An ecosystem is a community of living organisms (plants, animals and microbes) in conjunction with the nonliving components of their environment (things like air, water and mineral soil), interacting as a system.[2] These components are regarded as linked together through nutrient cycles and energy flows.[3] As ecosystems are defined by the network of interactions among organisms, and between organisms and their environment,[4] they can come in any size but usually encompass specific, limited spaces[5] (although some scientists say that the entire planet is an ecosystem).[6]

History and development

Arthur Tansley, a British ecologist, was the first person to use the term "ecosystem" in a published work.[fn 1][10] Tansley devised the concept to draw attention to the importance of transfers of materials between organisms and their environment.[11] He later refined the term, describing it as "The whole system, ... including not only the organism-complex, but also the whole complex of physical factors forming what we call the environment".[12] Tansley regarded ecosystems not simply as natural units, but as mental isolates.[12] Tansley later[13] defined the spatial extent of ecosystems using the term ecotope.
G. Evelyn Hutchinson, a pioneering limnologist who was a contemporary of Tansley's, combined Charles Elton's ideas about trophic ecology with those of Russian geochemist Vladimir Vernadsky to suggest that mineral nutrient availability in a lake limited algal production which would, in turn, limit the abundance of animals that feed on algae. Raymond Lindeman took these ideas one step further to suggest that the flow of energy through a lake was the primary driver of the ecosystem. Hutchinson's students, brothers Howard T. Odum and Eugene P. Odum, further developed a "systems approach" to the study of ecosystems, allowing them to study the flow of energy and material through ecological systems.[11]

Ecosystem processes

Energy and carbon enter ecosystems through photosynthesis, are incorporated into living tissue, transferred to other organisms that feed on the living and dead plant matter, and eventually released through respiration.[14] Most mineral nutrients, on the other hand, are recycled within ecosystems.[15]
Ecosystems are controlled both by external and internal factors. External factors, also called state factors, control the overall structure of an ecosystem and the way things work within it, but are not themselves influenced by the ecosystem. The most important of these is climate.[8] Climate determines the biome in which the ecosystem is embedded. Rainfall patterns and temperature seasonality determine the amount of water available to the ecosystem and the supply of energy available (by influencing photosynthesis).[8] Parent material, the underlying geological material that gives rise to soils, determines the nature of the soils present, and influences the supply of mineral nutrients. Topography also controls ecosystem processes by affecting things like microclimate, soil development and the movement of water through a system. This may be the difference between the ecosystem present in wetland situated in a small depression on the landscape, and one present on an adjacent steep hillside.[8]
Other external factors that play an important role in ecosystem functioning include time and potential biota. Ecosystems are dynamic entities—invariably, they are subject to periodic disturbances and are in the process of recovering from some past disturbance.[9] Time plays a role in the development of soil from bare rock and the recovery of a community from disturbance.[8] Similarly, the set of organisms that can potentially be present in an area can also have a major impact on ecosystems. Ecosystems in similar environments that are located in different parts of the world can end up doing things very differently simply because they have different pools of species present.[8] The introduction of non-native species can cause substantial shifts in ecosystem function.
Unlike external factors, internal factors in ecosystems not only control ecosystem processes, but are also controlled by them. Consequently, they are often subject to feedback loops.[8] While the resource inputs are generally controlled by external processes like climate and parent material, the availability of these resources within the ecosystem is controlled by internal factors like decomposition, root competition or shading.[8] Other factors like disturbance, succession or the types of species present are also internal factors. Human activities are important in almost all ecosystems. Although humans exist and operate within ecosystems, their cumulative effects are large enough to influence external factors like climate.[8]

Function and biodiversity

See also: Biodiversity and Ecosystem engineer
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Loch Lomond in Scotland forms a relatively isolated ecosystem. The fish community of this lake has remained stable over a long period until a number of introductions in the 1970s restructured its food web.[23]
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Spiny forest at Ifaty, Madagascar, featuring various Adansonia (baobab) species, Alluaudia procera (Madagascar ocotillo) and other vegetation.
Ecosystem processes are broad generalizations that actually take place through the actions of individual organisms. The nature of the organisms—the species, functional groups and trophic levels to which they belong—dictates the sorts of actions these individuals are capable of carrying out, and the relative efficiency with which they do so. Thus, ecosystem processes are driven by the number of species in an ecosystem, the exact nature of each individual species, and the relative abundance organisms within these species.[24] Biodiversity plays an important role in ecosystem functioning.[25]
Ecological theory suggests that in order to coexist, species must have some level of limiting similarity—they must be different from one another in some fundamental way, otherwise one species would competitively exclude the other.[26] Despite this, the cumulative effect of additional species in an ecosystem is not linear—additional species may enhance nitrogen retention, for example, but beyond some level of species richness, additional species may have little additive effect.[24] The addition (or loss) of species which are ecologically similar to those already present in an ecosystem tends to only have a small effect on ecosystem function. Ecologically distinct species, on the other hand, have a much larger effect. Similarly, dominant species have a large impact on ecosystem function, while rare species tend to have a small effect. Keystone species tend to have an effect on ecosystem function that is disproportionate to their abundance in an ecosystem.[24]

