Pat Willmer

LINKING BEES AND FLOWERS: HOW DO FLORAL COMMUNITIES STRUCTURE POLLINATOR COMMUNITIES?

Pollinators provide essential ecosystem services, and declines in some pollinator communities around the world have been reported. Understanding the fundamental components defining these communities is essential if conservation and restoration are to be successful. We examined the structure of plant-pollinator communities in a dynamic Mediterranean landscape, comprising a mosaic of post-fire regenerating habitats, and which is a recognized global hotspot for bee diversity. Each community was characterized by a highly skewed species abundance distribution, with a few dominant and many rare bee species, and was consistent with a log series model indicating that a few environmental factors govern the community. Floral community composition, the quantity and quality of forage resources present, and the geographic locality organized bee communities at various levels: (1) The overall structure of the bee community (116 species), as revealed through ordination, was dependent upon nectar resource diversity (defined as the variety of nectar volume-concentration combinations available), the ratio of pollen to nectar energy, floral diversity, floral abundance, and post-fire age. (2) Bee diversity, measured as species richness, was closely linked to floral diversity (especially of annuals), nectar resource diversity, and post-fire age of the habitat. (3) The abundance of the most common species was primarily related to post-fire age, grazing intensity, and nesting substrate availability. Ordination models based on age-characteristic post-fire floral community structure explained 39-50% of overall variation observed in bee community structure. Cluster analysis showed that all the communities shared a high degree of similarity in their species composition (27-59%); however, the geographical location of sites also contributed a smaller but significant component to bee community structure. We conclude that floral resources act in specific and previously unexplored ways to modulate the diversity of the local geographic species pool, with specific disturbance factors, superimposed upon these patterns, mainly affecting the dominant species.

Role of nesting resources in organising diverse bee communities in a Mediterranean landscape

Abstract.  1. The habitat components determining the structure of bee communities are well known when considering foraging resources; however, there is little data with respect to the role of nesting resources.

PARTITIONING OF POLLINATORS DURING FLOWERING IN AN AFRICAN ACACIA COMMUNITY

Competition for pollination is an important factor structuring flowering in many plant communities. We examined mechanisms reducing interspecific pollen flow in a community of 10 Acacia species in a highly seasonal savannah habitat in Tanzania. Partitioning is achieved, in part, through separation of flowering in space and seasonal time, and through interspecific differences in pollinator guilds. Nevertheless, coflowering Acacia species shared several pollinators; this means that interspecific pollen transfer is possible. We analyzed daily patterns of pollinator activity and pollen release in 10 Acacia assemblages containing a total of 10 Acacia species. Pollinator activity was scored using counts at flowers over constant time intervals throughout the day. Pollen availability was assessed using a simple method which allows quantification of pollen exposed on the surface of the Acacia inflorescence. Sympatric co-flowering Acacia species each show high intra- specific synchrony but release their pollen at different times of day. Pollinators rapidly harvest available pollen and move from one Acacia species to the next, following the daily sequence of pollen release. The activity of shared pollinators is structured throughout the day as a result of temporal patterns of pollen release across Acacia species. The observed temporal structuring of pollen release is compatible with patterns predicted to result from competitive displacement. Additional support for a competition-based explanation for this patterning comes from the observation that an Acacia species flowering without competitors

Why flower visitation is a poor proxy for pollination: measuring single-visit pollen deposition, with implications for pollination networks and conservation

Summary The relative importance of specialized and generalized plant-pollinator relationships is contentious, yet analyses usually avoid direct measures of pollinator quality (effectiveness), citing difficulties in collecting such data in the field and so relying on visitation data alone. We demonstrate that single-visit deposition (SVD) of pollen on virgin stigmas is a practical measure of pollinator effectiveness, using 13 temperate and tropical plant species. For each flower the most effective pollinator measured from SVD was as predicted from its pollination syndrome based on traditional advertisement and reward traits. Overall, c.n40% of visitors were not effective pollinators (range 0n78% for different flowers); thus, flowernpollinator relationships are substantially more specialized than visitation alone can reveal. Analyses at species level are crucial, as significant variation in SVD occurred within both higher-level taxonomic groups (genus, family) and within functional groups. Other measures sometimes used to distinguish visitors from pollinators (visit duration, frequency, or feeding behaviour in flowers) did not prove to be suitable proxies. Distinguishing between lpollinatorsr and lvisitorsr is therefore crucial, and true lpollination networksr should include SVD to reveal pollinator effectiveness (PE). Generating such networks, now underway, could avoid potential misinterpretations of the conservation values of flower visitors, and of possible extinction threats as modelled in existing networks.

How aggressive ant-guards assist seed-set in Acacia flowers

The phenomenon of ant-guarding on Acacia trees is probably the best known case of a mutualism between plants and animals, the ants conferring biotic defence against herbivores and perhaps against encroaching vegetation. However, as with many defence mutualisms, sometimes the interests of the plant and its defender conflict: for example, when they are in flower the Acacia trees require the presence and service of other insects to effect cross-pollination. How is pollinator access achieved in the face of aggressive ant-guards? Here we report that ants are deterred from young flowers at the crucial stage of dehiscence, allowing bees and other pollinators to visit and transfer pollen. This deterrence appears to be a response to a volatile chemical signal from young flowers, perhaps from the pollen itself. Ants patrol the young (undehisced) buds, and also return to the flowers after dehiscence, protecting the fertilized ovules and developing seeds. The outcome is a directly improved seed-set in the presence of ants (rather than an indirect extra reproductive resource allocation due to decreased defoliation).

