BIOSPHERE

relatively thin life-supporting stratum of the Earth's surface, extending from a few kilometres into the atmosphere to the deep-sea vents of the ocean. The biosphere is a global ecosystem composed of living organisms (biota) and the abiotic (nonliving) factors from which they derive energy and nutrients. Before the coming of life, the Earth was a bleak place, a rocky globe with shallow seas and a thin band of gaseslargely carbon dioxide, carbon monoxide, molecular nitrogen, hydrogen sulfide, and water vapour. It was a hostile and barren planet. This strictly inorganic state of the Earth is called the geosphere; it consists of the lithosphere (the rock and soil), the hydrosphere (the water), and the atmosphere (the air). Energy from the Sun relentlessly bombarded the surface of the primitive Earth, and in timemillions of yearschemical and physical actions produced the first evidence of life: formless, jellylike blobs that could collect energy from the environment and produce more of their own kind. This generation of life in the thin outer layer of the geosphere established what is called the biosphere, the zone of life, an energy-diverting skin that uses the matter of the Earth to make living substance. The biosphere is a system characterized by the continuous cycling of matter and an accompanying flow of solar energy in which certain large molecules and cells are self-reproducing. Water is a major predisposing factor, for all life depends on it. The elements carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, when combined as proteins, lipids, carbohydrates, and nucleic acids, provide the building blocks, the fuel, and the direction for the creation of life. Energy flow is required to maintain the structure of organisms by the formation and splitting of phosphate bonds. Organisms are cellular in nature and always contain some sort of enclosing membrane structure, and all have nucleic acids that store and transmit genetic information. All life on Earth depends ultimately upon green plants, as well as upon water. Plants utilize sunlight in a process called photosynthesis to produce the food upon which animals feed and to provide, as a by-product, oxygen, which most animals require for respiration. At first, the oceans and the lands were teeming with large numbers of a few kinds of simple single-celled organisms, but slowly plants and animals of increasing complexity evolved. Interrelationships developed so that certain plants grew in association with certain other plants, and animals associated with the plants and with one another to form communities of organisms, including those of forests, grasslands, deserts, dunes, bogs, rivers, and lakes. Living communities and their nonliving environment are inseparably interrelated and constantly interact upon each other. For convenience, any segment of the landscape that includes the biotic and abiotic components is called an ecosystem. A lake is an ecosystem when it is considered in totality as not just water but also nutrients, climate, and all of the life contained within it. A given forest, meadow, or river is likewise an ecosystem. One ecosystem grades into another along zones termed ecotones, where a mixture of plant and animal species from the two ecosystems occurs. A forest considered as an ecosystem is not simply a stand of trees but is a complex of soil, air, and water, of climate and minerals, of bacteria, viruses, fungi, grasses, herbs, and trees, of insects, reptiles, amphibians, birds, and mammals. Stated another way, the abiotic, or nonliving, portion of each ecosystem in the biosphere includes the flow of energy, nutrients, water, and gases and the concentrations of organic and inorganic substances in the environment. The biotic, or living, portion includes three general categories of organisms based on their methods of acquiring energy: the primary producers, largely green plants; the consumers, which include all the animals; and the decomposers, which include the microorganisms that break down the remains of plants and animals into simpler components for recycling in the biosphere. Aquatic ecosystems are those involving marine environments and freshwater environments on the land. Terrestrial ecosystems are those based on major vegetational types, such as forest, grassland, desert, and tundra. Particular kinds of animals are associated with each such plant province. Ecosystems may be further subdivided into smaller biotic units called communities. Examples of communities include the organisms in a stand of pine trees, on a coral reef, and in a cave, a valley, a lake, or a stream. The major consideration in the community is the living component, the organisms; the abiotic factors of the environment are excluded. A community is a collection of species populations. In a stand of pines, there may be many species of insects, of birds, of mammals, each a separate breeding unit but each dependent on the others for its continued existence. A species, furthermore, is composed of individuals, single functioning units