Ecosystem

Left: Coral reef ecosystems are highly productive marine systems.^[1] Right: Temperate rainforest, a terrestrial ecosystem.

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An ecosystem (or ecological system) is a system that

Ecosystems are controlled by external and internal factors. External factors such as climate, parent material which forms the soil and topography, control the overall structure of an ecosystem but are not themselves influenced by the ecosystem. Internal factors are controlled, for example, by decomposition, root competition, shading, disturbance, succession, and the types of species present. While the resource inputs are generally controlled by external processes, the availability of these resources within the ecosystem is controlled by internal factors. Therefore, internal factors not only control ecosystem processes but are also controlled by them.

Ecosystems are

Plants allow energy to enter the system through

Ecosystems provide a variety of goods and services upon which people depend, and may be part of. Ecosystem goods include the "tangible, material products" of ecosystem processes such as water, food, fuel, construction material, and

An ecosystem (or ecological system) consists of all the organisms and the abiotic pools (or physical environment) with which they interact.[3]^{[4]: 5 [2]: 458} The biotic and abiotic components are linked together through nutrient cycles and energy flows.^[5]

"Ecosystem processes" are the transfers of energy and materials from one pool to another.^[2]: 458 Ecosystem processes are known to "take place at a wide range of scales". Therefore, the correct scale of study depends on the question asked.^[4]: 5

The term "ecosystem" was first used in 1935 in a publication by British ecologist Arthur Tansley. The term was coined by Arthur Roy Clapham, who came up with the word at Tansley's request.^[6] Tansley devised the concept to draw attention to the importance of transfers of materials between organisms and their environment.^[4]: 9 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".^[3] Tansley regarded ecosystems not simply as natural units, but as "mental isolates".^[3] Tansley later defined the spatial extent of ecosystems using the term "ecotope".^[7]

Ecosystems are controlled by both 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. On broad geographic scales, climate is the factor that "most strongly determines ecosystem processes and structure".^[4]: 14 Climate determines the biome in which the ecosystem is embedded. Rainfall patterns and seasonal temperatures influence photosynthesis and thereby determine the amount of energy available to the ecosystem.^[8]: 145

Parent material determines the nature of the soil in an ecosystem, 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. For example, ecosystems can be quite different if situated in a small depression on the landscape, versus one present on an adjacent steep hillside.^{[9]: 39 [10]: 66}

Other external factors that play an important role in ecosystem functioning include time and potential

Unlike external factors, internal factors in ecosystems not only control ecosystem processes but are also controlled by them.[4]^: 16 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.^[13] Other factors like disturbance, succession or the types of species present are also internal factors.

Primary production is the production of organic matter from inorganic carbon sources. This mainly occurs through photosynthesis. The energy incorporated through this process supports life on earth, while the carbon makes up much of the organic matter in living and dead biomass, soil carbon and fossil fuels. It also drives the carbon cycle, which influences global climate via the greenhouse effect.

Through the process of photosynthesis, plants capture energy from light and use it to combine

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]: 157 The carbon and energy incorporated into plant tissues (net primary production) is either consumed by animals while the plant is alive, or it remains uneaten when the plant tissue dies and becomes detritus. In terrestrial ecosystems, the vast majority of the net primary production ends up being broken down by decomposers. The remainder is consumed by animals while still alive and enters the plant-based trophic system. After plants and animals die, the organic matter contained in them enters the detritus-based trophic system.^[15]

Ecosystem respiration is the sum of respiration by all living organisms (plants, animals, and decomposers) in the ecosystem.^[16] Net ecosystem production is the difference between gross primary production (GPP) and ecosystem respiration.^[17] In the absence of disturbance, net ecosystem production is equivalent to the net carbon accumulation in the ecosystem.

Energy can also be released from an ecosystem through disturbances such as wildfire or transferred to other ecosystems (e.g., from a forest to a stream to a lake) by erosion.

