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Effects of Past Global Change on Life Overview and Recommendations OVERVIEW The geologic record provides a unique, long-term history of changes in the global environment and of the impact of these changes on life. From the fact that organisms are intimately related to their environment, we can infer that environmental changes of the past have molded the history of life. The geologic record contains the paleontological evidence that confirms this inference for a wide range of temporal and spatial scales. Study of this record is providing a framework for evaluating the impact of present and future global change on the biosphere—a framework that is urgently needed for the formulation of public policy. What can be expected to happen to biotic communities when climatic zones shift or habitats shrink? As trends of global change progress, what thresholds may trigger sudden shifts between environmental states or cause catastrophic destruction of life? Lessons of the past will serve us well as we confront the future. The geologic record reveals how particular kinds of environmental change have caused species to migrate, become extinct, or give rise to new species. More generally, it shows that many kinds of species and ecosystems are naturally fragile, and therefore transient, whereas other kinds are inherently more stable. Many advances in the understanding of ancient ecosystems are interdisciplinary in nature. Accurate plate tectonic reconstructions are essential for the evaluation of circulation patterns for ancient atmospheres and oceans. Geochemical data help us to understand ancient atmospheric and oceanic compositions, as well as climates. Functional morphology and studies of fossil preservation reveal modes of life of extinct species, and knowledge of the ecological requirements of fossilized organisms complements sedimentological analyses in the reconstruction of ancient environments. Fossil plants are among the most important indicators of ancient terrestrial climates, and studies of microfossil assemblages and stable isotopes are critical for reconstructing the three-dimensional structure of
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Effects of Past Global Change on Life ancient oceans. Information about biotic productivity and other aspects of ancient ecosystems contributes to the understanding of secular changes in geochemical cycles. In all large-scale studies of ancient ecosystems, high-resolution stratigraphy is essential for establishing time scales. (Table 1 offers a simplified geologic time scale, which is designed to assist readers who are nongeologists.) INTRODUCTION The past few years have seen the emergence of a new interdisciplinary field of earth science that addresses the impact of large-scale environmental changes on ancient life. Exemplifying this development has been the maturation of the overlapping disciplines of TABLE 1 Simplified Geologic Time Scale Era Period Epoch Time (m.y. ago)a Cenozoic Neogene Holocene Past 10,000 years Pleistocene 1.6-0.01 Pliocene 5.3-1.6 Miocene 23.7-5.3 Paleogene Oligocene 34-23.7 Eocene 55-34 Paleocene 65-57.8 Mesozoic Cretaceous 144-65 Jurassic 208-144 Triassic 245-208 Paleozoic Permian 286-245 Carboniferous Pennsylvanian 320-286 Mississippian 360-320 Devonian 408-360 Silurian 438-408 Ordovician 505-438 Cambrian 544-505 Precambrian Proterozoic 2,500-544 Archean Prior to 2,500 NOTE: The time scale was initially devised based on paleontologic evidence, with each period and epoch representing a significant paleontologic change. Each of the epochs can be further subdivided (e.g., the Cenomanian age that is in the Cretaceous Period, with ages ranging from about 97.5 to 91 million years (m.y.) ago). a The relative numerical ages, based largely on radiometric determinations, are mostly from the Decade of North American Geology (1983) time scale issued by the Geological Society of America, with more recent modifications for the Cenozoic part of the record and for the Cambrian-Precambrian boundary reconstruction. Diverse new techniques have also fostered progress—improved methods for dating strata, for example, and new techniques for studying rates of evolution and extinction, as well as innovative ways of using isotopes to evaluate changes in environments, biological activity, and biogeochemical cycles.
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Effects of Past Global Change on Life paleogeography, paleoceanography, and paleoclimatology. The recent surge of interest in mass extinctions has helped to promote these developments, but their roots go much deeper. The success of the Deep-Sea Drilling Program and its successor, the Ocean Drilling Program, has opened new opportunities for research, as has recent progress in plate tectonic reconstruction. Diverse new techniques have also fostered progress—improved methods for dating strata, for example, and new techniques for studying rates of evolution and extinction, as well as innovative ways of using isotopes to evaluate changes in environments, biological activity, and biogeochemical cycles. Progress in all these areas has created a new framework for paleobiology, which entered a renaissance in the 1960s and is now well positioned to study the history of life in the context of a dynamic global environment. Patterns of evolution and extinction derived from fossil data are taking on new meaning in this context and have major implications for evolutionary biology and for studies of human-induced biotic change. The geologic record shows not only how the modern biosphere emerged in association with past global change, but also which kinds of species and biotic communities are most vulnerable to environmental change and which are most resilient. In this Overview, we offer examples of research that is emerging in the study of past changes in the global ecosystem and recommend fruitful areas for research. Detailed discussions of recent advances in understanding ancient environments, the life those environments supported, and reasons for the changes in both biotas and environments appear in the authored chapters that follow this Overview. The wide variety of methods employed to study the dynamics of ancient ecosystems illustrates the interdisciplinary nature of the subject. METHODS Functional morphology provides key biological information. For example, dental morphology reflects a mammal's diet, and the morphology of fossil leaves is an excellent indicator of ancient climatic conditions. Because these features reflect basic laws of physics, their testimony is as powerful for the past as for the future. Terrestrial pollen spectra and marine plankton assemblages offer pictures of climatic conditions that are especially detailed for the past several million years. Preservation of key materials is also of special value, as in the use of coal balls to study the fabric and composition of Pennsylvanian peat, or the use of deep-sea deposits to obtain nearly continuous records of oceanic life and environments. Stable isotopes and other geochemical signatures have been used in a variety of ways to investigate the dynamics of the oceans and climates. Oxygen isotopic composition of marine microfossils is the best indicator available for estimating ocean temperatures for the past 150 m.y. The oxygen isotope ratio (18O/16O) increases in the secreted skeletons with decreasing water temperature. Estimated ocean temperatures need to take into account that 16O evaporates from the sea surface more readily than 18O and accumulates preferentially in glacial ice. This information further allows the estimation of the volume of Cenozoic ice sheets. Oxygen isotopic ratios differ between summer-dwelling species and those representing other seasons and between surface water dwellers and forms that occupy deep, cool waters. Carbon isotopic ratios in deep-sea sediments shed light on productivity and rates of carbon burial. Concentrations of iron and manganese in deep water shales reflect degree of oxidation and, hence, ventilation of the deep-sea. Carbon isotopic ratios in paleosols appear to provide a proxy for past CO2 levels, as does the isotopic composition of specific biomarker organic molecules preserved in marine sediments. General circulation models have opened new possibilities for studying past changes in atmospheres and oceans. Models that couple oceans and atmospheres are especially valuable. Even imperfect global models can assist in simulating consequences of regional perturbations, such as the tectonic elevation of mountain systems and ocean barriers.
