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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.
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.