COMMUNITY ECOLOGY

study of the organization and functioning of communities, which are assemblages of interacting populations of the species living within a particular area or habitat. As populations of species interact with one another, they form biological communities. The number of interacting species in these communities and the complexity of their relationships exemplify what is meant by the term biodiversity. Structures arise within communities as species interact, and food chains, food webs, guilds, and other interactive webs are created. These relationships change over evolutionary time as species reciprocally adapt to one another through the process of coevolution. The overall structure of biological communities, the organization of interspecific interactions, and the effects the coevolutionary process has on the biological community are described below. Additional reading Community ecology Joseph Cone, Fire Under the Sea (1991), discusses the discovery of deep-sea vents and the unique communities that have evolved around them. Joel E. Cohen, Frdric Briand, and Charles M. Newman, Community Food Webs (1990), provides a technical and mathematical analysis of the structure of food webs and of the general patterns that have been observed in 113 previously published food webs for a wide variety of biological communities. Stuart L. Pimm, The Balance of Nature?: Ecological Issues in the Conservation of Species and Communities (1991), thoroughly analyzes the ways in which food webs structure biological communities. Wallace Arthur, The Green Machine: Ecology and the Balance of Nature (1990), gives a nontechnical account of some aspects of the structure of biological communities. E.O. Wilson and Frances M. Peter (eds.), Biodiversity (1988), evaluates the current state and future of the topic. E.O. Wilson, The Diversity of Life (1992), masterfully recounts the origin of biodiversity, its maintenance within biological communities, and the threats posed by current human activities. John Terborgh, Where Have All the Birds Gone?: Essays on the Biology and Conservation of Birds that Migrate to the American Tropics (1989), discusses how bird population and community ecology are affected by migration over large geographic scales and by the wholesale deforestation of temperate and tropical habitats. Robert E. Ricklefs and Dolph Schluter (eds.), Species Diversity in Ecological Communities: Historical and Geographical Perspectives (1993), explores the ecological bases for worldwide patterns in the diversity of species.Henry F. Howe and Lynn C. Westley, Ecological Relationships of Plants and Animals (1988); and Warren G. Abrahamson (ed.), Plant-Animal Interactions (1989), provide good introductory treatments of the variety of ways in which plants and animals interact. A more advanced treatment of the subject may be found in Peter W. Price et al. (eds.), Plant-Animal Interactions: Evolutionary Ecology in Tropical and Temperate Regions (1991).Friedrich G. Barth, Insects and Flowers: The Biology of a Partnership (1985; originally published in German, 1982), examines the wide variety of ways in which plants have evolved to be pollinated by insects and insects have evolved to exploit the resources offered by flowers. Vernon Ahmadjian and Surindar Paracer, Symbiosis: An Introduction to Biological Associations (1986); and A.E. Douglas, Symbiotic Interactions (1994), provide nontechnical treatments of some of the many ways in which various species have evolved in association with other species and how they have developed intimate and complex relationships. John N. Thompson, The Coevolutionary Process (1994), comprehensively treats the specialization and coevolution of species. Peter R. Grant, Ecology and Evolution of Darwin's Finches (1986), summarizes the long-term research on how Galpagos finches have radiated from a single species into many species with different niches within the biological communities on these isolated islands. John N. Thompson Evolution of the biosphere J.E. Lovelock, Gaia: A New Look at Life on Earth (1979, reissued 1987), a popular work, develops the idea that the biosphere is a single organism. A dated but comprehensive introduction to paleontology and the history of life can be found in Alfred Sherwood Romer, Vertebrate Paleontology, 3rd ed. (1966). Carroll Lane Fenton and Mildred Adams Fenton, The Fossil Book: A Record of Prehistoric Life, rev. and expanded ed. (1989), is an encyclopaedic documentation of the diversity of fossils worldwide. Patricia Vickers-Rich et al. (eds.), Vertebrate Palaeontology of Australasia (1991), summarizes the vertebrate fossil record and the major climatic and tectonic events in Australasia. Stephen Jay Gould, Wonderful Life: The Burgess Shale and the Nature of History (1989), an accessible work, traces the Cambrian explosion of multicellular life some 515 million years ago, centring on the discoveries made in western Canada. Rick Gore, The Cambrian Period: Explosion of Life, National Geographic, 184(4):120136 (October 1993), an excellent, brilliantly illustrated article, deals with the early history of life, focusing on the soft-bodied faunas of the Cambrian Period and the poorly known Chengjiang fossils from Yunnan, China. Patricia Vickers-Rich and Thomas Hewitt Rich, Wildlife of Gondwana (1993), chronicles the evolution of life on the southern continents over the past 500 million years. Patricia Vickers-Rich and Thomas Hewitt Rich, Australia's Polar Dinosaurs, Scientific American, 269(1):5055 (July 1993), discusses life at the poles 120 million years ago. Paul S. Martin and Richard G. Klein (eds.), Quaternary Extinctions: A Prehistoric Revolution (1984), is the most thorough treatment of Pleistocene extinctions to date, covering all theories and all geographic regions. Timothy Fridtjof Flannery, The Future Eaters: An Ecological History of the Australasian Lands and People (1994), recounts the ecological history of the biota of Australasia, detailing the consequences of the Pleistocene extinction. Ralph Molnar and Margaret O'Reagan,Dinosaur Extinctions, Australian Natural History, 22(12):560570 (1989), summarizes the plethora of theories concerning dinosaur extinctions. Fred Pearce, Turning Up the Heat (1989), is a readable account of the greenhouse effect and the natural cycles controlling the Earth's climate. An excellent, well-illustrated treatment of changing Earth climates with particular reference to the Australian flora can be found in Mary E. White, After the Greening: The Browning of Australia (1994). Coverage of the impact of human activities on the evolution of the biosphere can be found in Paul R. Ehrlich and Anne H. Ehrlich, The Population Explosion (1990), and in the books by