The Origin and Fate of Ecosystems

The first 3 billion years or so of life on earth were dominated by prokaryotes, life forms we collectively refer to as bacteria but that really include a vast array of simple organisms (for example, archaea, cyano-bacteria, and other forms having a similar grade of cellular organization). Such organisms must have formed ecosystems, with possible early chemoautotrophy giving way to later pho-toautotrophy as the major forms of primary energy capture. Although the organization and function of these earliest systems is poorly known, we can be sure that the economic aspects of life were transformed radically with the appearance first of large-bodied animals in the oceans (at the Proterozoic-Phanerozoic transition, roughly 500 to 600 million years ago) and later with the appearance of large plants on the land (in the Silurian and Devonian periods, roughly 360 to 440 million years ago). That is when the level of complexity that characterizes modern ecosystems first appeared. There is a controversy among paleontologists about how explosive the "Cambrian explosion" of animal life really was: was it truly a time of extremely rapid evolutionary innovation and divergence; or were these processes taking

Figure 4

Resistance Stability in Ecosystems

Figure 4

Resistance Stability in Ecosystems

[a] hub breakpoint

INTERACTIONS

Source: Miller, William III. 1996. "Ecology of Coordinated Stasis." Palaeogeography, Palaeoclimatology, Palaeoecology 127:177-190, fig. 3. (Reprinted with permission) Note: Resistance is one source of stability in ecosystems. Systems that exhibit resistance stability have a "defense-in-depth" structure of subsystems that can be sacrificed during disturbances without causing the collapse of the encompassing system. The system cleaves naturally at breakpoints, as in this diagram showing local systems organized around hub species. The local systems are in turn connected to form a larger regional ecosystem, with breakpoints at a larger scale.

INTERACTIONS

[a] hub breakpoint

Source: Miller, William III. 1996. "Ecology of Coordinated Stasis." Palaeogeography, Palaeoclimatology, Palaeoecology 127:177-190, fig. 3. (Reprinted with permission) Note: Resistance is one source of stability in ecosystems. Systems that exhibit resistance stability have a "defense-in-depth" structure of subsystems that can be sacrificed during disturbances without causing the collapse of the encompassing system. The system cleaves naturally at breakpoints, as in this diagram showing local systems organized around hub species. The local systems are in turn connected to form a larger regional ecosystem, with breakpoints at a larger scale.

place through the Late Proterozoic, with the "explosion" being more a sudden appearance of large, skeleton-bearing derivatives of previous evolution? The most reasonable interpretation is that the explosion was a major economic event more than anything else, involving the appearance of big, energy-hungry organisms, intricately organized networks of interactors, feedback loops with recuperators, storage sinks, and new ways to exploit opportunity, manipulate the environment, and maintain systems in equilibrium. The Cambrian explosion and the later appearance and spread of land plants were ecologic revolutions, and ecosystems were the things that were revolutionized.

Individual ecosystems are characterized by intervals of initial organization and establishment (called primary succession in local ecosystems), a life span that includes long periods of normal functioning and development punc tuated by disturbances and recoveries of various magnitudes, and a final interval of degradation and collapse. Ecologists have been interested mostly in the function and development of local systems, and they have paid little attention to the "birth" and "death" of larger, regional ecosystems. These are the times when evolutionary and ecologic processes intersect in many crucial ways, as migration, speciation, and extinction rates are all accelerated to produce what is known as turnover pulses. Most of the characteristics of established ecosystems, such as composition, organization, and dominant processes, are "discovered" and "formalized" during these pulses, yet little is known about exactly how this works in terms of the interplay of evolution and ecology. The biodiversity crisis is spiral-ing out of control, providing many grim opportunities to observe and finally to make generalizations about how these events actually work. If conservation efforts are ultimately fruitless, and many different kinds of ecosystem are degraded and collapse simultaneously, there could be a "surrendering of the lease" from the large, varied organisms that have characterized complex Phanerozoic ecosystems to the previous owners/operators of the biosphere—the prokaryotes

—William Miller III

See also: Bacteria; Biogeography; Carbon Cycle; Coevolution; Communities; Food Webs and Food Pyramids; Global Climate Change; Nitrogen Cycle; Nutrient/Energy Cycling; Oceanic Trenches; Positive Interactions; Succession and Successionlike Processes; Tropical Rain Forests

Bibliography

Golley, Frank B. 1993. A History of the Ecosystem Concept in Ecology. New Haven: Yale University Press; Margalef, Ramón. 1968. Perspectives in Ecological Theory. Chicago: University of Chicago Press; May, Robert M. 1974. Stability and Complexity in Model Ecosystems, 2d ed. Princeton: Princeton University Press; O'Neill, Robert V., et al. 1986. A Hierarchical Concept of Ecosystems. Princeton: Princeton

University Press; Pahl-Wostl, Claudia. 1995. The Dynamic Nature of Ecosystems: Chaos and Order Entwined. Chichester, U.K.: John Wiley; Pomeroy, Lawrence R., and James J. Alberts. 1988. Concepts of Ecosystem Ecology. New York: Springer-Verlag; Ulanowicz, Robert E. 1997. Ecology, the Ascendant Perspective. New York: Columbia University Press; Zhu-ravlev, Andrey Y. U., and Robert Riding. 2001. The Ecology of the Cambrian Radiation. New York: Columbia University Press.

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