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Effects of Past Global Change on Life Often such consequences can be tested against key features of the geologic record. Thus, certain empirical data may constrain models, whereas others test model results. At present, the effective utilization of models is often limited by a paucity of pertinent geologic data. In addition, some models fail, in detail, to square with geologic data. Any study of past changes in ecosystems demands a certain level of stratigraphic and chronological information. Such information is needed, for example, to determine whether similar events in widespread areas were contemporaneous. It is also essential for documenting global trends—for effectively collapsing data from many regions onto a single time line. Even at a single locality, one must know the approximate length of time separating two different conditions in order to calculate the rate of change that produced the second condition from the first. In general, chronological accuracy increases with decreasing geologic age. Special advantages are gained, for example, by working within the ranges of 14C dating, well-preserved glacial varves, and extant species. Farther back in the record, errors in correlation are frequently smaller than errors in actual dates. High-resolution stratigraphy based on widespread events of brief duration can yield correlations one or two orders of magnitude more accurate than conventional biostratigraphy. For example, chemical marker beds and changing isotopic ratios of carbon and oxygen, which reflect events that spanned less that 105 years, have contributed to a detailed global chronology for rapid environmental change and mass extinction at the Cenomanian-Turonian boundary, about 91 m.y. ago (see Kauffman, Chapter 3). Some events, such as volcanic eruptions and accumulations of chemical fallout from extraterrestrial impacts, have deposited widespread stratigraphic markers within less than a year (Toon et al., 1982). Quantitative statistical methods based on first and last stratigraphic occurrences of species are also yielding improved correlations. Calibration of sedimentation rates allows for estimation not only of rates of extinction but also of rates of biotic recovery. High rates of deposition yield an expanded stratigraphic record and therefore often improve the quality of both the record and its temporal resolution. Thus, for the terminal Cretaceous event at about 65 m.y. ago, the shallow (middle neritic) deposits exposed at El Kef, Tunisia, seem to offer a more accurate picture of the sequence of events than do deep-sea cores (see Keller and Perch-Nielsen, Chapter 4). For the terminal Ordovician crises about 440 m.y ago, intervals of biotic change have been estimated by using the numbers (including fractions) of graptolite zones that they span (see Berry et al., Chapter 2). The average duration of a zone (on the order of a million years) is estimated from radiometric ages for the boundaries of longer stratigraphic intervals. SHIFTS BETWEEN ENVIRONMENTAL STATES Many of the changes that have altered the global ecosystem in the course of Earth history can be viewed as shifts between environmental states. The most important shifts to affect the course of biotic evolution and the nature of the biosphere have been ones that are unique and unidirectional. Others have been components of episodic or periodic cycles, some of which have been superimposed on long-term trends. Periodic Cycles The controversial issue as to whether mass extinctions have occurred at equally spaced intervals has stimulated interest in the periodicity of geologic events. The most striking examples of periodic oscillations between environmental states in the recent past are those between glacial maxima and glacial minima during the past 2.5 m.y. These transitions, which have affected sea-level, climates, and biotas, have been linked to periodic changes in the Earth's axial and orbital rotations—the so-called Milankovich cycles. These cycles are best documented by foraminiferal fossils from deep-sea deposits, which exhibit relative
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Effects of Past Global Change on Life enrichment in 18O when polar glacial expansion preferentially sequesters 16O from the hydrosphere. For reasons not yet known, periodicities of ~41,000 years, reflecting the tilt cycle of the Earth's axis, dominated until about 0.8 m.y. ago, when periodicities of ~100,000 years, reflecting the shape of the Earth's orbit, began to prevail. Cycles in some pre-Neogene marine successions appear to reflect minor changes in sea-level or biotic productivity that were forced by Milankovich controls mediated by factors that remain poorly understood but may not always have entailed changes in ice volume. Certain Mesozoic lake deposits also contain evidence for Milankovitch-driven cyclicity, perhaps related to shifting monsoons or other patterns of rainfall and evaporation. Nonperiodic Cycles The most profound nonperiodic cycles of global change have been long-terms oscillations between what have been termed the ''hothouse" and the "icehouse" states for oceans and atmospheres. The term hothouse is preferred to "greenhouse" because the conditions described may not always result from greenhouse warming; the hothouse states are, however, characterized by warm polar regions and warm deep oceans. In contrast, the icehouse state entails cold (usually glacial) polar conditions and a frigid deep-sea that results from the descent of cold polar waters. The geologic record spanning the Eocene-Oligocene boundary documents the transition between a hothouse state and the icehouse state that has persisted to the present (Kennett et al., 1972). Much farther back in the geologic record, the interval spanning the Ordovician-Silurian boundary documents a similar transition, as well as the subsequent melting and retreat of glaciers and return of warmer conditions across broad regions (see Berry et al., Chapter 2). The Eocene-Oligocene Transition The recent ice age in the Northern Hemisphere constitutes only an intensification of the icehouse state that our planet entered about 34 