Arthur, cited in the community ecology section above. Timothy Fridtjof Flannery Evolution of the biosphere General features Life is characteristic of the Earth. The biospherewhich in relation to the diameter of the Earth is an extremely thin, life-supporting layer between the upper troposphere and the superficial layers of porous rocks and sedimentsis clearly visible from space; it is responsible for the blue and green colours seen in satellite photographs of the Earth. All known forms of life are based on nucleic acidprotein systems, although life systems involving different chemical components are theoretically possible. Life appears to have developed on the Earth as soon as conditions permitted. Cooling of the hot, primordial Earth was an important factor. In a universe in which extremes of temperature are the norm, most life-forms are restricted to a relatively narrow range of about 0 to 100 C. The abiotic elements of the biosphere have been profoundly shaped by life, just as life has been molded by the environmental conditions that surround it. The biosphere has grown over time. Seven hundred million years ago it was a narrow and possibly discontinuous band encompassing only the shallower parts of the ocean. Today it reaches high into the atmosphere and deep into the ocean, invading even the tiny spaces in porous rocks. Thus, from the troposphere, which extends from 10 to 17 kilometres (6.2 to 9.9 miles) above sea level, to the deepest parts of the ocean (11 kilometres below the sea), to many hundreds of metres into the rocks of the Earth's crust, life thrives. Even in the most hostile of the Earth's environmentsthe frozen and parched south polar desertalgae find refuge in tiny spaces in translucent rocks. The rocks provide shelter from the wind and focus the rays of the Sun, acting as a greenhouse and allowing biological activity to take place for a few weeks each year. At the other extreme, there are thermophilic (heat-loving) bacteria inhabiting deep-sea volcanic vents in which the water is heated under immense pressure to extremely high temperatures. Some researchers believe that some hyperthermophilic organisms existing in these vents can survive at temperatures above 300 C. If the temperature drops much below the boiling point, they die. Life is changed through the process of evolution. Evolution is an inevitable consequence of inheritance, genetic variation, and competition arising from the number of individuals exceeding available resources. The resultnatural selectionpermits the perpetuation of some traits over others. Through billions of years this process has resulted in a great diversification of life-forms. The history of life is characterized by an acceleration of evolutionary change and unpredictable periods of extinction, often followed by rapid diversification. There is still much debate over the causesand even the importanceof some of these trends and events. Perhaps the most hotly debated issues at present concern theories of extinction and diversification. In the early 1970s the evolutionary biologists Stephen Jay Gould and Niles Eldredge developed a model called punctuated equilibrium, which describes and explains some aspects of speciation (see evolution: Patterns and rates of species evolution: Reconstruction of evolutionary history: Gradual and punctuational evolution). This theory postulates that evolution does not progress at a steady rate but rather in bursts, as brief periods of rapid evolutionary change are followed by long periods of relative evolutionary stasis. The degree of interdependence between organic and inorganic elements of the biosphere and the importance of both negative and positive feedback mechanisms in the maintenance of life increasingly are being recognized. At one extreme the British physicist James Lovelock and the American microbiologist Lynn Margulis have argued that, because the elements of the biosphere are so interdependent and interrelated, the biosphere can be viewed as a single, self-regulating organism, which they call Gaia. The Gaia hypothesis postulates that the physical conditions of the Earth's surface, oceans, and atmosphere have been made fit and comfortable for life and have been maintained in this state by the biota themselves. Evidence includes the relatively constant temperature of the Earth's surface that has been maintained for the past 3.5 billion years despite a 25 percent increase in energy coming from the Sun during that period. The remarkable constancy of the Earth's oceanic and atmospheric chemistry for the past 500 million years also is invoked to support this theory. Also integral to the Gaia hypothesis is the crucial involvement of the biota in the cycling of various elements vital to life. The role that living things play in both the carbon and sulfur cycles is a good example of the importance of biological activity and the complex interrelationship of organic and inorganic elements in the biosphere (see biosphere: The organism and the environment: Resources of the biosphere: Nutrient cycling: The carbon cycle and The sulfur cycle). Although the Gaia concept has provided intriguing models of the biosphere, many researchers do not believe the biosphere to be as fully integrated as the Gaia hypothesis suggests. Geologic history and early life-forms Conditions prior to the emergence of life The Earth is approximately 4.6 billion years old. The oldest minerals known (zircon crystals) have been found in western Australia and are about 4.2 to 4.3 billion years old. The oldest known rocks have been found in Greenland and are 3.9 billion years old. They formed at a time when the Earth was fiery with volcanic activity and was pummeled by meteorites. During this time, sometimes referred to as the Hadean Eon, no atmosphere, ozone layer, continents, or oceans existed, and life could not be supported under such conditions (see geochronology: Geologic history of the Earth). The formation of the atmosphere is believed to have resulted from the release of gases from volcanic eruptions (one example of outgassing). (See atmosphere: Development of the Earth's atmosphere: Processes affecting the composition of the early atmosphere.) As the surface of the Earth cooled, water vapour in the newly formed atmosphere condensed to form the water of the oceans. Until 3.9 billion years ago the oceans may have been too hot to support life. By 2.8 billion years ago the first lightweight silica and aluminum rocks, which are typical of the continents, had formed. These rocks expanded rapidly so that by 2.6 billion years ago as much as 60 percent of the continental masses in existence today had formed, and the processes that permit continental drift had commenced (see plate tectonics).