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Ecology

Ecology is the study of environmental systems, or as it is sometimes called, the economy of nature. "Environmental" usually means relating to the natural, versus human-made world; the "systems" means that ecology is, by its very nature, not interested in just the components of nature individually but especially in how the parts interact. Ecology is technically an academic discipline, such as mathematics or physics, although in public or media use, it is often used to connote some sort of normative or evaluative issue as in something is “ecologically bad” or is or is not “good for the ecology”. More properly ecology is used only in the sense that it is an academic discipline, no more evaluative than mathematics or physics. When a normative or evaluative term is needed then it is more proper to use the term “environmental”, i.e., environmental quality or “environmentally degrading”. Most professional ecologists are not terribly unhappy when ecology is used in the normative sense, preferring the wider public awareness of environmental issues today compared to the widespread ignorance of three decades ago. 
The subject matter of ecology is normally divided onto four broad categories: physiological ecology, having to do with the response of single species to environmental conditions such as temperature or light; population ecology, usually focusing on the abundance and distribution of individual species and the factors that cause such distribution; community ecology, having to do with the number of species found at given location and their interactions; and ecosystems ecology, having to do with the structure and function of the entire suite of microbes, plants, and animals, and their abiotic environment, and how the parts interact to generate the whole. This branch of ecology often focuses on the energy and nutrient flows of ecosystems, and when this approach is combined with computer analysis and simulation we often call it systems ecology. Evolutionary ecology, which may operate at any of these levels but most commonly at the physiological or population level, is a rich and dynamic area of ecology focusing on attempting to understand how natural selection developed the structure and function of the organisms and ecosystems at any of these levels. 
Levels of organization of Ecology. (Credit: Erle Ellis)
Levels of organization of Ecology. (Credit: Erle Ellis)
Ecology is usually considered from the perspective of the specific geographic environment that is being studied a the moment: tropical rain forest, temperate grassland, arctic tundra, benthic marine, the entire biosphere, and so on. Thus you might study the population ecology of lions in an African savanna, an ecosystems study of a marine benthic environment, global nutrient budgets, and so on. The subject matter of ecology is the entire natural world, including both the living and the non living parts. Biogeography focuses on the observed distribution of plants and animals and the reasons behind it. More recently ecology has included increasingly the human-dominated world of agriculture, grazing lands for domestic animals, cities, and even industrial parks. Industrial ecology is a discipline that has recently been developed, especially in Europe, where the objective is to follow the energy and material use throughout the process of, e.g., making an automobile with the objective of attempting to improve the material and energy efficiency of manufacturing. For any of these levels or approaches there are some scientists that focus on theoretical ecology, which attempts to derive or apply theoretical or sometimes mathematical reasons and generalities for what is observed in nature, and empirical ecology, which is concerned principally with measurement. Applied ecology takes what is found from one or both of these approaches and uses it to protect or manage nature in some way. Related to this discipline is conservation biology. Plant ecology, animal ecology, and microbial ecology have obvious foci.
There are usually four basic reasons given to study and as to why we might want to understand ecology: first, since all of us live to some degree in a natural or at least partly natural ecosystem, then considerable pleasure can be derived by studying the environment around us. Just as one might learn to appreciate art better through an art history course so too might one appreciate more the nature around us with a better understanding of ecology. Second, human economies are in large part based on the exploitation and management of nature. Applied ecology is used every day in forestry, fisheries, range management, agriculture, and so on to provide us with the food and fiber we need. For example, in Argentina in many circles there is no difference between ecology and agriculture, which is essentially the ecology of crops and pastures. Third, human societies can often be understood very clearly from an ecological perspectives as we study, for example, the population dynamics (demography) of our own species, the food and fossil energy flowing through our society. Fourth, humans appear to be changing aspects of the global environment in many ways. Ecology can be very useful to help us understand what these changes are, what the implications might be for various ecosystems, and how we might intervene in either human economies or in nature to try to mitigate or otherwise alter these changes. There are many professional ecologists, who believe that these apparent changes from human activities have the potential to generate enormous harm to both natural ecosystems and human economies. Understanding, predicting and adapting to these issues could be the most important of all possible issue for humans to deal with. In this case ecology and environmentalism can be the same.
Since ecology by its very nature is an integrative discipline, science students preparing themselves professionally in the field are encouraged to take a broad suite of courses, mostly in the natural sciences and including physics, chemistry, and biology of many sorts but certainly including evolution, meteorology, hydrology, geography, and so on. Ecologists interested in human ecology are encouraged to take courses and undertake readings in agronomy, demography, human geography, sociology, economics, and so on. Since ecology is so broad there are many things that an ecologist might wish to do and to train for. Today many ecology courses are taught in biology departments, where the focus is often on population or community ecology and also individual species.
There are a number of classical areas of interest in ecology, and they revolve around questions similar to the following: how much is the photosynthesis of a hectare of land? How many animals of what types might that photosynthesis be able to support as a base for their food resources? How many species might “divide up” the land or food resources available? How do the species present change as the physical conditions change, for example as one ascends a mountain? What is the proportion of food that is passed on from each food or “trophic” level to the next? What are the mechanisms that control the populations, communities and ecosystems in some area? How are human activities impacting these natural systems?
Ecology should be more than just a set of ideas and principles that one might learn in a classroom or book but rather more a way of looking at the world which emphasizes the assessment and understanding of how the pieces fit together, how each influences and is influenced by the other pieces and how the whole operates in ways not really predictable from the pieces. When we are lucky we are able to capture these relations in conceptual, mathematical or, increasingly, computer models that allow us some sense of truly understanding the great complexity of nature, including as it is impacted by human activity. This is the goal of most ecologists.

