Bees constitute a 120 million year old monophyletic group that today includes approximately 20,000 described species worldwide. They rely almost exclusively on flowers for protein, lipids, and sugar throughout their life cycle. It has been estimated that 87.5% of all flowering plants (i.e. Angiosperms) are adapted to animal pollination, a large majority of which occurs through bees as they are the main pollinators in most ecosystems. The evolution of bees and flowering plants are therefore intrinsically linked. Bees have been recorded worldwide, in all continents and all habitats, wherever flowering plants are present.
A recent checklist collated by Kuhlmann et al. (2014) showed that western Palaearctic bee fauna totals about 3,370 species. Based on this checklist, and considering the European Red List boundaries, 1,965 species were assessed at the European level, and 1,900 species were assessed at the EU 27 level for the purposes of the European Red List of bees. These species are divided into six families and two groups:
(i) Apidae and Megachilidae form the group of long-tongued bees;
(ii) Andrenidae, Colletidae, Halictidae and Melittidae represent the short-tongued bees.
As there are approximately 20,000 species worldwide, Europe hosts about 10% of worldwide bee diversity, although the continent only represents 7% of global terrestrial habitats. The most prominent and diverse family of bees is the Apidae (561 species), which includes the Honey Bee and theBumblebees (Bombus spp.), while the least diverse family is the Melittidae with only 37 species.
The relatively high species diversity in Europe is partially explained by the presence of areas with a Mediterranean climate, as this climate provides the optimal conditions for bee diversity. Moreover, taxonomic research on bees in Europe has a very rich and well documented history and may skew the known diversity of bees, as many species have yet to be described in several other areas of the world. European authors, such as Linnaeus (1707-1778), Kirby (1759-1850), Latreille (1762-1833) and Lepeletier (1770-1845) were some of the first taxonomists to describe bees. Their type material, or original specimens, are still well conserved and available for study in European academic institutions.
The mapping of bee diversity in Europe highlights a general north-south positive gradient, with diversity increasing towards the Mediterranean area. Two features explain this pattern across Europe: (i) the more favourable energy/water balance of the Mediterranean areas which has resulted in extremely high floral diversity); and (ii) the likely role of these areas as refuges during the Quaternary glaciations.
Bee ecology can be characterised based on food, sociality and nesting requirements.
Bees can collect various resources from plants such as pollen, nectar, and less commonly, oil or perfumes; some materials used for nesting like resin, soil, and pieces of leaves and petals are also collected. Various foraging strategies have been described for bees mainly based upon the range of pollen collection from host plant(s). Bees collect pollen as a food source for their larvae and, in doing so, help pollinate the flowers of the plants upon which they forage. Some taxa display floral specificity, restricting their flower visits to closely related plant taxa (pollen specialists) while other bee species are more opportunistic, exploiting a wide range of different flowers (pollen generalists). Terms have been developed to describe the continuum in bee foraging strategies, from extreme specialization to extreme generalization: (i) monolecty (one host plant species); (ii) oligolecty (one host plant family) and (iii) polylecty (more than one host-plant family). Moreover, quantitative pollen requirement is relatively high for bees as they forage only on pollen for protein resources while other pollinators like syrphid flies or butterflies feed on alternative resources in the larval stage. Depending on both bee species and plant resources, from seven to 1,100 flowers or from 0.9 to 4.5 inflorescences are needed to rear a single larva.
Pollen is not the only food source for bees. Vascular plants produce substances such as nectar and oils. The extraction and gathering of such substances can require morphological adaptations. The morphological feature used for nectar collection is the labiomaxillary complex (i.e. the proboscis or tongue) that may be differentially shortened or elongated to reach nectar. Another non-pollen resource for bees is the plant itself, especially its tissues. Bees use resins, masticated leaves, cut petals, trichomes or other plant materials, sometimes along with mud and saliva, to construct nests in cavities or in the soil. Females can also use circular excisions of leaves and petals to line their brood cells (e.g., some Megachile s.l. species, some Osmiini species)or masticated leaves to hide the nest (e.g., some Osmia species).
