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Ecosystem

An ecosystem consists of a biological community and the abiotic factors on which it relies. These factors include sunlight, water, elements, and minerals. Energy flows one way through an ecosystem, starting as sunlight absorbed by primary producers, passing through several levels of consumers, and eventually dissipating as heat. Materials cycle through an ecosystem by alternating between biotic and abiotic stages.

The sun is the ultimate source of energy for most ecosystems. The distribution of solar energy around the world is dictated by the position of the sun and air and water movement. The variation in solar energy causes variation in temperature and rainfall in time and space, which in turn influences the type of ecosystem found in each place.

Ecosystems contain interconnected food chains known as food webs through which energy flows. Each food chain consists of a sequence of predator-prey relationships at different trophic levels. Each predator species can have more than one prey species and vice versa. Primary producers, which provide 99 percent of all organic material, are photosynthetic plants and algae. Primary consumers, or herbivores, eat primary producers; secondary consumers, or carnivores, eat herbivores. Tertiary consumers eat other carnivores. Most ecosystems contain no more than two carnivorous trophic levels, because only about 10 percent of the energy contained in the biomass at one level is passed on to the next.

Detrivores, or decomposers, form yet another trophic level by scavenging or decomposing dead organic material. Decomposers are capable of gaining energy from materials that no other animals can, such as cellulose or nitrogenous waste, and may consume up to 90 percent of primary production (energy produced by plants) in forests.

Trophic levels are characterized by their productivity. Gross productivity is the rate at which energy is assimilated by organisms. Gross productivity minus the amount of energy left over after the cost of metabolic activity is net productivity. Net productivity can be measured as accumulated bio-mass, which is the total dry weight of organic materials, and is the energy that is available to organisms at the next trophic level. The difference between gross and net productivity limits the number of trophic levels in an ecosystem. At some point, there is not enough residual energy to support a healthy population of predators.

The loss of energy at each trophic level would seem to dictate a pyramidal structure in which each trophic level contains less biomass than the one beneath it. However, productivity is a function of both biomass and reproduction rate. For example, a phytoplankton population often reproduces fast enough to support a larger population of zooplankton.

Whole ecosystems can also be measured for their productivity. Algal beds and reefs, due to their rapid reproduction rates, are the most productive ecosystems on Earth. Temperate forests, however, contain the most bio-mass. Swamps and marshes rank as high as tropical rain forests in productivity, whereas the desert and the open ocean rank the lowest. Cultivated land has only average productivity.

Materials flow through ecosystems in biogeochemical cycles. These cycles include the atmosphere, the lithosphere (Earth's crust), and the hydrosphere (bodies of water). Decomposers play an important role in material cycling by separating inorganic materials, such as nitrogen, from organic compounds. A generalized biogeochemical cycle consists of available and unavailable organic components and available and unavailable inorganic components.

Inorganic materials become organic through assimilation and photo-synthesis. Organic materials become inorganic due to respiration, decomposition, and excretion. Sedimentation causes inorganic material to become unavailable, whereas erosion releases it. Fossilization stores organic material as fossil fuel, whereas erosion and combustion release fossil fuels as inorganic material.

The water cycle plays a significant role in terrestrial ecosystems because it is the major component, by weight, of all organisms. Water evaporates from oceans, rivers, and lakes and transpires from plants into the atmosphere. Precipitation occurs over land when the atmospheric water condenses, followed by runoff and percolation through the soil into ground water. Eventually, the water returns to the atmosphere by evaporation or transpiration, but in the meantime it can be assimilated by organisms.

Nitrogen also plays a vital role in ecosystems because it is necessary for the synthesis of both amino and nucleic acids, which makeup proteins and DNA. In order for plants to assimilate nitrogen, it must be in the inorganic form of nitrate (NO₃). Bacteria convert ammonia (NH₃) or ammonium (NH₄) into nitrate by nitrification, which requires the addition of oxygen.

	Available	Unavailable
Organic	Organisms	Coal, petroleum oil, natural gas
	Detritus
Inorganic	Atmosphere
	Soil	Minerals in rocks
	Water

Plants convert nitrates into ammonium in order to synthesize organic compounds. Decomposers complete the cycle by ammonification, the separation of inorganic nitrogen from dead organic material. Nitrogen is lost from this cycle by denitrification, in which bacteria break down nitrates into oxygen and nitrogen in poorly aerated soils. Nitrogen is added to the cycle by nitrogen-fixing bacteria, which incorporate atmospheric nitrogen into organic compounds.

The fundamental element in organic molecules is carbon. Plants assimilate carbon from the atmosphere in the form of carbon dioxide, which is broken down during photosynthesis to produce oxygen and carbohydrate. Respiration in plants and animals reverses this process by using carbohydrate to fuel the conversion of oxygen into carbon dioxide. Thus, carbon may be thought of as cycling between gaseous and organic states.

Phosphorus is necessary for the synthesis of ATP (adenosine triphosphate) and nitrogen-containing molecules called nucleotides. Phosphorus is separated from organic compounds by decomposers or excreted by animals as phosphates. Plants and algae then assimilate phosphates from the soil and water to produce organic compounds.

Humans affect the function of ecosystems in many ways. One effect is the increase in atmospheric carbon dioxide from the burning of fossil fuels, which was negligible until industrialization. Another is the diversion of water from rivers and ground water into reservoirs. Industrial fertilization has increased the level of phosphates in many waterways, causing blooms of phytoplankton that choke the oxygen out of the water. The long-term effects of human influence on ecosystems remain to be determined.