The superiority of bumblebees to honeybees as pollinators: insect visits to raspberry flowers

Abstract. 1 The behaviour and activity patterns of Apis mellifera and of five species of Bombus were analysed in relation to climatic variables and nectar quality on three varieties of unsprayed cultivated raspberry (Rubus idaeus) in eastern Scotland. 2 Stages of floral morphology and reward were similar for the three varieties: young flowers offered both nectar and pollen, but medium and old flowers offered nectar only, in diminishing quantities. 3 A wide range of insects visited raspberry flowers, but bees were dominant, bumblebees being responsible for about 60% of all visits and honeybees making up most of the remaining percentage. All bees had substantial pollen deposited on their bodies during visits, though few specifically collected it. 4 Bombus spp. were found to favour young (receptive) flowers strongly, especially early in the morning when pollen was most abundant: whilst Apis visited unselectively. Bumblebees also foraged over substantially longer periods of the day, and in poorer weather, some being present at most times of observation; and they foraged more quickly in terms of flower visits per minute. 5 Bombus carried more pollen on their bodies than Apis, and also deposited more pollen on raspberry stigmas, with B.lapidarius and B.terrestris being particularly effective and also being the most abundant species. All bumblebees also foraged over a longer range, moving between canes and rows more frequently than did honeybees. 6 Bumblebees are therefore likely to be substantially more important as pollinators of raspberries than are honeybees, especially as raspberries though moderately self‐fertile may exhibit metaxenia. Reasons why Bombus may be the preferred pollinator in most sites of raspberry cultivation are discussed, together with implications for present and future growers.

Behavioral, ecological, and physiological determinants of the activity patterns of bees

Publisher Summary This chapter discusses the behavioral, ecological, and physiological determinants of the activity patterns of bees. An activity pattern is the change in levels of a particular activity through time. Bees are an excellent taxon within which to investigate both the issues of constraint and currency. Their physiological constraints are better understood than those of almost any other invertebrate, allowing an unparalleled opportunity to integrate physiology into behavioral ecology. Many bees require elevated body temperatures ( T b ) to fly, and hence thermal properties of their environment significantly constrain their activity. However, many species are heterothermic, being able to elevate their T b endothermically when necessary, which gives some degree of escape from the usual thermal constraints on other entirely ectothermic insects. The chapter discusses the factors structuring activity patterns in a wide range of bees. Observed activity patterns are generated by interactions between properties of the bees themselves (intrinsic factors) and properties of their environment (extrinsic factors). Intrinsic factors include physiological differences between species and between the sexes. Extrinsic factors are the fluctuating properties of the environment, and are either abiotic or biotic.

Abiotic and biotic factors influencing nest-site selection by Halictus rubicundus, a ground-nesting halictine bee

1. The nest‐site selection behaviour of the bee Halictus rubicundus (Christ) was examined both within and across sites in the U.K. Females utilized a range of edaphic and microclimatic conditions when choosing a site to excavate a nest. Factors with broad tolerances included slope and hardness; those with much narrower limits included aspect, soil humidity and soil particle composition.

Pollination ecology of acacias (Fabaceae, Mimosoideae)

We review the pollination ecology of acacias worldwide, discussing (1) the rewards provided to flower visitors, (2) the temporal patterns of flowering and reward provision and (3) the taxonomic composition of flower visitor assemblages. The flowers of most acacias (including all members of the subgenus Phyllodineae) offer only pollen to flower visitors and floral nectar is limited to a minority of species in the subgenera Acacia and Aculeiferum. The most important pollinators of acacias are social and solitary bees, although other insects and nectar-feeding birds are important in specific cases. Acacias that secrete nectar attract far more species-rich assemblages of flower visitors, although many of these are probably not important as pollinators. Most acacias in the subgenus Phyllodineae have long-lived protogynous flowers, without clear daily patterns in reward provision and visitation. In contrast, most members of the other two subgenera have flowers that last for a single day, appear to be protandrous and have clear daily patterning in reward provision and visitation. The generality of these patterns should not be assumed until the pollination ecology of many more phyllodinous acacias has been studied, particularly in arid environments. The accessibility of the floral rewards in acacia flowers makes them important examples of two general issues in plant communities—the partitioning of shared pollinators and the evolution of floral ant repellents. SBogy as G. N.

The potential for interspecific competition between two abundant defoliators on oak: leaf damage and habitat quality

Abstract. 1. The pedunculate oak, Quercus robur L., suffers high annual levels of spring defoliation in Wytham Woods, Oxon. The two major defoliators, Tortrix viridana L. and Operophtera brumata L., sometimes reach high enough densities to completely defoliate trees.