m.y. ago, at the end of the Eocene Epoch. Fossil floras and vertebrate faunas reveal that early in Eocene time, subtropical conditions extended north of the Arctic Circle and that southeastern England and the Paris Basin (45 to 50°N) supported tropical rain forests. Fossil floras are, in fact, the most valuable indicators of terrestrial climates for the past 100 m.y. Not only does the taxonomic composition of fossil floras reflect climatic conditions, but so does leaf morphology, especially the percentage of species with smooth, as opposed to jagged or lobed, leaf margins; this percentage varies linearly in the modern world with mean annual temperature (Figure 1). Leaf morphology and cuticular structure also provide a guide to precipitation conditions. Fossil floras show that the Eocene-Oligocene climatic shift was profound at middle and high latitudes in both hemispheres. As warm-adapted floral elements disappeared from these regions, other types of vegetation, adapted to colder and drier conditions, expanded (see Christophel, Chapter 10). Climates actually did not undergo a simple shift between Early Eocene and Early Oligocene time. The tropical flora of England began to disappear at the end of Early Eocene time, as global temperatures began to cool, especially at high latitudes. By Late Eocene time, woodland savanna had already become the dominant vegetation of midcontinental North America (see Webb and Opdyke, Chapter 11). Whether the particular temporal pattern observed for North America characterized other continents remains uncertain, in part because of uncertain dating and in part because in some regions, such as Australia, a floral record is missing for much of the Eocene. It is now widely agreed that the plate tectonic separation of Australia from Antarctica was a primary trigger of climatic changes near the end of the Eocene (and continuing separation caused further climatic changes after Eocene time). This event created the
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Effects of Past Global Change on Life FIGURE 1 Estimated changes in temperature in four areas over the course of Cenozoic time, based on percentages of species or fossil terrestrial plants having smooth leaf margins. Especially evident is dramatic cooling near the end of the Eocene Epoch (after Wolfe, 1978). incipient circum-Antarctic current, which began to isolate the Antarctic continent from warm waters flowing from the north. The resulting cooling of surface waters led to the formation of cool deep waters. Enrichment of 18O in both planktonic and deep-sea benthic foraminifera and an influx of ice-rafted sediments indicate a significant, although temporary, expansion of the East Antarctic ice sheet at this time (see Kennett and Stott, Chapter 5). North Atlantic deep water (NADW), which is less dense than South Atlantic deep water (SADW), began to form slightly later, when rifting separated Greenland from Europe, permitting Arctic waters to descend into the North Atlantic (Schnitker, 1980). There is no question that climates cooled at middle and high latitudes, but four major questions remain: Fossil floras indicate that temperate conditions extended to high latitudes in both hemispheres during Early Eocene time. How was so much heat transported from the equator toward the poles? During this very warm interval, were the tropics warmer than, cooler than, or comparable to the tropics today? How did the oceanographic changes during the Eocene-Oligocene transition shut down the heat transport system that had existed previously? How did biotas throughout the world respond to the environmental changes? Strong wind-driven currents cannot account for most of the meridional transport during the Eocene because, being dependent on steep thermal gradients, such currents are self-limiting. Most likely, a primary transport mechanism was the poleward flow of warm, saline subsurface water masses that formed at low latitudes. Whether the flux was sufficient to depress tropical temperatures below their modern levels remains a major question. Another is how the new system of thermohaline circulation thwarted this heat flux. Also at issue is the role of the greenhouse effect in producing the widespread warmth
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Effects of Past Global Change on Life of the Early Eocene. If mean global temperature was well above that of the modern world, greenhouse warming is perhaps the most likely cause. If warmer high latitudes were accompanied by cooler equatorial conditions, then heat transport may have been the dominant control on the latitudinal temperature gradient. More generally, one can ask how the secular changes in the greenhouse capacity of the atmosphere have interacted with increasing solar luminosity, continental geographies, and orogenic uplift to produce the significant climatic oscillations recorded throughout geologic time. The Younger Dryas Cooling Events in the North Atlantic during the earliest stages of deglaciation between 14,000 and 11,400 years ago represent another example of change in thermohaline circulation. The North Atlantic is part of a circuit that extends to the Pacific—the so-called great conveyor belt (Broecker and Denton, 1989). North Atlantic deep water flows southward and is entrained in the Antarctic Current. It then passes into and through the Indian Ocean and the South Pacific to the North Pacific, where it upwells and returns by surface currents to the North Atlantic. There it loses heat that warms the climate of northern Europe and sinks again. During the emergence from the most recent glacial maximum, there was a brief expansion of glaciers between 11,400 and 10,200 years ago known as the Younger Dryas event. The cause of this reversal was in part a change in the flow of meltwater from North America to the ocean: Surface salinities declined in the North Atlantic because meltwater was diverted from the Mississippi drainage eastward to the St. Lawrence. Mixture with these buoyant waters reduced the density of the waters flowing northward into the North Atlantic and interrupted the formation at the surface of relatively dense NADW until the meltwater pulse had