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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 that include biotic and 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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The hierarchy of biological classification's eight major taxonomic ranks, which is an example of definition by genus and differentia. Life is divided into domains, which are subdivided into further groups. Intermediate minor rankings are not shown.
Pictured left: The hierarchy of biological classification's eight major taxonomic ranks, which is an example of definition by genus and differentia. Life is divided into domains, which are subdivided into further groups. Intermediate minor rankings are not shown.

Life (cf. biota) is a characteristic that distinguishes objects that have signaling and self-sustaining processes (i. e., living organisms) from those that do not, either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate. Biology is the science concerned with the study of life.

Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means. A diverse array of living organisms (life forms) can be found in the biosphere on Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information.

It is still a challenge for scientists and philosophers to define life in unequivocal terms. Defining life is difficult—in part—because life is a process, not a pure substance. Any definition must be sufficiently broad to encompass all life with which we are familiar, and it should be sufficiently general that, with it, scientists would not miss life that may be fundamentally different from life on Earth.

Evidence suggests that life on Earth has existed for about 3.7 billion years, with the oldest traces of life found in fossils dating back 3.4 billion years. All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. There are many different hypotheses regarding the path that might have been taken from simple organic molecules via pre-cellular life to protocells and metabolism. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models that combine both categories.

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Amphiprion ocellaris (Clown anemonefish) in Heteractis magnifica (Sea anemone).jpg

Ocellaris clownfish often live symbiotically with the Heteractis magnifica sea anemone, using them for shelter and protection.

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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...

...soil ecology is the study of the interactions among soil organisms, and between biotic and abiotic aspects of the soil environment? It is particularly concerned with the cycling of nutrients, formation and stabilization of the pore structure, the spread and vitality of pathogens, and the biodiversity of this rich biological community.
(Pictured left: a type of soil known as ultisol, common throughout the American South (especially near the piedmont), and colloquially known as "red clay soil". Photo taken in the Piedmont region of North Carolina.)
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To halt the decline of an ecosystem, it is necessary to think like an ecosystem.

—Douglas P. Wheeler, EPA Journal, September-October 1990

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The Journal of Biogeography is a peer-reviewed scientific journal in biogeography that was established in 1974. It covers aspects of spatial, ecological, and historical biogeography.

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