Food chains are representative of the eating relationships between species within an ecosystem or a particular living place. Many types of food chains or webs are applicable depending on habitat or environmental factors. Every known food chain begins with a type of autotroph, an organism that is able to manufacture its own food, whether it be a plant or some kind of unicellular organism.
Organisms represented in food chains
In almost all food chains, light energy from the sun is utilized by autotrophs, i.e., producers, such as plants in a process called photosynthesis combining water and carbon dioxide to produce glucose, C 6 H 12 O 6 . Carbon dioxide and water are both low in energy, but glucose, capable of storing the sun’s energy, is high in energy, which can be expended for cellular processes, growth, and development. Photosynthesis is a process of transforming the light energy of the sun into the chemical energy of glucose. The glucose is used to make all of the plant’s carbohydrates, including other sugars, starch, and cellulose, fats, and proteins. Proteins can be made using nitrates, sulfates, and phosphates in the soil. When autotrophs are eaten by heterotrophs, i.e., consumers, such as animals the carbohydrates, fats, and proteins contained in them become energy sources for the heterotrophs.
Carbohydrates, fats, and proteins are used almost universally as energy sources by living organisms. An important exception is lithotrophy, the utilization of inorganic compounds, especially minerals such as sulfur or iron, for energy. In some lithotrophs, minerals are used simply to power processes for making organic compounds from inorganic carbon sources. In a few food chains, e.g., near hydrothermal vents in the deep sea, autotrophs are able to produce organic compounds without sunlight, through a process similar to photosynthesis called chemosynthesis, using a carbon source such as carbon dioxide and a chemical energy source such as hydrogen sulfide, H 2 S, or molecular hydrogen, H 2 . Unlike water, the hydrogen compounds used in chemosynthesis are high in energy. Other lithotrophs are able to directly utilize inorganic substances, e.g., iron, hydrogen sulfide, elemental sulfur, or thiosulfate, for some or all of their energy needs.
Carbon dioxide recycles in the carbon cycle when carbohydrates, fats, and proteins are oxidized to release carbon dioxide and water. Oxygen released by photosynthesis is utilized in respiration to produce energy. Dead organisms are consumed by detritivores, scavengers, and decomposers, and their nutrients are recycled.
Food web
Food chains are overly simplistic as representatives of the relationships of living organisms in nature."Energy flows from one consumer to the other", showing only one pathway of energy and material transfer. Most consumers can feed on multiples species and, in turn, can be fed upon by multiple other species. For a snake, the prey might be a mouse, a lizard, or a frog, and the predator might be a bird of prey or a badger. The relations of detritivores and parasites are seldom adequately characterized in such chains as well.
A food web is a series of related food chains displaying the movement of energy and matter through an ecosystem. The food web is divided into two broad categories: the grazing web, beginning with autotrophs, and the detrital web, beginning with organic debris. There are many food chains contained in these food webs. In a grazing web, energy and nutrients move from plants to the herbivores consuming them to the carnivores or omnivores preying upon the herbivores. In a detrital web, plant and animal matter is broken down by decomposers, e.g., bacteria and fungi, and moves to detritivores and then carnivores.
There are often relationships between the detrital web and the grazing web. Mushrooms produced by decomposers in the detrital web become a food source for deer, squirrels, and mice in the grazing web. Earthworms eaten by robins are detritivores consuming decaying leaves.
Flow of food chains
Food energy flows from one organism to the next and to the next and so on. Organisms in a food chain are grouped into trophic levels, based on how many links they are removed from the primary producers. In trophic levels there may be one species or a group of species with the same predators and prey. Autotrophs such as plants or phytoplankton are in the first trophic level; they are at the base of the food chain. Herbivores, i.e., primary consumers, are in the second trophic level. Carnivores, i.e., secondary consumers, are in the third. Omnivores are found in the second and third levels. Predators preying upon other predators are tertiary consumers or secondary carnivores, and they are found in the fourth trophic level. The less numerous organisms in the higher levels are generally larger and more ferocious, although parasites and pathogens are important exceptions. Beginning in the second level, decomposers can be herbivores or carnivores when their food is derived from plants or animals.
It is often the case that the biomass of each trophic level decreases from the base of the chain to the top. This is because energy is lost to the environment with each transfer as entropy increases. About eighty to ninety percent of the energy is expended for the organism’s life processes or is lost as heat or wastes. Only about ten to twenty percent of the organism’s energy is generally passed to the next organism. The amount can be less than one percent in animals consuming less digestible plants, and it can be as high as forty percent in zooplankton consuming phytoplankton. Graphic representations of the biomass or productivity at each tropic level are called ecological pyramids or trophic pyramids. The transfer of energy from primary producers to top consumers can also be characterized by energy flow diagrams.
Some producers, especially phytoplankton, are able to reproduce quickly enough to support a larger biomass of grazers. This is called an inverted pyramid, caused by a longer lifespan and slower growth rate in the consumers than in the organisms being consumed, with phytoplankton living just a few days, compared to several weeks for the zooplankton eating the phytoplankton and years for fish eating the zooplankton. A pyramid of energy, reflecting the energy or kilojoules in each level, is representative of the true relationships of the phytoplankton, zooplankton, and fish, showing phytoplankton as the largest section, then zooplankton as a smaller section, and fish as the smallest section.
In a pyramid of numbers, the number of consumers at each level decreases significantly, so that a single top consumer, e.g., a polar bear or a human, will be supported by a million separate producers, e.g., phytoplankton.
There is usually a maximum of four or five links in a food chain, although food chains in aquatic ecosystems are frequently longer than those on land. Eventually, all the energy in a food chain is lost as heat.
History of food webs
Food webs serve as a framework to help ecologists organize the complex network of interactions among species observed in nature. Perhaps the earliest graphical depiction of a food web was by Lorenzo Camerano in 1880, followed independently by those of Pierce and colleagues in 1912 and Victor Shelford in 1913. Two food webs about herring were produced by Victor Summerhayes and Charles Elton and Alister Hardy in 1923 and 1924. After Charles Elton's use of food webs in his 1927 synthesis, they became a central concept in the field of ecology. The utilization of the common currency of energy flow along links in a flow was emphasized in Raymond Lindeman’s work, initiating the extensive analysis of energy and material flows that are a core activity of ecosystem ecology.
Interest in food webs increased after Robert Paine's experimental and descriptive study of intertidal shores suggesting that food web complexity was key to maintaining species diversity and ecological stability. Many theoretical ecologists, including Sir Robert May and Stuart Pimm, were prompted by this discovery and others to examine the mathematical properties of food webs. According to their analyses, complex food webs should be highly unstable. The apparent paradox between the complexity of food webs observed in nature and the mathematical fragility of food web models is currently an area of intensive study and debate. The paradox may be due partially to conceptual differences between persistence of a food web and equilibrial stability of a food web. Current research points to important roles of non-random structure in the connections within the food web that develop as food webs assemble over long periods of time, of patterns in the strengths of interactions among species within the food web, of variable strengths of species interactions as species abundances change, and of spatial variation in the environment creating food webs of different structures that are connected by movement of individuals and materials, in the creation and persistence of complex food webs.
See also
- Balance of Nature
- Biodiversity
- Ecology
- Ecosystem
- Earth science
- Food systems
- Natural environment
- Nature
- List of feeding behaviours
- Antipredator adaptations
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