identifiable as organisms. Beyond this level, the units of the biosphere are those of the organism: organ systems composed of organs, organs of tissues, tissues of cells, cells of molecules, and molecules of atomic elements and energy. The progression, therefore, proceeding upward from atoms and energy, is toward fewer units, larger and more complex in pattern, at each successive level. This article focuses on the makeup of the biosphere and examines the relationships between its principal components, including man. The characteristics and dynamics of biological populations and communities are dealt with, as are the interactions that constitute the primary stabilizing links among the constituent organisms. Due attention is also given to the distribution patterns of these biotic units and to the processes that produced such patterns. The major aquatic and terrestrial ecosystems of the Earth are treated in some detail. Other points include energy transformations and transfers within the biosphere and the cyclic flow of materials needed for life. For the development, methodology, and applications of the study of interrelations of organisms with their environment and each other, see ecology. Further treatment of the various aquatic and terrestrial environments is provided in ocean, lake, river, continental landform, Arctic, and Antarctica. For a discussion of the origin of life on Earth and the varieties of and commonalities among organisms, see life and Earth, pregeologic history of. The characteristics and classifications of living organisms are covered in detail in algae, amphibian, angiosperm, animal, annelid, arachnid, arthropod, aschelminth, bacteria, bird, bryophyte, chordate, cnidarian, crustacean, dinosaur, echinoderm, fern, fish, flatworm, fungus, gymnosperm, insect, lamp shell, mammal, mollusk, moss animal, plant, protist, protozoa, reptile, sponge, and virus. David M. Gates The Editors of the Encyclopdia Britannica relatively thin life-supporting stratum of the Earth's surface, extending from a few kilometres into the atmosphere to the deep-sea vents of the oceans. The biosphere is a global ecosystem composed of living organisms (biota) and the abiotic (nonliving) factors from which they derive energy and nutrients. The biosphere can be broken down into segments of abiotic and biotic components, called ecosystems. Oceans, lakes, and wetlands are aquatic ecosystems, while forests, deserts, and tundras are terrestrial ecosystems. Through these systems, energy flows and chemicals essential to life are cycled. The biosphere itself can be studied as a worldwide ecosystem through which the interconnectedness of all life and life-supporting systems on the Earth can be understood. Organisms in the biosphere are classified into trophic levels, or feeding relationships, that constitute the food chain. Primary producers, or autotrophs, are those organisms that convert energy from the Sun (photoautotrophs) or from inorganic substances (chemoautotrophs) to produce organic compounds. Green plants make up the largest group of primary producers. The next trophic level is that of primary consumers, or herbivores (organisms that eat plants or algae). Secondary consumers are carnivores that feed on herbivores, while tertiary consumers feed on secondary consumers, and so on. Detritivores, or decomposers, are consumers that feed on organic detritus. The process of energy flow occurs across the trophic levels. Energy enters the system through solar radiation, which primary producers convert to chemical energy (organic compounds) by the process of photosynthesis. Of the light energy that strikes the Earth, only about 1 percent is used in photosynthesis. Energy flows unidirectionally through the food chain and is dissipated at each successive stage; roughly 10 percent of energy is transferred from one trophic level to the next. Unlike energy, which flows into and out of the system, chemicals are recycled in the biosphere. Elements essential to life such as carbon, nitrogen, phosphorus, and sulfur are drawn from the geosphere, or nonliving world, which consists of the atmosphere (air), the hydrosphere (water), and the lithosphere (rocks and soil of the terrestrial surface). Once taken up by organisms, the elements cycle between biotic and abiotic states according to their biogeochemical cycles. The cycling of water is also necessary to the maintenance of life. The biotic portion of ecosystems can be broken down into communitiesi.e., assemblages of populations of different species that live in proximity and may interact with one another. Populations, still smaller organizational units, are groups of individuals of the same species located in a particular geographic area. Environmental conditions such as temperature, water availability, light, and periodic disturbances affect the distribution of organisms, and interactions between the species themselves further influence the composition of the community. Interspecific interactions include competition, antagonism, and predation. Interactions with the