Biogeography

The distributions of the various groups of bees (tribes to families) are indicated in Table 26-1, which is a modified version of a table presented by Michener (1979a). The following is an explanation of the columns.

1. Aust. Australia, including Tasmania, New Guinea, the Bismarck Archipelago, and nearby islands such as the Solomons.

3. Orient. The oriental faunal region, i.e., tropical Asia from Sri Lanka, India and Pakistan below the Himalayas, across southeastern Asia to Vietnam and southeastern China, also Taiwan, the Philippines, and western Indonesia. (Most of China and Japan are in the eastern part of the palearctic region, not in the oriental region. Likewise, the mountainous parts of northern India and of Pakistan and its western area in Baluchistan are palearc-tic.)

4. Madag. Madagascar.

5. Afr. Sub-Saharan Africa (Africa north of the Sahara is palearctic).

6. Palear. The palearctic faunal region, including Europe, northern Africa, Turkey and the Middle East, northern India and Pakistan, most of China, and Japan. Although much of Pakistan is oriental, the western Baluchistan area is palearctic.

7. Nearct. The nearctic faunal region, including the Mexican plateau and surrounding mountains. Seemingly nearctic areas on the mountains of Chiapas, Mexico, and Guatemala are explained separately in the text but are not included under the term "nearctic" for present purposes.

8. Neotr. The neotropical faunal region from tropical Mexico southward through South America, excluding regions 9 and 10.

9. Antill. The Greater and Lesser Antilles, excluding Trinidad which is included in region 8.

10. Arauc. The Araucanian region, i.e., Chile and adjacent parts of western and southern Argentina.

Across the bottom of Table 26-1, the totals show the numbers of higher-category (tribe to family) bee taxa in each region. Because the areas represented in Table 26-1 often grade into one another, arbitrary decisions were often necessary. Michener (1979a) provided details not repeated here. Radchenko and Pesenko (1994) provided a similar table, except that their columns are for the usual six biotic areas; comparison with Table 26-1 may be useful.

The place of origin of bees remains obscure, but one can speculate as to both the place and its climate. The xeric interior of the old continent of Gondwanaland, particularly West Gondwanaland (Africa-South America), has been suggested as the area of origin of angiosperms (Raven and Axelrod, 1974). It presumably had a seasonal temperate climate. Xeric regions, especially those with sandy soils, are commonly areas of abundance for sphe-coid wasps, most ofwhich nest in the ground. It is not unlikely that bees arose from these wasps in such a place, or, if bees already existed, that they radiated in such a place. Most of them have retained their association with xeric areas and have been, compared to the angiosperms, relatively unsuccessful in adapting to humid climates.

The complete absence of unusual archaic bees in New Zealand may indicate that there were no bees in Gond-wanaland when New Zealand became isolated by the splitting of that continent in the late Jurassic, some 157 myBP (Smith, Smith, and Funnell, 1994). New Zealand was one of the early fragments to separate from Gondwanaland. As noted in Section 25, the bees of New Zealand appear to be the few that arrived and established themselves after overwater dispersal from Australia; the New Zealand bees all belong to Australian genera. The distance now is about 1,400 km.

The distributions of most bee taxa are not disjunct, and where disjunctures do occur they are limited to neighboring continents, e.g., holarctic taxa whose distributions could have been attained during warmer times with the continents in their present positions. Similarly, many tropical groups occur in Africa and from Sri Lanka and India eastward across southern Asia, often with little differentiation between forms in the two areas. An earlier, more humid (not wet-tropical but savanna) climate across the Arabian peninsula, southern Iran, and western Pakistan would connect or nearly connect these areas for the bees concerned, even with the continents in their present positions. Since more humid conditions undoubtedly existed in this area in the not very distant past, this disjuncture, like that for cool-temperate forms across Bering Strait, is easy to understand. Some of the taxa involved are Braunsapis, Ctenoplectra,, Megachile (three or more subgenera), Pachyhalictus, Tetralonia (Thygatina), Thrinchostoma, and Xylocopa (various subgenera).

Some taxa are found on most or all continents and even many islands, and obviously have considerable potential for crossing major water barriers. The dispersal of some such groups probably preceded the present arrangement of the continents. Numerous taxa, by occurring on both sides of a major physiogeographical or climatic barrier, provide some information on their antiquity or their dispersal ability.

Many organisms inhabit Mediterranean and desertic climates ofboth North and South America and are absent from the intervening tropics. These amphitropical distributions have long been a subject ofinterest (Raven, 1963; Raven and Axelrod, 1974). In North America most am-phitropical bees are primarily Sonoran and Chihuahuan desertic elements, although Caupolicana and Ptilothrix each has an eastern North American species. In South America, amphitropical bees occur either in the Argenti-nan-southern Brazilian area, often in its more xeric parts, or in the Araucanian region, or both. Some amphitropi-cal genera and subgenera are Anthophorula, Caupolicana, Martinapis, Protoxaea,, Ptilothrix, and Zikanapis.

Since there is no evidence for an arid corridor through the neotropics during the Tertiary (or at any other time), long-distance dispersal probably accounts for am-phitropical distributions, perhaps facilitated by local xeric areas, as suggested by Michener (1954). The eleva-

Table 26-1. Summary of Distribution of Families, Subfamilies, and Tribes of Bees.

Areas (column headings) are explained in the text. Plus signs indicate presence, neither diversity nor abundance necessarily implied; m indicates marginal presence, i.e., the taxon enters the area only marginally, or one or a few of its species extend well into the area but not halfway across it. The second symbols in some of the notations are subjective indications of relative diversity of the taxon in those areas, + indicating more diverse, — less diverse, than indicated for the taxon by a simple + without a second symbol. Introductions by human agency are ignored.

123 4 567 89 10

Taxon Aust NZ Orient Madag Afr Palear Nearct Neotr Antill Arauc

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