[edit] Ecosystem goods and services

Main articles: Ecosystem services and Ecological goods and services
See also: Ecosystem valuation and Ecological yield
Ecosystems provide a variety of goods and services upon which people depend.[27] Ecosystem goods include the "tangible, material products"[28] of ecosystem processes—food, construction material, medicinal plants—in addition to less tangible items like tourism and recreation, and genes from wild plants and animals that can be used to improve domestic species.[27] Ecosystem services, on the other hand, are generally "improvements in the condition or location of things of value".[28] These include things like the maintenance of hydrological cycles, cleaning air and water, the maintenance of oxygen in the atmosphere, crop pollination and even things like beauty, inspiration and opportunities for research.[27] While ecosystem goods have traditionally been recognized as being the basis for things of economic value, ecosystem services tend to be taken for granted.[28] While Gretchen Daily's original definition distinguished between ecosystem goods and ecosystem services, Robert Costanza and colleagues' later work and that of the Millennium Ecosystem Assessment lumped all of these toBrown2007"/> gether as ecosystem services.[29] A variety of definitions exist: F. Stuart Chapin and coauthors define it as "the application of ecological science to resource management to promote long-term sustainability of ecosystems and the delivery of essential ecosystem goods and services",[30] while Norman Christensen and coauthors defined it as "management driven by explicit goals, executed by policies, protocols, and practices, and made adaptable by monitoring and research based on our best understanding of the ecological interactions and processes necessary to sustain ecosystem structure and function"[27] and Peter Brussard and colleagues defined it as "managing areas at various scales in such a way that ecosystem services and biological resources are preserved while appropriate human use and options for livelihood are sustained".[31]
Although definitions of ecosystem management abound, there is a common set of principles which underlie these definitions.[30] A fundamental principle is the long-term sustainability of the production of goods and services by the ecosystem;[30] "intergenerational sustainability [is] a precondition for management, not an afterthought".[27] It also requires clear goals with respect to future trajectories and behaviors of the system being managed. Other important requirements include a sound ecological understanding of the system, including connectedness, ecological dynamics and the context in which the system is embedded. Other important principles include an understanding of the role of humans as components of the ecosystems and the use of adaptive management.[27] While ecosystem management can be used as part of a plan for wilderness conservation, it can also be used in intensively managed ecosystems[27] (see, for example, agroecosystem and close to nature forestry).

Classifying ecosystems into ecologically homogeneous units is an important step towards effective ecosystem management.[42] A variety of systems exist, based on vegetation cover, remote sensing, and bioclimatic classification systems.[42] American geographer Robert Bailey defines a hierarchy of ecosystem units ranging from microecosystems (individual homogeneous sites, on the order of 10 square kilometres (4 sq mi) in area), through mesoecosystems (landscape mosaics, on the order of 1,000 square kilometres (400 sq mi)) to macroecosystems (ecoregions, on the order of 100,000 square kilometres (40,000 sq mi)).[43]
Bailey outlined five different methods for identifying ecosystems: gestalt ("a whole that is not derived through considerable of its parts"), in which regions are recognized and boundaries drawn intuitively; a map overlay system where different layers like geology, landforms and soil types are overlain to identify ecosystems; mulitvariate clustering of site attributes; digital image processing of remotely sensed data grouping areas based on their appearance or other spectral properties; or by a "controlling factors method" where a subset of factors (like soils, climate, vegetation physiognomy or the distribution of plant or animal species) are selected from a large array of possible ones are used to delineate ecosystems.[44] In contrast with Bailey's methodology, Puerto Rico ecologist Ariel Lugo and coauthors identified ten characteristics of an effective classification system: that it be based on georeferenced, quantitative data; that it should minimize subjectivity and explicitly identify criteria and assumptions; that it should be structured around the factors that drive ecosystem processes; that it should reflect the hierarchical nature of ecosystems; that it should be flexible enough to conform to the various scales at which ecosystem management operates; that it should be tied to reliable measures of climate so that it can "anticipat[e] global climate change; that it be applicable worldwide; that it should be validated against independent data; that it take into account the sometimes complex relationship between climate, vegetation and ecosystem functioning; and that it should be able to adapt and improve as new data become available".[42]