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Integrative levels, scope, and scale of organization

Ecosystems regenerate after a disturbance such as fire, forming mosaics of different age groups structured across a landscape. Pictured are different seral stages in forested ecosystems starting from pioneers colonizing a disturbed site and maturing in successional stages leading to old-growth forests.
The scope of ecology covers a wide array of interacting levels of organization spanning micro-level (e.g., cells) to planetary scale (e.g., ecosphere) phenomena. Ecosystems, for example, contain populations of individuals that aggregate into distinct ecological communities. It can take thousands of years for ecological processes to mature through and until the final successional stages of a forest. The area of an ecosystem can vary greatly from tiny to vast. A single tree is of little consequence to the classification of a forest ecosystem, but critically relevant to the smaller organisms living in and on it.[14] Several generations of an aphid population can exist over the lifespan of a single leaf. Each of those aphids, in turn, support diverse bacterial communities.[15] The nature of connections in ecological communities cannot be explained by knowing the details of each species in isolation, because the emergent pattern is neither revealed nor predicted until the ecosystem is studied as an integrated whole. Some ecological principles, however, do exhibit collective properties where the sum of the components explain the properties of the whole, such as birth rates of a population being equal to the sum of individual births over a designated time frame

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Coevolution

Ecological interactions can be divided into host and associate relationships. A host is any entity that harbors another that is called the associate.[143] Host and associate relationships among species that are mutually or reciprocally beneficial are called mutualisms. If the host and associate are physically connected, the relationship is called symbiosis. Approximately 60% of all plants, for example, have a symbiotic relationship with arbuscular mycorrhizal fungi. Symbiotic plants and fungi exchange carbohydrates for mineral nutrients.[144] Symbiosis differs from indirect mutualisms where the organisms live apart. For example, tropical rainforests regulate the Earth's atmosphere. Trees living in the equatorial regions of the planet supply oxygen into the atmosphere that sustains species living in distant polar regions of the planet. This relationship is called commensalism because many other host species receive the benefits of clean air at no cost or harm to the associate tree species supplying the oxygen.[145] The host and associate relationship is called parasitism if one species benefits while the other suffers. Competition among species or among members of the same species is defined as reciprocal antagonism, such as grasses competing for growth space.[146]

Parasites: A harvestman arachnid is parasitized by mites. This is parasitism because the harvestman is being consumed as its juices are slowly sucked out while the mites gain all the benefits traveling on and feeding off of their host.
Popular ecological study systems for mutualism include, fungus-growing ants employing agricultural symbiosis, bacteria living in the guts of insects and other organisms, the fig wasp and yucca moth pollination complex, lichens with fungi and photosynthetic algae, and corals with photosynthetic algae. Nevertheless, many organisms exploit host rewards without reciprocating and thus have been branded with a myriad of not-very-flattering names such as 'cheaters', 'exploiters', 'robbers', and 'thieves'. Although cheaters impose several host cots (e.g., via damage to their reproductive organs or propagules, denying the services of a beneficial partner), their net effect on host fitness is not necessarily negative and, thus, becomes difficult to forecast.