largely ended. Shifts in the isotopic composition of fossil planktonic foraminifera document this sequence of events, especially in the Gulf of Mexico region (see Flower and Kennett, Chapter 12). Plankton assemblages responded to changes in the salinity of surface waters. The Younger Dryas event illustrates how even a relatively small-scale perturbation of thermohaline circulation can have global oceanographic and climatic consequences. The Terminal Ordovician Transition Studies of events at the close of the Ordovician Period, about 440 m.y. ago, illustrate how it is also possible to interpret general causes and consequences of major climatic fluctuations for pre-Cenozoic intervals (see Berry et al., Chapter 2). These events are associated with one of the largest mass extinctions of all time. The stratigraphic record offers evidence that continental glaciation began at this time in the supercontinent of Gondwanaland and that the buildup of glaciers lowered sea-level by at least 50 m. Paleomagnetic data and environmental reconstructions reveal that Gondwanaland was moving over the South Pole. Marine fossils indicate a transition to a hothouse state: Hirnantian fauna, which had previously been restricted to cool water masses of the deep-sea and to high latitudes, expanded over broad regions of the ocean, replacing warm-adapted taxa that became extinct. Shifts to Hothouse Intervals A shift in the opposite direction, from the icehouse to the hothouse state, is associated with the development of warmer polar regions and deep ocean waters that are warm, sluggish, and dysaerobic to anoxic. Such a shift will automatically eliminate much life in the deep-sea. During the mid-Cretaceous highstand of sea-level, anoxic conditions extended upward into the deep portions of epicontinental seas, and it was during this interval that environmental perturbations produced pulses of biotic destruction that constitute the Cenomanian-Turonian mass extinction (see Kauffman,
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Effects of Past Global Change on Life Chapter 3). Near the end of the Ordovician Period, upward advection of toxic anoxic waters associated with expansion of the oxygen minimum zone may have caused major extinctions of midwater planktonic graptolites, while cooling also eliminated marine taxa (see Berry et al., Chapter 2). Value of High Latitude Biotas Stratigraphic records at high latitudes are often critical to understanding patterns and causes of global climatic change. They also document the radiation of cold-adapted biotas during icehouse intervals and the extinction of these biotas during transitions to the hothouse state. It is enlightening to compare the middle to late Cenozoic transition from warm-adapted to cold-adapted terrestrial biotas at northern and southern high latitudes (see Askin and Spicer, Chapter 9). Apparently because higher taxa in the north spread from seasonally arid middle-latitude regions, angiosperms that occupied the new cold climates of the Northern Hemisphere were all deciduous or capable of dormancy. Antarctica, in contrast, became a center of evolutionary innovation as it grew increasingly isolated with the rifting apart of Australia and South America. Here evergreen rain forests prevailed in relatively warm coastal areas. Both northern and southern polar regions suffered drastic declines in floral diversity during the climatic cooling trend that began in the Middle Eocene. Today, only mosses and lichens grow along ice-free margins of Antarctica. In cold temperate and boreal zones of the Northern Hemisphere, the mixed coniferous forest that is widespread today became well established early in the Miocene at a time of widespread nonglacial climates. Grasses assumed a prominent role in floras of northern high latitudes near the Miocene-Pliocene transition (about 5.3 m.y. ago), when the taiga and tundra expanded dramatically. Unidirectional Shifts Whereas most large-scale environmental transitions in Earth history have been reversed after an interval of time, others have represented unreversed net secular trends. The composition of ancient soils, for example, points to a buildup of atmospheric oxygen from about 1% of the present atmospheric level (PAL) at about 2200 m.y. ago to about 15% PAL at about 1900 m.y. ago (see Knoll and Holland, Chapter 1). This shift, which may have been affected by a complex feedback system involving the marine geochemistry of iron and phosphorus, must have dramatically increased the production of nitrates and thus permanently altered patterns of productivity in the oceans. Carbon-13 enrichment of carbonates and buried organic matter during the interval between 850 and 580 m.y. ago probably reflects accelerated burial of organic carbon, especially near the end of this interval. Thus, it may also reflect an increase in the partial pressure of atmospheric oxygen, as may a contemporaneous shift in the isotopic composition of marine sulfates. New data continue to support the hypothesis that atmospheric oxygen levels increased both at the beginning and at the end of the long Proterozoic Eon and had important consequences for biological evolution. RATES OF TRANSITION During the past few years, new evidence from high-resolution stratigraphy has revealed that many important environmental changes and biotic responses were more sudden than previously believed. The most notable example is the group of events that ended the Cretaceous Period. Most of these appear to have occurred in a crisis, perhaps measured in months rather than years, that many experts believe resulted from the impact of a comet or a meteorite—or from two or more related crises of this type. Evidence of this event, and of other sudden (though less dramatic) changes in the global ecosystem, has led to a resurgence of catastrophism as a paradigm to explain some fraction of the change in the earth system through time. How large a shift toward catastrophism is justified is a matter
