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Energy and Food

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A Florida Panther
 
A "food chain" can be defined as a simple, linear relationship of food sources in a given environment. But in most situations, biologists see examples in nature where the interchange of food sources do not follow such a simple arrangement. Instead, an organism may be observed to hunt, kill and consume smaller animals, feed on a larger animal in a group the next day, or eat plants (such as fruits or leaves) many days later. In other words, many animals eat different kinds of animals and plants, not just one exclusive source. In this way, an organism is not limited to one food source and can acquire the energy and nutrients it needs from multiple sources. Evolutionary theory predicts that if a species simply consumes one food source, and that food source disappears, the reliant species either has to adapt to eating a new source, or become extinct. To avoid this situation, many animals (and humans) consume plants and animals from more than one trophic level. As a result, a "food web" can be set up, where energy is transferred between trophic levels.
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Phytoplankton
 
The energy (from food) an organism consumes is mostly burned for the purposes of movement and keeping warm. In terms of the trophic levels, this is energy "wasted" (heat radiated by an animal out to the surrounding environment) and not passed on to the next level as consumable food. For instance, imagine that the sun's rays (the ultimate source of energy in the "food chain") striking a portion of the ocean on the Earth's surface is equal to one million energy units. Much of that energy is absorbed by the ocean's water molecules, but the amount left over is intercepted by phytoplankton and turned into food by photosynthesis. At this first trophic level, the amount of energy the phytoplankton store as food is 20,000 units. At the next trophic level, the zooplankton consume the phytoplankton, but just 2,000 units of energy is passed on as food in the bodies of the tiny animals. Small fish (at the next trophic level) scoop up the little zooplankton, but their bodies only represent 10% of the previous level's energy units - therefore, the amount of energy left is now 200 units. By the time large fish at their trophic level eat the smaller fish, 20 energy units remain. Finally, at the point where humans (at the final trophic level) catch these fish, only 1 or 2 energy units are left. As this example shows, the approximate efficiency at which energy is transferred through the "food chain" from one level to the next highlights the great amount of energy lost in an ecosystem, and how a relatively small amount of energy is actually available to an organism in the form of food that it needs to survive and perpetuate.
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A Hypothetical Biomass Pyramid
 
Although the previous example shows the available energy from one trophic level to the next, it is also useful to think of the food producers and consumers as part of a "biomass pyramid," in which the comparative masses of consumers and food can be represented simultaneously. Instead of energy, the approximate masses of the animals and plants involved are "stacked up" in pyramidal fashion, with the primary producers (plants) at the bottom, and the last (generally largest) carnivore in the "food chain" at the top. The poor energy-transfer efficiency from one trophic level to the next manifests itself by the end of the chain, where a mere one-hundredth of a percent of the "base" of the pyramid remains. In this example, if sharks were able to feed on zooplankton, the trophic levels in between the two would be "skipped over," and energy transfer would be made more efficient in that ecosystem. In the case of humans, entire fields of grain are planted and harvested for the purpose of feeding cattle, and the cattle in turn are raised for the production of meat products. However, between the stage of the primary producer (grain) to the cattle (herbivore), 90% of the energy supplied by the grain is lost in the raising of the cows. From the cattle to the humans, the same efficiency also holds; just 10% of the energy stored in the beef is passed on to people. But if the grain was directly fed to people, the amount of energy that would have been used (and wasted) by cattle would be used by people, essentially "saving" a great deal of available energy and biomass. This concept is particularly important in international food organizations, as world population continues to increase, and the difficulty of properly feeding everyone becomes more crucial. By "skipping over" levels of the biomass pyramid, less energy is wasted, and more available energy - food - becomes accessible to humans.
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Schematic of the Carbon Cycle
 
Carbon is an unusual element. Not only is it very abundant on our planet, but it has one particular feature that makes it different from any other element; it has a strong affinity for itself. This property means that carbon atoms easily bond with other carbon atoms, and can form long, complex chains. As the "backbone" of literally hundreds of thousands of compounds, the ability of carbon to link together into chains is the basis for organic compounds - and the essential component for the building blocks of life. But carbon (like many other elements on Earth) moves through the environment in a cyclical fashion. This "carbon cycle" encorporates the actions of plants, animals, the ocean, atmosphere and geosphere. Shown here is a diagram of the carbon exchange between these systems; the numbers indicate the mass of carbon exchanged in gigatons (10 to the power of 15 tons) per year. Inorganic carbon dioxide gas in the atmosphere is readily taken up by land-based plants and the ocean (since carbon dioxide readily dissolves in water) where phytoplankton reside, and the action of photosynthesis converts this carbon into organic form. The organic carbon by primary producers such as land plants and phytoplankton, introduce and supply their respective food webs with carbon, until the animals at higher trophic levels exhale carbon dioxide gas, or expire. As living material dies and decays, microbes and bacteria consume animal tissues (as well as dead plants), and produce methane and carbon dioxide in the process. On land, this gas is both discharged into the atmosphere, or is retained within the soil. On the dark, cold, ocean bottom, the carbon dioxide produced by decomposition is not available for photosynthesis; instead, it combines with aqueous metal ions (such as calcium and magnesium) to form sediments, although most of it remains dissolved in deep waters as bicarbonate ions. Limestone, a sedimentary rock containing calcium carbonate, is a major "storage area" of Earth's surface carbon. As for all rocks, plate tectonics recycles the limestone at subduction zones, where the rock melts and mixes with the magma in the upper mantle. The carbon in rising magma (that reaches the Earth's land surface through volcanoes and vents) is emitted to the atmosphere as carbon monoxide and carbon dioxide. Another process of atmospheric carbon gain is the burning of fossil fuels by humans. Through metamorphic processes (such as heating and compressing), organic material can be converted into carbon-rich "fossil fuels", such as natural gas and oil. The combustion of these compounds with oxygen also produces carbon dioxide and carbon monoxide. Natural (and man-made) fires also release the same gases. This atmospheric carbon is reabsorbed by oceanic and terrestrial plants, and the carbon cycle begins again.
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Schematic of the Nitrogen Cycle
 
Although carbon is important in the formation of organic compounds - and is essential for life on Earth - the element nitrogen is also vital. Nitrogen forms simple chemicals called amino acids, the essential building blocks of all proteins, enzymes, and especially DNA. Although three-fourths of our atmosphere is in the form of diatomic nitrogen gas, the gas itself is very unreactive. Plants and animals simply cannot absorb the gas directly from the atmosphere. Instead, lightning strikes split the nitrogen molecule into free nitrogen, which immediately reacts with oxygen in the air to form nitrogen oxides. Some of these nitrogen oxide gases dissolve in rainwater and eventually percolate into the soil. Certain forms of bacteria that cling to roots within the soil convert (or "fix") this inorganic nitrogen into organic forms (ammonia and nitrate ions) that plants can absorb. The nutrients needed for plant growth are drawn from the soil from the roots to the leaves. Therefore, any organism (including humans) consuming the nuts, leaves, seeds, roots, tubercules, or fruits of plants can digest this organic form of nitrogen. Any organic waste or dead creatures are decomposed by bacteria that return nitrogen back to the atmosphere, or re-fix the nitrogen to the soil again. In agriculture, soils are generally not rich enough in fixed nitrogen to sustain repetitive crop yields year after year; as a result, farmers use compost heaps or add industrally, mass-produced fertilizers such as ammonium nitrate (containing high amounts of organic nitrogen), to enhance the soil.

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