Types of ecosystems

• Aquatic ecosystem • Forest • Greater Yellowstone Ecosystem • Human ecosystem • Large marine ecosystem • Littoral zone • Marine ecosystem • Riparian zone • River ecosystem • Subsurface Lithoautotrophic Microbial Ecosystem • Urban ecosystem • Movile Cave • Desert

Aquatic ecosystem

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An estuary mouth and coastal waters, part of an aquatic ecosystem
An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems.[
Marine ecosystems cover approximately 71% of the Earth's surface and contain approximately 97% of the planet's water. They generate 32% of the world's net primary production.[1] They are distinguished from freshwater ecosystems by the presence of dissolved compounds, especially salts, in the water. Approximately 85% of the dissolved materials in seawater are sodium and chlorine. Seawater has an average salinity of 35 parts per thousand (ppt) of water. Actual salinity varies among different marine ecosystems.[2]
Classes of organisms found in marine ecosystems include brown algae, dinoflagellates, corals, cephalopods, echinoderms, and sharks. Fishes caught in marine ecosystems are the biggest source of commercial foods obtained from wild populations.[1]
Environmental problems concerning marine ecosystems include unsustainable exploitation of marine resources (for example overfishing of certain species), marine pollution, climate change, and building on coastal areas.[1]

FOREST
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending on various cultural definitions, what is considered a forest may vary significantly in size and have different classifications according to how and of what the forest is composed.[1] A forest is usually an area filled with trees but any tall densely packed area of vegetation may be considered a forest, even underwater vegetation such as kelp forests, or non-vegetation such as fungi,[2] and bacteria. Tree forests cover approximately 9.4 percent of the Earth's surface (or 30 percent of total land area), though they once covered much more (about 50 percent of total land area). They function as habitats for organisms, hydrologic flow modulators, and soil conservers, constituting one of the most important aspects of the biosphere.
A typical tree forest is composed of the overstory (canopy or upper tree layer) and the understory. The understory is further subdivided into the shrub layer, herb layer, and also the moss layer and soil microbes. In some complex forests, there is also a well-defined lower tree layer. Forests are central to all human life because they provide a diverse range of resources: they store carbon, aid in regulating the planetary climate, purify water and mitigate natural hazards such as floods. Forests also contain roughly 90 percent of the world's terrestrial biodiversity

Human ecosystem

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An aerial view of a human ecosystem. Pictured is the city of Chicago
Human ecosystems are complex cybernetic systems that are increasingly being used by ecological anthropologists and other scholars to examine the ecological aspects of human communities in a way that integrates multiple factors as economics, socio-political organization, psychological factors, and physical factors related to the environment.

Marine ecosystem

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Coral reefs form complex marine ecosystems with tremendous biodiversity.
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Here, we can see different types of starfish, coral reefs and fishes in the Great Barrier Reef.
Marine ecosystems are among the largest of Earth's aquatic ecosystems. They include oceans, salt marsh and intertidal ecology, estuaries and lagoons, mangroves and coral reefs, the deep sea and the sea floor. They can be contrasted with freshwater ecosystems, which have a lower salt content. Marine waters cover two-thirds of the surface of the Earth. Such places are considered ecosystems because the plant life supports the animal life and vice-versa. See food chains.
Marine ecosystems are very important for the overall health of both marine and terrestrial environments. According to the World Resource Center, coastal habitats alone account for approximately 1/3 of all marine biological productivity, and estuarine ecosystems (i.e., salt marshes, seagrasses, mangrove forests) are among the most productive regions on the planet. In addition, other marine ecosystems such as coral reefs, provide food and shelter to the highest levels of marine diversity in the world.[1]
Marine ecosystems usually have a large biodiversity and are therefore thought to have a good resistance against invasive species. However, exceptions have been observed, and the mechanisms responsible in determining the success of an invasion are not yet clear.[

