community was done in the north‐western Chihuahuan Desert, North America at sites in five habitats (riparian, mesquite forest, abandoned field, grassland, and desert scrub) that either had been intensely grazed by cattle in the previous year or not been grazed for 22+ years. Species density and species composition of bees did not change in response to grazing, abundance and proportion of singleton species (those represented by one specimen) did. In all habitats, other than the riparian, there was lower overall abundance and a greater proportion of singleton species at sites that had been recently grazed than at sites that had not been grazed since 1979. The proportion of singletons was greater in recently grazed than in long‐term ungrazed areas, but pollen‐specialist species did not respond more strongly than pollen generalists. The substantial variation in bee abundance and rarity was probably associated with differences in floral resources. Overall, lower bee abundance in grazed areas reflects the continuous removal by grazers of the flowers that bees use. Despite reduced bee abundance due to long‐term grazing, species richness, and composition in this desert bee community remained high.

Host range evolution in a selected group of osmiine bees (Hymenoptera: Megachilidae): the Boraginaceae-Fabaceae paradox

Bees are extraordinarily diverse with respect to host plant choice and adaptation. Recent findings suggest that bee host range might be largely governed by evolutionary constraints related to pollen digestion or flower recognition and handling. In the present study, we applied phylogenetic inference to investigate whether such constraints underlie host plant choice in bees of the Annosmia-Hoplitis group (Megachilidae) and to what extent these bees have evolved specialized adaptations for pollen collection. We demonstrate that most pollen specialist species exclusively exploit either Boraginaceae or Fabaceae, whereas all pollen generalists harvest pollen from both Boraginaceae and Fabaceae. The counterintuitive affinity towards these two plant families, which are neither closely related nor share similar flower morphologies, demonstrates that pollen host choice is considerably constrained in this group of bees. We hypothesize that this Boraginaceae-Fabaceae paradox might be the result of (1) similar secondary metabolites in the pollen of both families; (2) metabolites that can be detoxified by the same physiological tools; or (3) similar pollen nutrient composition. Contrary to the widely held belief that specialized adaptations for pollen collection are rare among bees, such adaptations are common in the Annosmia-Hoplitis bees, where they have evolved several times independently to exploit flowers of widely different morphologies.

Closely related pollen generalist bees differ in their ability to develop on the same pollen diet: evidence for physiological adaptations to digest pollen

1. Given the enormous quantitative pollen requirements of bees and their high efficiency in pollen removal, flowers should balance the need to attract bees for pollination on the one hand and to restrict pollen loss to bees on the other hand. Although various morphological flower traits have been identified that reduce excessive pollen losses to bees, the question of whether pollen might also be chemically protected remains largely unexplored. 2. In this study we compared the larval performance of the two very closely related and highly pollen generalist solitary bee species Osmia bicornis and Osmia cornuta on four different pollen diets. 3. Despite their very large pollen diet breadth, the two bee species showed striking differences in their ability to develop on pollen of the same plant species. Osmia bicornis developed well on Ranunculus pollen but failed to do so on Echium pollen, whereas the reverse held true for O. cornuta with the exception of two larvae grown on Ranunculus pollen that developed into dwarfish adults. Both bee species performed well on Sinapis pollen, while neither of the two species managed to develop on Tanacetum pollen. 4. The observed differences in larval survival of these two Osmia species when reared on the same pollen diet as well as their failure to develop on Tanacetum pollen clearly demonstrate that bees require physiological adaptations to cope with the unfavourable chemical properties of certain pollen. 5. Our results show a remarkable analogy of bee–flower relationships with herbivore–plant interactions and possibly indicate that the pollen of certain plant taxa might be chemically protected.