biotic and abiotic components of their ecosystems have shaped the distribution and evolution of species, resulting in a diverse array of organisms. These organisms contribute to the steady-state environment of each ecosystem and, thus, to the maintenance of biospheric processes. Disturbances, both natural and man-made, to even seemingly small parts of the system may have significant and far-reaching effects. Additional reading General works Mitchell B. Rambler, Lynn Margulis, and Ren Fester (eds.), Global Ecology: Towards a Science of the Biosphere (1989), describes and interprets the biosphere and the processes that occur within it. Richard J. Huggett, Climate, Earth Processes, and Earth History (1991), discusses the changing of the biosphere over time. William K. Purves, Gordon H. Orians, and H. Craig Heller, Life: The Science of Biology, 4th ed. (1994), treats such topics as general ecology, the biosphere, and the origin of life. Paul R. Ehrlich and Jonathan Roughgarden, The Science of Ecology (1987), describes organisms in their environments. Lawrence E. Joseph, Gaia: The Growth of an Idea (1990), is a simple explanation of the Gaia hypothesis. Leslie A. Real and James H. Brown (eds.), Foundations of Ecology: Classic Papers with Commentaries (1991), provides a historical perspective of the major issues in ecology. Nigel Pears, Basic Biogeography, 2nd ed. (1985), examines the distribution and abundance of life on the Earth. Lynn Margulis and Lorraine Olendzenski (eds.), Environmental Evolution (1992), discusses the interaction of life and the abiotic components of the Earth, as well as the evolution of life as a consequence of changes to the biosphere over time.The environmental physiology of animals is covered in F. Harvey Pough, John B. Heiser, and William N. McFarland, Vertebrate Life (1989); Knut Schmidt-Nielsen, Animal Physiology: Adaptation and Environment, 4th ed. (1990); and Philip C. Withers, Comparative Animal Physiology (1992). J. Prothero and K.D. Jurgens, An Energetic Model of Daily Torpor in Endotherms, Journal of Theoretical Biology, 121(4):403416 (1986), explores the advantages to endothermic animals of entering torpor. Cynthia Carey et al. (eds.), Life in the Cold: Ecological, Physiological, and Molecular Mechanisms (1993), investigates the interactions of organisms with cold environments from ecological and physiological perspectives. Harold Heatwole and Janet Taylor, Ecology of Reptiles (1987), sets forth the major environmental parameters that influence the lives of animals, using reptilian examples. Knut Schmidt-Nielsen, F.R. Hainsworth, and D.E. Murrish, Counter-Current Heat Exchange in the Respiratory Passages: Effects on Water and Heat Balance, Respiration Physiology, 9(7):263276 (May 1970), describes animals' utilization of the nasal passages as a means to allow thermoregulation and simultaneously conserve water. R.S. Seymour, How Sea Snakes May Avoid the Bends, Nature, 250(5466):489490 (Aug. 9, 1974), outlines the physical and physiological influences affecting animals that breathe air on the surface but dive to great depths. David W. Goodall, Ecosystems of the World (1977 ), discusses terrestrial and aquatic ecosystems. Michael B. Thompson The organism and the environment Michael Begon, John L. Harper, and Colin R. Townsend, Ecology: Individuals, Populations, and Communities, 2nd ed. (1990); Robert E. Ricklefs, Ecology, 3rd ed. (1990); Charles J. Krebs, Ecology: The Experimental Analysis of Distribution and Abundance, 4th ed. (1994); and the work by Ehrlich and Roughgarden, cited above, are well-written textbooks that provide good general descriptions of energy flow and nutrient cycling through ecosystems, population biology, and community ecology.D.L. DeAngelis, Dynamics of Nutrient Cycling and Food Webs (1992), is a mathematical treatment of the rates of energy flow and nutrient cycling through ecosystems. B. Bolin and R.B. Cook (eds.), The Major Biogeochemical Cycles and Their Interactions (1983), discusses the major global patterns of nutrient cycling for all the major nutrients. An in-depth treatment of how the major nutrients necessary for plant growth move in cycles between plants and the soil can be found in F.J. Stevenson, Cycles of Soil: Carbon, Nitrogen, Phosphorus, Sulfur, Micronutrients (1986). A review of the groups of microorganisms and their various roles in the nitrogen cycle is Janet I. Sprent and Peter Sprent, Nitrogen Fixing Organisms: Pure and Applied Aspects (1990).David A. Dunnette and Robert J. O'brien (eds.), The Science of Global Change: The Impact of Human Activities on the Environment (1992), reports on the ways in which human activities are influencing biogeochemical cycles and climate change. Robert L. Peters and Thomas E. Lovejoy (eds.), Global Warming and Biological Diversity (1992); and Peter M. Kareiva, Joel G. Kingsolver, and Raymond B. Huey (eds.), Biotic Interactions and Global Change (1993), investigate how human-induced global changes affect organisms, population, species, communities, and ecosystems. John N. Thompson