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Effects of Past Global Change on Life of current debate. On a smaller scale, ice core data are now revealing that during the past 40,000 years the climate of Greenland has changed dramatically for intervals of just 10 to 20 years (Taylor et al., 1993). Sudden Shifts and Gradual Trends Two issues of timing are especially difficult to resolve. One is whether important events were protracted. The other is whether protracted events were pulsatile. An incomplete geologic record can give the false appearance of suddenness for an event that was actually protracted. Similarly, an imperfect record can give the false appearance of simultaneity for physical events, such as onsets or terminations of glaciation in two or more regions. A worldwide chronology for the multiple glaciations near the end of Proterozoic time has yet to be established, for example. Events of severe extinction that appear to have been protracted or to have occurred in multiple steps warrant statistical scrutiny. A key issue is the completeness of the records of taxa before their final disappearance. Imperfect records can produce an illusion of gradual or multistep extinction for a group of taxa that actually died out simultaneously: the so-called Signor-Lipps effect. For some major events, however, the geologic record is of sufficiently high quality to document a stepwise or pulsatile pattern. Eight steps of extinction, for example, have been identified for the Cenomanian-Turonian crisis (about 91 m.y. ago). The Ordovician crisis (about 440 m.y. ago) had two principal phases, each possibly lasting hundreds of thousands of years (see Berry et al., Chapter 2). The first pulse, at or near the Rawtheyan-Hirnantian stage boundary, coincided with glacial expansion. The second occurred within the Hirnantian (latest Hirnantian Ordovician) age, during the glacial maximum. The geologic record documents numerous changes in the global ecosystem that spanned many millions of years. For these trends, the record, though imperfect, is too extensive to be masking a single dramatic event. We may nonetheless have difficulty in distinguishing between gradual and stepwise patterns for such trends. The classic example of this kind of trend is the climatic transition toward cooler and drier conditions on many continents between Eocene and Pleistocene times. During this time, prevailing biomes over broad regions shifted from tropical forest through savanna to grassland and steppe (see Christophel, Chapter 10; Webb and Opdyke, Chapter 11). While the terminal Eocene transition described earlier was a major early step in this trend (and was itself a complex event), the degree to which later changes were stepwise is not well established. It is, however, clear that net rates of change varied from place to place. One of the difficulties in resolving the details is in distinguishing between global changes and regional changes that resulted from such events as tectonic uplift in the American West or the Himalayan region. The most recent geologic record offers special opportunities to establish temporal resolution for events on very short time scales. Studies of glacial varves suggest that the Younger Dryas cooling episode that interrupted deglaciation in the Northern Hemisphere between about 11,400 and 10,200 years ago developed during an interval of less than 300 years and may have ended during an interval of less than 20 years (Dansgaard et al., 1989). The Nature of Thresholds Even a gradual environmental change can result in a sudden change of state when a threshold is crossed. The growth and contraction of glaciers are inherently unstable processes because glaciers have a higher albedo than land. One result is that the birth of a relatively small glacier can plunge a high latitude region into an interval of widespread glaciation. The development of the Antarctic cryosphere during the Eocene-Oligocene transition is such an episode. Cooling during the Late Eocene eventually proceeded to a
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Effects of Past Global Change on Life point where the cryosphere expanded (and it has never returned to its previous state). Another example is the abrupt shift of meltwater drainage and the resulting change in thermohaline circulation patterns that may have triggered and terminated the Younger Dryas. Ecological limitations of organisms yield thresholds of biotic response to environmental change. For example, global circulation models suggest that during the Cretaceous Period, surface water temperatures and salinities across much of the Tethyan Ocean may have come to exceed the tolerance of most modern reef-building corals (30°C and 3.7% salinity). A shift past critical limits may explain the corals' relatively sudden loss of dominance to rudist bivalves in the central Tethyan reefs during Albian time, about 113 to 98 m.y. ago (see Barron, Chapter 6). At the end of the Westphalian age of the Pennsylvanian Period, drier climates swept across North America and Europe, with profound consequences for vegetation (see DiMichele and Phillips, Chapter 8). As loss of swamp habitat reached a critical threshold, the arborescent lycopods that had been a conspicuous feature of landscapes for tens of millions of years disappeared rapidly. When climates amenable to swamp formation returned, ferns and tree ferns rose to dominance in these environments. The paleobotanical record shows repeated evidence for the adaptation of plants to particular environments, followed by extinction when habitats were disrupted. Thresholds appear to have been crossed for antelopes, micromammals, and members of the human family close to 2.5 m.y. ago in Africa, with the rapid shrinkage of forests during the onset of the modern ice age. Forest-dependent species within all three groups disappeared over a broad area during an interval on the order of 10,000 years, and many species adapted to open, grassy habitats made their first appearances (Vrba, 1985). Within the human family, it appears that gracile australopithecines died out because they had depended on forests for food and refuge (see Stanley, Chapter 14). In the manner of modern chimpanzees, these animals presumably slept in trees and fled into them when threatened by predators. The characterization of paleofloras at fossil hominid sites using carbon isotopic ratios of