In

The sequence of consumption—from plant to herbivore, to carnivore—forms a food chain. Real systems are much more complex than this—organisms will generally feed on more than one form of food, and may feed at more than one trophic level. Carnivores may capture some prey that is part of a plant-based trophic system and others that are part of a detritus-based trophic system (a bird that feeds both on herbivorous grasshoppers and earthworms, which consume detritus). Real systems, with all these complexities, form food webs rather than food chains which present a number of common, non random properties in the topology of their network.^[18]

The carbon and nutrients in dead organic matter are broken down by a group of processes known as decomposition. This releases nutrients that can then be re-used for plant and microbial production and returns carbon dioxide to the atmosphere (or water) where it can be used for photosynthesis. In the absence of decomposition, the dead organic matter would accumulate in an ecosystem, and nutrients and atmospheric carbon dioxide would be depleted.^[19]: 183

Decomposition processes can be separated into three categories—leaching, fragmentation and chemical alteration of dead material. As water moves through dead organic matter, it dissolves and carries with it the water-soluble components. These are then taken up by organisms in the soil, react with mineral soil, or are transported beyond the confines of the ecosystem (and are considered lost to it).^{[20]: 271–280} Newly shed leaves and newly dead animals have high concentrations of water-soluble components and include sugars, amino acids and mineral nutrients. Leaching is more important in wet environments and less important in dry ones.^{[10]: 69–77}

Fragmentation processes break organic material into smaller pieces, exposing new surfaces for colonization by microbes. Freshly shed

The chemical alteration of the dead organic matter is primarily achieved through bacterial and fungal action. Fungal hyphae produce enzymes that can break through the tough outer structures surrounding dead plant material. They also produce enzymes that break down lignin, which allows them access to both cell contents and the nitrogen in the lignin. Fungi can transfer carbon and nitrogen through their hyphal networks and thus, unlike bacteria, are not dependent solely on locally available resources.^[19]: 186

Decomposition rates vary among ecosystems.^[21] The rate of decomposition is governed by three sets of factors—the physical environment (temperature, moisture, and soil properties), the quantity and quality of the dead material available to decomposers, and the nature of the microbial community itself.^[19]: 194 Temperature controls the rate of microbial respiration; the higher the temperature, the faster the microbial decomposition occurs. Temperature also affects soil moisture, which affects decomposition. Freeze-thaw cycles also affect decomposition—freezing temperatures kill soil microorganisms, which allows leaching to play a more important role in moving nutrients around. This can be especially important as the soil thaws in the spring, creating a pulse of nutrients that become available.^[20]: 280

Decomposition rates are low under very wet or very dry conditions. Decomposition rates are highest in wet, moist conditions with adequate levels of oxygen. Wet soils tend to become deficient in oxygen (this is especially true in wetlands), which slows microbial growth. In dry soils, decomposition slows as well, but bacteria continue to grow (albeit at a slower rate) even after soils become too dry to support plant growth.^[19]: 200

Ecosystems are dynamic entities. They are subject to periodic disturbances and are always in the process of recovering from past disturbances.

Disturbance also plays an important role in ecological processes. F. Stuart Chapin and coauthors define disturbance as "a relatively discrete event in time that removes plant biomass".^[22]: 346 This can range from herbivore outbreaks, treefalls, fires, hurricanes, floods, glacial advances, to volcanic eruptions. Such disturbances can cause large changes in plant, animal and microbe populations, as well as soil organic matter content. Disturbance is followed by succession, a "directional change in ecosystem structure and functioning resulting from biotically driven changes in resource supply."^[2]: 470

The frequency and severity of disturbance determine the way it affects ecosystem function. A major disturbance like a volcanic eruption or glacial advance and retreat leave behind soils that lack plants, animals or organic matter. Ecosystems that experience such disturbances undergo primary succession. A less severe disturbance like forest fires, hurricanes or cultivation result in secondary succession and a faster recovery.^[22]: 348 More severe and more frequent disturbance result in longer recovery times.

From one year to another, ecosystems experience variation in their biotic and abiotic environments. A drought, a colder than usual winter, and a pest outbreak all are short-term variability in environmental conditions. Animal populations vary from year to year, building up during resource-rich periods and crashing as they overshoot their food supply. Longer-term changes also shape ecosystem processes. For example, the forests of eastern North America still show legacies of cultivation which ceased in 1850 when large areas were reverted to forests.^[22]: 340 Another example is the methane production in eastern Siberian lakes that is controlled by organic matter which accumulated during the Pleistocene.^[26]