DESERT
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than 400 millimetres (16 in).[1] A common definition distinguishes between true deserts, which receive less than 250 millimetres (10 in) of average annual precipitation, and semideserts or steppes, which receive between 250 millimetres (10 in) and 400 to 500 millimetres (16 to 20 in).[1][2]
Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation.[1] In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates.[

Desert features

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Satellite view of Al-Dahna desert in Saudi Arabia showing different depositional features
Sand covers only about 20% of Earth's deserts. Most of the sand is in sand sheets and sand seas—vast regions of undulating dunes resembling ocean waves "frozen" in an instant of time. In general, there are five forms of deserts: • Mountain and basin deserts • Hamada deserts, which consist of plateau landforms • Regs, which consist of rock pavements • Ergs, which are formed by sand seas • Intermontane Basins
Nearly all desert surfaces are plains where eolian deflation—removal of fine-grained material by the wind—has exposed loose gravels consisting predominantly of pebbles but with occasional cobbles.
The remaining surfaces of arid lands are composed of exposed bedrock outcrops, desert soils, and fluvial deposits including alluvial fans, playas, desert lakes, and oases. Bedrock outcrops occur as small mountains surrounded by extensive erosional plains.
Several different types of dunes exist. Barchan dunes are produced by strong winds blowing across a level surface and are crescent-shaped. Longitudinal or seif dunes are dunes that are parallel to a strong wind that blows in one general direction. Transverse dunes run at a right angle to the constant wind direction. Star dunes are star-shaped and have several ridges that spread out around a point.
Oases are vegetated areas moistened by springs, wells, or by irrigation. Many are artificial. Oases are often the only places in deserts that support crops and permanent habitation.

Urban ecosystem

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Bangkok
Urban ecosystems are the cities, towns, and urban strips constructed by humans.
This is the growth in the urban population and the supporting built infrastructure has affected both urban environments and also on areas which surround urban areas. These include semi or 'peri-urban' environments that fringe cities as well as agricultural and natural landscapes.
Scientists are now developing ways to measure and understand the effects of urbanisation on human and environmental health.
By considering urban areas as part of a broader ecological system, scientists can investigate how urban landscapes function and how they affect other landscapes with which they interact. In this context, urban environments are affected by their surrounding environment but also affect that environment. Knowing this may provide clues as to which alternative development options will lead to the best overall environmental outcome.
CSE's urban ecosystem research is focused on: • Understanding how cities work as ecological system • Developing sustainable approaches to development of city fringe areas that reduce negative impact on surrounding environments • Developing approaches to urban design that provide for health and opportunity for citizens.

Greater Yellowstone Ecosystem

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Bison grazing near Gibbon River at Madison in Yellowstone National Park
The Greater Yellowstone Ecosystem is one of the last remaining large, nearly intact ecosystems in the northern temperate zone of the Earth[1] and is partly located in Yellowstone National Park. Conflict over management has been controversial, and the area is a flagship site among conservation groups that promote ecosystem management. The Greater Yellow Ecosystem (GYE) is one of the world's foremost natural laboratories in landscape ecology and geology and is a world-renowned recreational site. It is also home to the animals of Yellowstone.

History

Yellowstone National Park boundaries were arbitrarily drawn in 1872 in hopes of including all regional geothermal basins in the area. No other landscape considerations were incorporated. By the 1970s, however, the grizzly bear's (Ursus arctos) range in and near the park became the first informal minimum boundary of a theoretical Greater Yellowstone Ecosystem that included at least 16,000 square kilometres (4,000,000 acres). Since then, definitions of the greater ecosystem's size have steadily grown larger. A 1994 study listed the size as 76,890 square kilometres (19,000,000 acres), while a 1994 speech by a Greater Yellowstone Coalition leader enlarged that to 80,000 square kilometres (20,000,000 acres).
In 1985 the United States House of Representatives Subcommittees on Public Lands and National Parks and Recreation held a joint subcommittee hearing on Greater Yellowstone, resulting in a 1986 report by the Congressional Research Service outlining shortcomings in interagency coordination and concluding that the area's essential values were at risk.
Other federally managed areas within the GYE include Gallatin, Custer, Caribou-Targhee, Bridger-Teton and Shoshone National Forests, the John D. Rockefeller, Jr. Memorial Parkway, as well as the National Elk Refuge and Grand Teton National Park. The GYE also encompasses some privately held lands surrounding those managed by the U.S. Government. Outside of Yellowstone National Park, ten distinct wilderness areas have been established in the National Forests since 1966 to ensure a higher level of habitat protection than is normally mandated.