paleosols reveals an acceleration at about 2.5 m.y. ago in the long-term Neogene shift from closed forests toward grassy habitats (Cerling, 1992). Apparently no fossil site supported a closed canopy forest after this time. Although the preceding examples highlight the role of environmental thresholds in promoting extinction, at times environmental change has opened up new evolutionary possibilities. For example, increases in the partial pressure of oxygen must have crossed crucial thresholds for the evolution of life during Archean and Proterozoic time (see Knoll and Holland, Chapter 1). Increased partial pressure of oxygen to 1, 10, and essentially 100% of present-day levels would have cleared the environmental path for the evolution of aerobic bacteria and mitochondria-bearing eukaryotes, obligately photosynthetic eukaryotes, and large animals, respectively. In addition, an increase to levels much closer to those of today may have permitted the evolution of large animals. (As animals evolved complex circulatory and respiratory systems, they would have been able to expand into less oxygen-rich regions of the ocean and to enclose their tissues within thick mineralized shells.) PATTERNS OF BIOTIC RESPONSE An adverse change in the environment can cause species to migrate, or if migration to a suitable habitat is impossible, it can lead to their extinction. Patterns of migration and extinction for ancient biotas are of particular interest because they yield predictions as to how modern communities may respond to future global change. Environmental changes of the past have also had positive effects on certain surviving forms—especially ecological opportunists—or have triggered adaptive radiations within an impoverished ecosystem or a newly expanding habitat. Interactions between species intensify biotic responses to
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Effects of Past Global Change on Life 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
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Effects of Past Global Change on Life often point to causes. In global mass extinctions such as the terminal Ordovician, Cenomanian-Turonian, and terminal Cretaceous events, tropical taxa, including reef communities, suffered preferentially. This is consistent with the idea that climatic cooling played a major role in extinction, but it may also reflect the typically narrow niche breadth of tropical taxa and the high degree of interdependence among species. High-resolution stratigraphic and paleoenvironmental studies are crucial for understanding mass extinctions. Such studies reveal that a major extinction in deep-sea benthic assemblages at the end of the Paleocene, the most profound of the last 90 m.y. for this habitat, resulted from rapid warming of the deep oceans in conjunction with global warming (see Kennett and Stott, Chapter 5). Extinctions that removed between 35 and 50% of deep-sea taxa occurred in less than 2000 years, equal to the time required for the deep water to circulate through its basins. For a few thousand years, ocean circulation underwent fundamental changes that affected the deep-sea biota. High-resolution study of this event has illustrated, first, how events that are geologically brief but not instantaneous can strongly alter the ecosystem and, second, how such changes can be largely decoupled from events in other segments of the biosphere. Patterns of extinction can point to particular causes of mass extinction. For example, the severe extinction of western Atlantic bivalve mollusks during the onset of the modern ice age seems to have eliminated all strictly tropical species of southern Florida; all survivors have broad thermal tolerances, ranging well beyond the tropics today. Here, a thermal filter seems clearly to have operated (see Stanley and Ruddiman, Chapter 7). Similarly, that climatic change was the ultimate cause of the previously discussed severe extinction of African mammals at the start of the modern ice age (-2.5 m.y. ago) is supported not only by the evidence that forest habitats shrank at this time but also by the fact that forest-adapted species were the primary victims (Vrba, 1985). Some patterns of extinction have characterized higher taxa in more than one mass extinction. A striking aspect of the terminal Cretaceous extinction of planktonic foraminifera was the disappearance of species with large, complex, highly ornamented skeletons (see Keller and Perch-Nielsen, Chapter 4). Survivors were inherently small species or species that became dwarfed during the crisis. Deep-water planktonic species also died out first and in the largest numbers. These patterns must be taken into account in any analysis of the proximate causes of extinction. Most species of planktonic foraminifera that became extinct during the Late Eocene to Early Oligocene extinction were also complex, highly ornamented species. These were also largely warm-adapted taxa, which is compatible with evidence that climatic changes were the primary cause of this global crisis. If there is one general pattern for extinctions, it is the rate of environmental change and not necessarily its magnitude that places most populations in jeopardy. This consideration is highly relevant to global changes predicted for the next century. If current models are correct, the magnitude of change will not be unusual on a geological time scale, but the rate of change may be. Evolutionary Turnover Evidence of causation also comes from the nature of species that immigrate into a region or originate within it during or soon after a pulse of extinction. In other words, the disappearance of some species and the appearance of others during a brief episode of evolutionary turnover should offer compatible testimony about environmental change. Thus, not only did forest-adapted species of antelopes preferentially die out in Africa about 2.5 m.y. ago, but newly appearing species were virtually all adapted to grassy habitats. Simultaneously, the apparently semiarboreal gracile australopithecines gave way to early Homo, which had helpless infants and could not have climbed trees habitually for refuge (see Stanley, Chapter 14).
