Effects of Past Global Change on Life (1995)
Chapter: Extinction
environmental change by producing chain reactions of extinction. On the other hand, interactions promote the diversification of certain taxa when others on which they depend diversify.
Migration
The intercontinental migration of Cenozoic land mammals has produced numerous natural experiments of faunal mixing. In interpreting the fossil record of these events it is often difficult to trace the causes of dispersal in detail. Correlation of the deep-sea oxygen isotopic record with pulses of mammalian migration between Eurasia and North America implicates land bridges produced by glacially controlled eustatic lowering of sea-level (see Webb and Opdyke, Chapter 11). The subsequent spread of taxa throughout new regions may have been influenced by climatic or other environmental changes. In addition, it is not always clear whether the excessive rates of extinction that have typified regions being invaded by new species have resulted from habitat change or adverse species interactions. What is clear at present is that during the Cenozoic there had been a strong correlation between mammalian turnover and changes in sea-level and climate.
The behavior of plant associations during floral migration has recently attracted much attention, partly because of its implications for floral changes during future global warming and partly because new evidence has contradicted the traditional view that modern biomes are ancient, coadapted associations. It appears that during the glacially induced climatic and eustatic fluctuations of the Pennsylvanian Period, coal swamp floras retained their ecological structure through many cycles of expansion and contraction (see DiMichele and Phillips, Chapter 8). Perhaps this can be attributed in part to the discrete character of the moist coal swamp environment, which did not easily exchange species with neighboring habitats. On the other hand, the pollen record of the past 20,000 years reveals that modern forest biomes of the temperate zone are transitory associations, not long-standing ones (see Webb, Chapter 13). Today in eastern North America, for example, Pinus (pines) and Quercus (oaks) have largely complementary geographic distributions outside the coastal plain, but this pattern has developed since the rapid contraction of glaciers about 10,000 years ago. During the most recent glacial maximum, pines and oaks were both largely restricted to a small region of the southeastern United States. Independent migration of plant species during future climatic changes could have important consequences for negative interactions between species of both plants and animals. Additional evidence of biotic mixing comes from small areas of Australia, where the present blending of floral provinces seems to have resulted from mid-Miocene warming.
Exactly what happened to tropical rain forests during Pleistocene glacial maxima remains unclear. Limited palynological and paleogeomorphological data suggest that the Amazon rain forest was considerably reduced in area during the last glacial maximum. Also, present geographic occurrences of certain taxa of plants and animals have been interpreted as relict distributions produced by fragmentation of rain forests during glacial maxima. This possibility needs further study, as does the more general question of coherence of rain forest communities during the past 2.5 m.y.
In the modern marine realm, many species that lived together in shallow water Pliocene environments of eastern North America are now confined to separate depth zones. Increased seasonality (especially colder winter temperatures) since the onset of the recent ice age has driven thermally intolerant forms into deeper waters (see Stanley and Ruddiman, Chapter 7).
Extinction
In recent years, numerous biological patterns—biases against certain kinds of taxa— have been detected for particular episodes of extinction. Commonalities among victims
