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Effects of Past Global Change on Life
FIGURE 7.3 Stratigraphic ranges of antelope species in sub-Saharan Africa (I: immigrants; L:living; for identities of species, see Vrba, 1988). Most of the extinctions close to 2.5 Ma were of forest-adapted species. Most species that appeared at this time or shortly thereafter were savanna dwellers.
ably, this resulted from two conditions: (1) many species of mammals were endemic to Africa, and (2) the vegetational changes were continental in scale. Numerous forest-adapted species of antelopes died out close to 2.5 Ma, but there was a positive evolutionary response to the climatic change as well (Vrba, 1985). Several new species of antelopes adapted to savannas appeared through speciation events at about 2.5 Ma, or within 200,000 to 300,000 years thereafter (Figure 7.3). At the same time, several species adapted to grasslands invaded from Eurasia, apparently because forest barriers disappeared, at least temporarily. These species and others that evolved slightly later constitute the modern antelope fauna of the African savannas. Micromammals appear to have changed in a parallel fashion (Wesselman, 1985); in Ethiopia, numerous forest-dwelling species died out and were replaced by species adapted to drier conditions. African monkeys experienced a high rate of evolutionary turnover at about the same time, although the relationship of this change to the vegetational transition remains obscure (Delson, 1985). The dramatic effects on the history of the human family are discussed in Chapter 14 of this volume.
We have already noted the floral change along the northwestern border of the Mediterranean, where the decline of cypress-dominated forests beginning slightly before 3 Ma apparently resulted primarily from aridification but may have entailed climatic cooling as well. This region is marginal to the trade wind belt and is far more arid today than northwestern Europe. In the latter, more northerly region, cooling of climates and also divergence of seasonal extremes of temperature apparently had a stronger effect on biotas than did aridification.
We have previously noted the disappearance of the subtropical Malayan component from the flora of northwestern Europe in two pulses centered at about 3.1 and 2.5 Ma. This was not simply a matter of regional extermination: numerous endemic species suffered extinction (Leopold, 1967). As Reid and Reid (1915) noted long ago, at the time of the climatic change, southern Eurasia was spanned from east to west by a barrier of seas, deserts, and mountains unbreached by river valleys. At the same time, the increasingly dry climate of central Asia blocked eastward migration. The result was a lethal trap for many warm-adapted species, including palms. Other species, which were more widely distributed before climatic changes began, disappeared from Europe but survived in North America or China.
The floral change began earlier in northwestern Europe than in Poland (Leopold, 1967). Extinction continued into the Pleistocene Epoch, especially in Poland, but ceased almost entirely during the latter part of the Pleistocene. Floral compositions have continued to fluctuate markedly with shifts of climate, however, as a result of oscillatory migration. In Poland, for example, temperate elements have been more conspicuous during glacial minima (including the Holocene) and arctic-alpine elements during glacial maxima. In other words, the heaviest extinction occurred during the Pliocene, when numerous vulnerable species were present, and declined into the Pleistocene. By the latter part of the Pleistocene, vulnerable species had disappeared and floras consisted of species that could tolerate climatic fluctuations. Today, as a result of extinction and regional extermination, the European flora is markedly impoverished.
Mammals suffered much less heavy extinction in Europe than in Africa during the Pliocene. The strongest pulse of extinction in Europe appears to have occurred shortly after 2.0 Ma, before the existence of the faunas labeled "Tegelen"; about 15% of all known mammal species died out (Kurtén, 1968). The relatively high survivorship for the entire Pliocene Epoch probably reflected the nature of the climatic and floral changes in Europe. Whereas throughout the African continent the forest biome was constricted to a degree that was lethal for many kinds of animals, in Europe, forests changed in composition and