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Effects of Past Global Change on Life Not only during the Pliocene but throughout much of the Cenozoic Era, mammals experienced pulses of evolutionary turnover that produced stepwise net increases in the relative number of species adapted to savannas, grasslands, or steppes. The first pulse of evolutionary turnover came at the end of the Eocene, when the extinction event that also affected the marine realm removed numerous browsers, including the huge, rhino-like titanotheres. New mammalian taxa included numerous taxa adapted to eating coarse fodder (see Webb and Opdyke, Chapter 11). By mid-Miocene time, the diversification of taxa—adapted to grassy habitats—had produced the greatest North American land mammal diversity of all time, in savannas that were the biotic equivalents of those in Africa today. Continuation of the trend produced drier grasslands and steppes with lower mammalian diversities later in the Neogene. Although post-Eocene pulses of turnover for Cenozoic mammals have not yet been shown to correlate well with particular floral shifts, they have been correlated with isotopic evidence of glacial expansion. Efforts to associate turnover with global climatic changes are complicated by regional trends produced by major tectonic events, such as the uplift of the Sierra Nevada, the Colorado Plateau, and the Himalayan Plateau. Similarly, although the spectacular diversification of grasses and other plants adapted to dry, seasonal habitats clearly resulted from the general post-Eocene climatic trends, intervals of diversification have not as yet been associated with pulses of extinction of moist-adapted forms. Delayed Recovery Severe extinctions that are not largely offset by simultaneous immigration or speciation result in impoverished ecosystems that sometimes persist for millions of years. Several factors can contribute to delayed recovery. Sometimes a delay results from a dearth of taxa capable of responding to the opportunity created by severe extinction. A striking example is the absence throughout Mississippian and Pennsylvanian times of a framework-building reef community to replace the tabulate-stromatoporoid community that had been devastated in the Late Devonian mass extinction (James, 1984). Contrasting with this situation was the rapid diversification of sclerophyllous terrestrial plants (forms with reduced leaves and thickened cuticles) in Australia after the Eocene (see Christophel, Chapter 10). These taxa seem to have originated in nutrient-poor soils at the margins of Paleogene rain forests and were, in effect, poised for rapid evolutionary response when, because of aridification, soils deteriorated over a broad area of the continent. Similarly, the mammals' evolutionary recovery from severe Late Eocene extinction in the Northern Hemisphere was accelerated by the fact that a variety of mammalian taxa with high-crowned teeth adapted for grazing on coarse vegetation had already evolved during the Eocene, prior to the severe climatic change (see Webb and Opdyke, Chapter 11). For reasons that remain to be explained, small brachiopods that occupied the chalky seafloor of western Europe attained their former diversity within about 1 m.y. after the terminal Cretaceous extinction (see Figure 2). In general, delayed recovery from severe extinction has typified the marine realm. After the severe extinction of Late Eocene and Early Oligocene times, for example, marine faunas remained relatively impoverished throughout the Oligocene. Delayed marine recovery appears to have two primary causes. One is the inherently slow rate of adaptive radiation that characterizes many taxa of marine animals. The other is the typical failure of postcrisis conditions in the marine realm to stimulate the adaptive radiation of new kinds of taxa adapted to these conditions—to provide a new resource base comparable to productive savannas on the land (see Stanley and Ruddiman, Chapter 7). Even as overall mammalian diversity declined in North America after mid-Miocene time, certain mammalian and other taxa favored directly or indirectly by aridification underwent spectacular adaptive radiations: songbirds and Old World rats and mice—two groups that included many species that fed on the seeds of the
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Effects of Past Global Change on Life FIGURE 2 Diversity of small brachiopods in the chalk of western Europe. About three-quarters of the species died out suddenly in the terminal Cretaceous extinction, but a larger number of new species then originated very rapidly, during the next million years (from Johansen, 1988). FIGURE 3 Proliferation of adaptive radiation upward from the base of the food web in terrestrial ecosystems that were favored by the global trend toward aridification during the past 25 m.y. (from Stanley, 1990). Many songbirds and Old World rats and mice feed on the seeds of taxa of grasses, herbs, and weeds that diversified dramatically during the spread of grasslands. The large majority of modern snake species belong to the family Colubridae, and many of these feed on rats and mice or on songbird eggs and chicks.
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Effects of Past Global Change on Life rampantly radiating herbs and grasses—as well as colubrid snakes, which prey on rats and mice and songbird chicks and eggs (see Figure 3). Pre-Cenozoic marine faunas offer many additional examples of delayed recovery. The few small planktonic foraminifera that survived the terminal Cretaceous extinction died out sequentially during the first 200,000 years or so of Paleocene time, while new species evolved (see Keller and Perch-Nielsen, Chapter 4). The new forms were initially small, simple, and unornamented. Planktonic foraminifera did not recover their pre-extinction level of diversity until late in the Early Paleocene. Very low d13C values and low vertical d13C gradients indicate that the crisis produced low productivity in surface waters of the ocean. Drastic reduction in the abundance of calcareous nannofossils offers similar testimony, although a small number of opportunistic species experienced regional blooms. The planktonic realm began to recover only after 250,000-300,000 years. Exactly when shallow water benthic invertebrates began to rebound from the terminal Cretaceous crisis is unknown, but their recovery occupied most of Paleocene time (Hansen, 1984). Recovery of marine life from the Cenomanian-Turonian crisis, earlier in the Cretaceous, was also slow, in part because of the loss of basic elements of the ecosystem, such as numerous taxa of reef-building rudists (see Kauffman, Chapter 3). During the terminal Ordovician mass extinction, the cool-adapted Hirnantian fauna expanded geographically but did not diversify appreciably (see Berry et al., Chapter 2). Reradiation of graptolites and decimated benthic faunas was slow during the early phases of deglaciation. Few graptolite species survived to initiate radiations after the