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Ecology (from Greek: οἶκος, "house", or "environment"; -λογία, "study of") is the branch of biology which studies the interactions among organisms and their environment. Objects of study include interactions of organisms with each other and with abiotic components of their environment. Topics of interest include the biodiversity, distribution, biomass, and populations of organisms, as well as cooperation and competition within and between species. Ecosystems are dynamically interacting systems of organisms, the communities they make up, and the non-living components of their environment. Ecosystem processes, such as primary production, pedogenesis, nutrient cycling, and niche construction, regulate the flux of energy and matter through an environment. These processes are sustained by organisms with specific life history traits. Biodiversity means the varieties of species, genes, and ecosystems, enhances certain ecosystem services.

Ecology is not synonymous with environmentalism, natural history, or environmental science. It overlaps with the closely related sciences of evolutionary biology, genetics, and ethology. An important focus for ecologists is to improve the understanding of how biodiversity affects ecological function. Ecologists seek to explain:

  • Life processes, interactions, and adaptations
  • The movement of materials and energy through living communities
  • The successional development of ecosystems
  • The abundance and distribution of organisms and biodiversity in the context of the environment.

Ecology has practical applications in conservation biology, wetland management, natural resource management (agroecology, agriculture, forestry, agroforestry, fisheries), city planning (urban ecology), community health, economics, basic and applied science, and human social interaction (human ecology). For example, the Circles of Sustainability approach treats ecology as more than the environment 'out there'. It is not treated as separate from humans. Organisms (including humans) and resources compose ecosystems which, in turn, maintain biophysical feedback mechanisms that moderate processes acting on living (biotic) and non-living (abiotic) components of the planet. Ecosystems sustain life-supporting functions and produce natural capital like biomass production (food, fuel, fiber, and medicine), the regulation of climate, global biogeochemical cycles, water filtration, soil formation, erosion control, flood protection, and many other natural features of scientific, historical, economic, or intrinsic value.

The word "ecology" ("Ökologie") was coined in 1866 by the German scientist Ernst Haeckel. Ecological thought is derivative of established currents in philosophy, particularly from ethics and politics. Ancient Greek philosophers such as Hippocrates and Aristotle laid the foundations of ecology in their studies on natural history. Modern ecology became a much more rigorous science in the late 19th century. Evolutionary concepts relating to adaptation and natural selection became the cornerstones of modern ecological theory.

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View of the crescent moon through the top of the earth's atmosphere. Photographed above 21.5°N, 113.3°E. by the International Space Station crew Expedition 13 over the South China Sea, just south of Macau.
Pictured left: View of the crescent moon through the top of the earth's atmosphere. Photographed above 21.5°N, 113.3°E. by the International Space Station crew Expedition 13 over the South China Sea, just south of Macau.

The atmosphere of the Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Kármán line, at 100 km (62 mi), also is often regarded as the boundary between atmosphere and outer space.

Air is the name given to atmosphere used in breathing and photosynthesis. While air content and atmospheric pressure varies at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases, often referred to as trace gases, which include greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Air also contains a variable amount of water vapor, on average around 1%. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide [SO2].

In general, air pressure and density decrease in the atmosphere as height increases. However, temperature has a more complicated profile with altitude. Because the general pattern of this profile is constant and recognizable through means such as balloon soundings, temperature provides a useful metric to distinguish between atmospheric layers. In this way, Earth's atmosphere can be divided into five main layers. From highest to lowest, these layers are the Exosphere, Thermosphere, Mesosphere, Stratosphere and Troposphere.

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Paddlefish Polyodon spathula.jpg
Credit: U.S. Fish and Wildlife Service

Drawing of an American paddlefish (Polyodon spathula). Also called the Mississippi paddlefish or spoonbill, they live in slow-flowing waters of the Mississippi River drainage system and may grow to 7 feet (220 cm) and weigh up to 220 pounds (100 kg). They appear to have been extirpated from Lake Erie and its tributaries.

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Johannes Eugenius Bülow Warming (November 3, 1841 – April 2, 1924), known as Eugen Warming, was a Danish botanist and a main founding figure of the scientific discipline of ecology. Warming wrote the first textbook (1895) on plant ecology, taught the first university course in ecology and gave the concept its meaning and content. Warming wrote a number of textbooks on botany, plant geography and ecology, which were translated to several languages and were immensely influential at their time and later. Most important were Plantesamfund and Haandbog i den systematiske Botanik.

The book Plantesamfund was based on Warming’s lectures on plant geography at the University of Copenhagen. It gives an introduction to all major biomes of the world. Warming’s aim, and his major lasting impact on the development of ecology, was to explain how nature solved similar problems (drought, flooding, cold, salt, herbivory, etc.) in similar way, despite using very different ‘raw material’ (species of different origin) in different regions of the world. This was a remarkably modern view – completely different from the merely descriptive floristic plant geography prevailing during his time.

Did you know...

Dmawsoni Head shot.jpg
...the Antarctic cod, or Antarctic toothfish, of the fish family Nototheniidae, is famous for producing antifreeze glycoprotein that allows it to survive in the ice-laden waters of the Southern Ocean surrounding Antarctica? With a heartbeat once every six seconds, research involving Antarctic toothfish may lead to advances in cardiac medicine involving conditions where human hearts beat slowly during certain medical procedures or fail to beat fast enough due to hypothermia.
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Oh Beautiful for smoggy skies, insecticided grain, For strip-mined mountain's majesty above the asphalt plain. America, America, man sheds his waste on thee, And hides the pines with billboard signs, from sea to oily sea.
— George Carlin

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The American Naturalist is a monthly scientific journal that was founded in 1867 and is associated with the American Society of Naturalists. It is published by the University of Chicago Press. The journal covers ecology, evolutionary biology, population, and integrative biology research.

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