Sociality is very variable among bees. Species can be solitary, social or kleptoparasitic, although the majority of the species are solitary. In strictly solitary species, each female builds a nest by herself, prepares the nest for her larvae and then dies without ever having contact with either her offspring or with her conspecifics, except of course for mating. The social species include the most familiar species such as the Honey Bee and the bumblebees, yet at world scale they represent only 6% of the bee species diversity and are restricted to two families (Apidae and Halictidae). The eusocial species are made up of colonial family groups, with reproductive division of labour, cooperative brood care and overlapping generations. These colonies typically show a bias in the degree of reproduction where only a small number of individuals lay the majority of eggs; the other individuals are sterile, or lay unfertilized male eggs and mainly care for the offspring. The last category includes bees that do not collect pollen but are parasites of other bee species. Parasitic, cuckoo or kleptoparasitic bees make up approximately 20% of described species globally. In Europe, three out of the six families of wild bees include at least one genus of species with exclusively parasitic ways of life (Apidae, Halictidae and Megachilidae). Cuckoo bees can be very variable in their host spectrum; some are exclusively linked to one host-bee species, while others have two to numerous hosts and are then considered generalists. Host generalist cuckoo bees are host specialised at the individual level, while host specialists can switch among the hosts without considerable change to their specialized strategy during their evolution.
Concerning the nesting behaviour of bees, a distinction can first be made between the ground-nesting digger species and the ones that nest away from the ground or in existing underground cavities. The ground-nesting digger species account for over half of all the species in the world. In these taxa, the ground texture, sun exposure and degree of slope can be important factors in determining nest sites. Some species such as Dasypoda hirtipes (Melittidae), Andrena fuscipes (Andrenidae) and Panurgus calcaratus (Andrenidae), have a preference for well-exposed sandy sites. Other species, such as Colletes cunicularius (Colletidae) and Andrena vaga (Andrenidae), are strictly dependent on sandy ground (psammophilous species). Among the non-digging species, some excavate their galleries in plants (e.g., stems), while others do so in rock crevices, soil or abandoned nests. Other species build potter nests in mud, or in plant resin or fibres. For example, Megachile parietina builds cells with dried mud on rock sides or walls. In Icteranthidium and Anthidiellum, the females harvest resin from conifers to build brood cells and attach them to a support they made of mineral or plant materials. Finally, many long-tongued bees (Apidae and Megachilidae) nest in a variety of existing holes, either naturally made or created by another organism. Some species even nest exclusively in empty snail (Helicidae) shells (e.g., Osmia bicolor and Osmia aurulenta). A number of species of the genus Bombus nest in much larger cavities. Bombus terrestris, for example, can set up home underground in abandoned rodent galleries. B. hypnorum often nest in old bird nests installed in tree hollows. The carder bees, Bombus pascuorum and Bombus muscorum, for example, nest in accumulations of dried grassland plant litter on the ground surface.
By collecting floral resources (i.e., pollen, nectar) for feeding, bees carry pollen on their bodies and transfer it from flower to flower. They can fertilize plants this way through pollination, allowing the plants to reproduce sexually. The ubiquity of bees and their tight association with flowering plants makes their role in pollination a global keystone in wild and agricultural ecosystem dynamics.
In terms of global agricultural production volumes, 35% comes from crops that depend (to a greater or lesser extent) on pollinators, mainly insects (i.e. one third of human food is mainly from plants pollinated by insects). Out of the 124 main cropsgrown for global human consumption, 87 (70%) require insect pollination for seed production (e.g., carrots, onions, garlic) and to enhance product quality and yields (e.g., coffee, nuts, many fruits). Bees provide an ecosystem service in the form of crop pollination estimated to be 153 billion Euros a year worldwide and 22 billion Euros a year in Europe.
Enhanced bee pollination can lead to benefits such as increased production, better crop quality and shelf life, yield stability and higher commercial value for many entomophilous crops (e.g., strawberries, and apples). Bee species diversity is also important as recent studies show that wild bees are responsible for a greater proportion of the pollination service previously attributed to domesticated honey bees (Apis mellifera). In addition, some crop plants can only be pollinated by a restricted number of species hence the loss of bee biodiversity can lead to loss of plant diversity.