Pollen hosts of western palaearctic bees of the genus Colletes (Hymenoptera: Colletidae): the Asteraceae paradox

To assess the pollen hosts of 60 western palaearctic bee species of the genus Colletes (Colletidae), we microscopically analysed 1336 pollen loads of collected females. Twenty-six species (43.3%) were found to be specialized at the level of plant family, subfamily or genus. Thirty-four species (56.7%) proved to be pollen generalists to varying degrees, visiting the flowers of up to 15 different plant families. Flowers of the subfamily Asteroideae (Asteraceae) are by far the most important pollen source, contributing 23.6% to the pollen-plant spectrum of the whole bee genus. The high significance of Asteroideae pollen is due to the large number of specialists: 14 Colletes species belonging to four different taxonomic groups harvest pollen exclusively or predominantly on flowers of the Asteroideae. By striking contrast, Asteroideae pollen plays only a marginal role in the diets of the pollen generalists: it was recorded in only 2.7% of the pollen loads and in seven out of the 34 pollen generalists. Among the few generalists exploiting Asteroideae for pollen, three closely related species have ancestors which were possibly specialized on Asteraceae. The pattern of use of Asteroideae pollen by the Colletes bees supports recent findings that this pollen possesses unfavourable or protective properties, which render its digestion difficult, and suggests that bees need physiological adaptations to successfully utilize it.

PATTERNS OF HOST-PLANT CHOICE IN BEES OF THE GENUSCHELOSTOMA: THE CONSTRAINT HYPOTHESIS OF HOST-RANGE EVOLUTION IN BEES

To trace the evolution of host-plant choice in bees of the genus Chelostoma (Megachilidae), we assessed the host plants of 35 Palearctic, North American and Indomalayan species by microscopically analyzing the pollen loads of 634 females and reconstructed their phylogenetic history based on four genes and a morphological dataset, applying both parsimony and Bayesian methods. All species except two were found to be strict pollen specialists at the level of plant family or genus. These oligolectic species together exploit the flowers of eight different plant orders that are distributed among all major angiosperm lineages. Based on ancestral state reconstruction, we found that oligolecty is the ancestral state in Chelostoma and that the two pollen generalists evolved from oligolectic ancestors. The distinct pattern of host broadening in these two polylectic species, the highly conserved floral specializations within the different clades, the exploitation of unrelated hosts with a striking floral similarity as well as a recent report on larval performance on nonhost pollen in two Chelostoma species clearly suggest that floral host choice is physiologically or neurologically constrained in bees of the genus Chelostoma. Based on this finding, we propose a new hypothesis on the evolution of host range in bees.

Spatial predictability and resource specialization of bees (Hymenoptera: Apoidea) at a superabundant, widespread resource

Abstract For reciprocal specialization (coevolution) to occur among floral visitors and their host plants the interactions must be temporally and spatially persistent. However, studies repeatedly have shown that species composition and relative abundance of floral visitors vary dramatically at all spatial and temporal scales. We test the hypothesis that, on average, pollen specialist bee species occur more predictably at their floral hosts than pollen generalist bee species. Taxonomic floral specialization reaches its extreme among species of solitary, pollen-collecting bees, yet few studies have considered how pollen specialization by floral visitors influences their spatial constancy. We test this hypothesis using an unusually diverse bee guild that visits creosote bush ( Larrea tridentata ), the most widespread, dominant plant of the warm deserts of North America. Twenty-two strict pollen specialist and 80+ generalist bee species visit Larrea for its floral resources. The sites we sampled were separated by 0.5 to >1450 km, and spanned three distinct deserts and four vegetation zones. We found that species of Larrea pollen specialist bees occurred at more sites and tended to be more abundant than generalists. Surprisingly, spatial turnover was high for both pollen specialist and generalist bee species at all distances, and species composition of samples from sites 1–5 km apart varied as much as repeat samples made at single sites. Nevertheless, the pattern of bee species turnover was not haphazard. As distance among sites increased faunal similarity of sites decreased. Faunal similarities among sites within 250 km of each other were generally greater than if randomly distributed over all sites (the null model). No single ecological category of species (widespread, localized, Larrea pollen specialist, floral generalist) accounted for this spatial predictability. Evidently, concordant local distribution patterns of many ecologically diverse species contribute to the non-random spatial pattern. The ecological dominance of creosote bush does not confer obvious ecological advantages to its specialist floral visitors. Spatial turnover is comparable to that found for bee guilds from other biogeographic regions of the world and is not therefore limited to those bee species that inhabit highly seasonal climates, such as deserts. Philopatry and differences in bloom predictability among sites are probably more important causes for spatial turnover of bee species than are interspecific competition for nest sites or floral resources.