sulfide by bacteria living in those oxygen-depleted water masses. Physiological research on modern marine organisms, aimed at understanding current environmental change (e.g., Pörtner et al., 2005), allows Earth scientists to predict the biological consequences of such an event on end-Permian biological diversity. Indeed, paleobiological data show that extinctions did not affect all Permian animals equally. For example, groups whose living relatives were vulnerable to the physiological consequences of sea acidification disappeared at rates much higher than those physiologically well buffered against such environmental perturbations. Extinctions on land are consistent with the predicted effects of rapid climate change (summarized in Knoll et al., 2007). Continuing research on Earth’s great intervals of biological upheaval will increasingly integrate insights from paleobiology, stratigraphy, high-precision geochronology, and geochemistry with physiology and models generated to help understand current issues of global change.

What Governs the History of Biological Diversity?

Major extinctions have clearly influenced the history of plant and animal life, but what, fundamentally, controls the observed pattern of diversity increase from the Cambrian to today (Figure 3.15)? Quantification of diversity change through time on land and in the oceans remains a subject of active research and debate, but many Earth scientists would agree that the modern world (at least in preindustrial times) harbors more species of land plants, more species of land animals, and more species of marine animals than any previous moment in our planet’s history (e.g., Benton and Emerson, 2007). Attempts to model diversity history employ logistic equations, which imply biologically or physically imposed limits to diversification (e.g., Sepkoski, 1984), or exponential equations, which imply persistent diversity increases, episodically knocked back by mass extinctions (Stanley, 2007).

The tension between these classes of models focuses attention on a great and unsolved problem. What are the relative roles of genetic innovation, ecology, and physical Earth history in governing the long-term history of life? The answer certainly requires macroecological insights from biologists, but the questions are necessarily framed by paleontologists. And rapidly emerging insights into the physical history of the Earth surface system provide, for the first time, the proper environmental framework to address the issue. Has primary production increased through time, and if so what have been its consequences? What are the consequences of sea-level change, episodically flooding and exposing continental interiors, on species origination and extinction in the marine realm (Peters, 2005)? Did the rules of community construction change when flowering plants evolved the capacity to use animals to ensure the faithful spread of pollen from one plant to the next? How did the ecological relationships that undergird community diversity reform following episodes of mass extinction? Detailed analyses of community organization in systems as disparate as Pleistocene coral reefs, Cenozoic mammals, and Carboniferous forests promise important insights into ecology and evolution that cannot be made solely on the basis of the short-term observations and experiments available to biologists (Jackson and Erwin, 2006).

Summary

Earth’s surface environment is obviously altered by large-scale geological processes (Questions 4 and 5), but it is also affected continuously and pervasively by the activities of life forms. Likewise, Earth’s geological evolution and infrequent catastrophic events, such as meteorite impacts, have clearly affected the evolution of life. But even when we can document extinctions and major evolutionary changes, we cannot yet sort out the causes. To what extent were they caused by geological as opposed to biological processes? Which environmental conditions were responsible for which extinctions or changes in biological form and function? We know that the composition of Earth’s atmosphere, especially its high concentration of oxygen, is a major consequence of the presence of life, one that made possible the evolution of more complex organisms. But exactly how other geological events have affected evolution, and how much control life has had on climate, are still topics of debate.

Life processes and Earth processes also interact locally. Erosion rates, climate, and weathering rates affect the habitability of specific regions of Earth, and the ecosystems themselves in turn affect erosion rates, climate, and weathering processes. Understanding the



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