crisis, and brachiopods began to diversify only after sea-level began to rise and climates became warmer. Trilobite faunas remained impoverished until late in the Early Silurian, with most species tolerating a wide range of environmental conditions. RECOMMENDATIONS The issues and examples cited in previous pages demonstrate the phenomenological richness of past environmental changes and biological responses to it. Earth scientists are now presented with opportunities and needs to reconstruct and interpret these interactions in unprecedented temporal and spatial detail. Some of their findings will shed light on future global change. Special attention should be given to the most recent segment of the geologic record, because it can be studied in great detail and reveals how present conditions have developed; however, older intervals that document key events also warrant study. The results will benefit evolutionary biology by bringing to light fundamental aspects of evolution and extinction, and will provide a perspective for anticipating the environmental and biotic consequences of future global change scenarios. We present the following specific recommendations. Expand interdisciplinary research that elucidates the geologic history of the biosphere in the context of earth system science—research that reveals how environments have changed on a global scale and how life has responded. Key intervals that warrant attention are the following: intervals marked by major transitions between environmental states, some of which have dramatically transformed the biosphere; intervals marked by very rapid environmental change; intervals characterized by warmer conditions than those of the present—conditions that may resemble those produced by future global warming; and events that have produced the modern world since the latest glacial maximum in the Northern Hemisphere, about 20,000 years ago. Features of special importance include:
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Effects of Past Global Change on Life distribution of landmasses, shallow seas, deep oceans, and biogeographic provinces; for the terrestrial realm: the location of climatic zones and mountain belts; and for the oceans: three-dimensional structure, including major currents, thermohaline circulation, patterns of upwelling, and the global influence of polar regions. Identify secular changes in biogeochemical cycles, including reservoir sizes and fluxes, and evaluate the consequences of these changes that are of particular importance to the documentation of past environmental change. Topics deserving high priority include the following: the history of photosynthetic productivity in both terrestrial and marine environments; the history of atmospheric CO2 and other greenhouse gases; and the Precambrian history of the atmosphere. Identify and interpret patterns of extinction, migration, and evolution of life during intervals of environmental change. Taxonomic patterns are critically important, but so are patterns based on functional and ecological groupings of organisms. Construct conceptual and numerical models that portray the earth system as it existed during key geologic intervals. Emphasis should be given to the following: causal explanations for changes between environmental states (crossing of environmental thresholds) that affected the biosphere; environmental consequences of changes in terrestrial topography and in land-sea configurations; modeling that couples the ocean and the atmosphere; synergistic interactions between building of models and gathering of the data required to constrain and test these models; and factors that amplify the influence of Milankovich cycles. Improve existing, and develop new, techniques for characterizing ancient environments and for determining the ecological roles of species in these environments. Approaches of special importance include the following: improved methodologies for characterizing the environmental tolerances of fossilized taxa; synthetic studies that focus on both plants and animals, for example, or both macrobacteria and microbacteria; and innovative isotopic, elemental, and organic geochemical techniques for environmental reconstruction. Apply high-resolution stratigraphy and develop new techniques for dating and correlation in order to improve the chronological framework for studying ancient ecosystems. Those areas deserving increased emphasis include the following: new or improved isotopic approaches to dating and correlation; dating of widespread events that were sudden, cyclical, or of great biotic consequence; quantitative correlation; refined biostratigraphic techniques; and studies that integrate physical and biological approaches.
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Effects of Past Global Change on Life REFERENCES Broecker, W. S., and G. H. Denton (1989). The role of ocean-atmosphere reorganizations in glacial cycles, Geochimica et Cosmochimica Acta 53, 2465-2501. Cerling, T. E. (1992). Development of grasslands and savannahs in East Africa during the Neogene, Palaeogeography, Palaeoclimatology, Palaeoecology 97, 241-247. Dansgaard, W., W. C. White, and S. J. Johnson (1989). The abrupt termination of the Younger Dryas climatic event, Nature 339, 532-534. Hansen, T. A. (1984). Early Tertiary radiation of marine molluscs and the long-term effects of the Cretaceous-Tertiary Boundary, Paleobiology 14, 37-51. James, N. (1984). Reefs, in Facies Models, R. G. Walker, ed., Geoscience Canada Reprint Series 1, 2nd edition, pp. 229-244. Johansen, M. B. (1988). Brachiopod extinctions in the Upper Cretaceous to lowermost Tertiary chalk of northwest Europe, Revista Espanola de Paleontologia, nº Extraordinario, 41-56. Kennett, J. P., R. E. Burns, J. E. Andrews, M. Churkin, T. A. Davies, P. Dumitricia, A. R. Edwards, J. S. Galehouse, G. H. Packham, and G. J. Van der Lingen (1972). Australian-Antarctic continental drift, paleo-circulation changes, and Oligocene deep-sea erosion, Nature 239, 51-55. Schnitker, D. (1980). West Atlantic circulation during the past 120,000 years, Annual Reviews of Earth and Planetary Sciences 8, 343-370. Stanley, S. M. (1990). Adaptive radiation and macroevolution, Systematics Association Special Volume 42, 1-16. Taylor, K. C., et al. (1993). The ''flickering switch" of late Pleistocene climatic change, Nature 361, 432-436. Toon, O. B., J. B. Pollack, T. P. Ackerman, R. P. Turco, C. P. McKay, and M. S. Liu (1982). Evolution of an impact-generated dust cloud and its effects on the atmosphere , Geological Society of America Special Paper 190, 187-200. Vrba, E. S. (1985). African Bovidae: Evolutionary events since the Miocene, South African Journal of Science 81, 263-266. Wolfe, J. A. (1978). A paleobotanical interpretation of Tertiary climates in the Northern Hemisphere, American Scientist 66, 694-703.
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Effects of Past Global Change on Life BACKGROUND
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Representative terms from entire chapter: