The present distribution of bee taxa depends on (1) the climatic and vegetational factors considered in Section 24 and (2) intercontinental and other barriers and the Tertiary and probably late Cretaceous continental movements considered in Section 26. But present distribution also depends on bees' ability to disperse and to reach suitable areas under their own power.
Because bees fly well, one might think that they would be rather successful at crossing barriers, such as water or areas that are climatically or vegetationally inhospitable. A female bee usually mates early in adult life and carries enough sperm cells in her spermatheca to last for part or all of her reproductive life. One can therefore assume that, except for those ofthe few highly social bees (Meliponini, Apini), individual females transported across a barrier might be able to nest, reproduce, and establish a population. The ability of a bee in a new area to fly about and perhaps find suitable nest sites and food sources also would seem to enhance the probability of establishment.
Nonetheless, distributional data suggest that most groups of bees are not particularly good at crossing major barriers. Most bees fly only in good weather; accordingly, they are likely to be in their nests during storm winds. Moreover, individuals of successive generations of solitary to primitively social species commonly return to the same nesting site, so that they tend to be quite sedentary (Mich-ener, 1974a). Thus for the majority of kinds of bees, dispersal has been by slow spread across continents or to nearby land masses, or by transport on moving continents. The Antillean and eastern Indonesian faunas, however, show that scattered islands between continents can serve as stepping stones for many taxa. (Western Indonesian islands were so recently part of the continent ofAsia that no over-water dispersal is needed to explain their bee faunas.)
The monumental work on bees ofMadagascar (Pauly et al., 2001) reports 244 species from that island. Although there are certainly new species to be found, the fauna is far better known than previously. Nearly all species and several genera are restricted to Madagascar. Major groups that one might expect in Madagascar but that are absent include the Colletinae, Andrenidae except one species of Melitturgini, Melittidae except Meganomiinae, Osmiini except Heri-ades and Stenoheriades, and Nomadini.
Some evidence suggests that solitary to primitively social bees that nest in wood or stems are more likely to cross water barriers than are those that nest in the ground, presumably because wood and stems containing nests are sometimes carried above water in floating islands of vegetation. The bee faunas of oceanic islands, however, include minute forms that nest in soil in addition to the moderate-sized to large, wood-nesting species. For example, on the oceanic islands of the Pacific (Fiji, Samoa, Hawaii, Micronesia, etc.) there are a few moderate-sized, wood-nesting Lithurgus and Megachile, perhaps carried to some of the islands by nests in Polynesian boats. Otherwise, the fauna consists mostly of small, ground-nesting Homalictus (Halictini) or of Hylaeus; the latter nest in wood, in stems, or in holes in rocks or soil. It is only among the small forms, especially Hylaeus in Hawaii
(Perkins, 1899) and Homalictus-Echthralktus in Samoa (Perkins and Cheesman, 1928), that there has been significant evolution in these isolated oceanic islands. Thus it seems that dispersal, presumably by wind at least for Homalictus, must have favored small forms, whereas the larger ones probably came later and perhaps in some cases with the help of humans. The Galápagos Islands lack small bees and have only one large form, the wood-nesting Xylocopa darwini Cockerell, which belongs to the tropical American subgenus Neoxylocopa.
Less-isolated island groups frequently have richer faunas, including small as well as larger bees, both those that nest in soil and those that nest in wood. For example, the rather large Megachile (Creightonella) frontalis (Fabricius) [and its subspecies or allied species M. atrata (Smith)] ranges across 6,400 km from Sumatra to the Solomon Islands; this is a ground-nesting species, and presumably the adults have flown or been blown across the water barriers. The Antillean fauna has probably all arrived across the water. Yet it includes not only bees that nest in wood and minute bees, but also moderate-sized to large, ground-nesting bees such as Agapostemon, Anthophora, Caupolicana, Centris, and Melissodes. They probably were blown from Yucatan or elsewhere by hurricanes or other storms. The New Zealand bee fauna consists largely of middle-sized ground-nesting Leioproctus rather closely related to those of Australia, about 1,400 km away. It is the smallest fauna of any substantial land area except for arctic and antarctic regions; rare over-water dispersal seems to be the only reasonable explanation for its fauna.
The highly social bees (Meliponini, Apini) present special biogeographical problems. They disperse by swarming or by absconding as colonies, not by the action of individuals, most of which are in any case nonreproductive workers. In Meliponini, a new colony is established by individuals from the parent colony that go back and forth provisioning the new nest before a young queen goes there. Thus dispersal by flight across even a few hundred meters of water would be impossible. The absence of Meliponini from the Greater Antilles (except for one species probably introduced by human agency), in spite of their abundance on the Caribbean mainland shores, supports this view, although there are fossil meliponines from Hispaniola. In view of the distribution of Meli-ponini in the East Indies, as far east as the Solomon Islands and south to Australia, these bees must occasionally be carried across substantial water barriers as colonies in natural rafts or perhaps even in hollow logs floating in the sea. They store food supplies, and those that inhabit hollow logs often close entrances with waterproof resin under unfavorable conditions, and therefore might survive weeks of drifting. In the Apini, dispersal is by swarms or migrating colonies that may fly for distances of perhaps dozens ofkilometers or—across habitable country where they can stop—hundreds of kilometers. Yet traversal of a broad ocean by an organized swarm would be impossible. There are three Apis species in the Philippines, but the water gaps there were much narrower or absent when the sea level was lowered during the glacial periods.
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