Biogeography

The word biogeography is an amalgamation of biology and geography. Biogeography is the comparative study of the geographic distribution of organisms and the corresponding evolution of their traits in space and time.[151] The Journal of Biogeography was established in 1974.[152] Biogeography and ecology share many of their disciplinary roots. For example, the theory of island biogeography, published by the mathematician Robert MacArthur and ecologist Edward O. Wilson in 1967[153] is considered one of the fundamentals of ecological theory.[154]
Biogeography has a long history in the natural sciences where questions arise concerning the spatial distribution of plants and animals. Ecology and evolution provide the explanatory context for biogeographical studies.[151] Biogeographical patterns result from ecological processes that influence range distributions, such as migration and dispersal.[154] and from historical processes that split populations or species into different areas.[155] The biogeographic processes that result in the natural splitting of species explains much of the modern distribution of the Earth's biota. The splitting of lineages in a species is called vicariance biogeography and it is a sub-discipline of biogeography. There are also practical applications in the field of biogeography concerning ecological systems and processes. For example, the range and distribution of biodiversity and invasive species responding to climate change is a serious concern and active area of research in context of global warming

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Trophic levels

The Greek root of the word troph, τροφή, trophē, means food or feeding. Links in food-webs primarily connect feeding relations or trophism among species. Biodiversity within ecosystems can be organized into vertical and horizontal dimensions. The vertical dimension represents feeding relations that become further removed from the base of the food chain up toward top predators. A trophic level is defined as "a group of organisms acquiring a considerable majority of its energy from the adjacent level nearer the abiotic source." [91]:383 The horizontal dimension represents the abundance or biomass at each level.[92] When the relative abundance or biomass of each functional feeding group is stacked into their respective trophic levels they naturally sort into a 'pyramid of numbers'.[93] Functional groups are broadly categorized as autotrophs (or primary producers), heterotrophs (or consumers), and detrivores (or decomposers). Autotrophs are organisms that can produce their own food (production is greater than respiration) and are usually plants or cyanobacteria that are capable of photosynthesis but can also be other organisms such as bacteria near ocean vents that are capable of chemosynthesis. Heterotrophs are organisms that must feed on others for nourishment and energy (respiration exceeds production).[1] Heterotrophs can be further sub-divided into different functional groups, including: primary consumers (strict herbivores), secondary consumers (carnivorous predators that feed exclusively on herbivores) and tertiary consumers (predators that feed on a mix of herbivores and predators).[94] Omnivores do not fit neatly into a functional category because they eat both plant and animal tissues. It has been suggested that omnivores have a greater functional influence as predators because relative to herbivores they are comparatively inefficient at grazing.[95]

A trophic pyramid (a) and a food-web (b) illustrating ecological relationships among creatures that are typical of a northern Boreal terrestrial ecosystem. The trophic pyramid roughly represents the biomass (usually measured as total dry-weight) at each level. Plants generally have the greatest biomass. Names of trophic categories are shown to the right of the pyramid. Some ecosystems, such as many wetlands, do not organize as a strict pyramid, because aquatic plants are not as productive as long-lived terrestrial plants such as trees. Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy.[1]
The decomposition of dead organic matter, such as leaves falling on the forest floor, turns into soils containing minerals and nutrients that feed into plant production. The total sum of the planet's soil ecosystems is called the pedosphere where a very large proportion of the Earth's biodiversity sorts into other trophic levels. Invertebrates that feed and shred larger leaves, for example, create smaller bits for smaller organisms in the feeding chain. Collectively, these are the detrivores that regulate soil formation. Tree roots, fungi, bacteria, worms, ants, beetles, centipedes, spiders, mammals, birds, reptiles, amphibians and other less familiar creatures all work to create the trophic web of life in soil ecosystems.
As organisms feed and migrate through soils they physically displace materials, which is an important ecological process called bioturbation. Bioturbation helps to aerate the soils, thus stimulating hetertrophic growth and production. Biomass of soil microorganisms are influenced by and feed back into the trophic dynamics of the exposed solar surface ecology. Paleoecological studies of soils places the origin for bioturbation to a time before the Cambrian period. Other events, such as the evolution of trees and amphibians moving into land in the Devonian period played a significant role in the development of the ecological trophism in soils

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