————————————————- Food web From Wikipedia, the free encyclopedia A freshwater aquatic and terrestrial food web. A food web (or food cycle) depicts feeding connections (what eats what) in an ecological communityand hence is also referred to as a consumer-resource system. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs. To maintaintheir bodies, grow, develop, and to reproduce, autotrophs produce organic matter from inorganicsubstances, including both minerals and gases such as carbon dioxide.
These chemical reactionsrequire energy, which mainly comes from the sun and largely by photosynthesis, although a very small amount comes from hydrothermal vents and hot springs. A gradient exists between trophic levels running from complete autotrophs that obtain their sole source of carbon from the atmosphere, to mixotrophs(such as carnivorous plants) that are autotrophic organisms that partially obtain organic matter from sources other than the atmosphere, and complete heterotrophs that must feed to obtain organic matter.
The linkages in a food web illustrate the feeding pathways, such as where heterotrophs obtain organic matter by feeding on autotrophs and other heterotrophs. The food web is a simplified illustration of the various methods of feeding that links an ecosystem into a unified system of exchange. There are different kinds of feeding relations that can be roughly divided into herbivory, carnivory, scavenging andparasitism. Some of the organic matter eaten by heterotrophs, such as sugars, provides energy.
Autotrophs and heterotrophs come in all sizes, from microscopic to many tonnes – from cyanobacteria togiant redwoods, and from viruses and bdellovibrio to blue whales. Charles Elton pioneered the concept of food cycles, food chains, and food size in his classical 1927 book “Animal Ecology”; Elton’s ‘food cycle’ was replaced by ‘food web’ in a subsequent ecological text. Elton organized species into functional groups, which was the basis for Raymond Lindeman’s classic and landmark paper in 1942 on trophic dynamics.
Lindeman emphasized the important role of decomposer organisms in a trophic system of classification. The notion of a food web has a historical foothold in the writings of Charles Darwin and his terminology, including an “entangled bank”, “web of life”, “web of complex relations”, and in reference to the decomposition actions of earthworms he talked about “the continued movement of the particles of earth”. Even earlier, in 1768 John Bruckner described nature as “one continued web of life”. ————————————————-
Food webs are limited representations of real ecosystems as they necessarily aggregate many species into trophic species, which are functional groups of species that have the same predators and prey in a food web. Ecologists use these simplifications in quantitative (or mathematical) models of trophic orconsumer-resource systems dynamics. Using these models they can measure and test for generalized patterns in the structure of real food web networks. Ecologists have identified non-random properties in the topographic structure of food webs. Published examples that are used in meta analysis are of variable quality with omissions.
However, the number of empirical studies on community webs is on the rise and the mathematical treatment of food webs usingnetwork theory had identified patterns that are common to all. Scaling laws, for example, predict a relationship between the topology of food web predator-prey linkages and levels of species richness. Trophic levels Main article: Trophic level A trophic pyramid (a) and a simplified community food web (b) illustrating ecological relations among creatures that are typical of a northern Boreal terrestrial ecosystem. The trophic pyramid roughly represents the biomass (usually measured as total dry-weight) at each level.
Plants generally have the greatest biomass. Names of trophic categories are shown to the right of the pyramid. Some ecosystems, such as many wetlands, do not organize as a strict pyramid, because aquatic plants are not as productive as long-lived terrestrial plants such as trees. Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy.  Food webs have trophic levels and positions. Basal species, such as plants, form the first level and are the resource limited species that feed on no other living creature in the web.
Basal species can be autotrophs ordetritivores, including “decomposing organic material and its associated microorganisms which we defined as detritus, micro-inorganic material and associated microorganisms (MIP), and vascular plant material. “:94 Most autotrophs capture the sun’s energy in chlorophyll, but some autotrophs (the chemolithotrophs) obtain energy by the chemical oxidation of inorganic compounds and can grow in dark environments, such as the sulfur bacterium Thiobacillus, which lives in hot sulfur springs.
The top level has top (or apex) predators which no other species kills directly for its food resource needs. The intermediate levels are filled with omnivores that feed on more than one trophic level and cause energy to flow through a number of food pathways starting from a basal species.  ————————————————- In the simplest scheme, the first trophic level (level 1) is plants, then herbivores (level 2), and then carnivores (level 3). The trophic level is equal to one more than the chain length, which is the number of links connecting to the base.
The base of the food chain (primary producers or detritivores) is set at zero.  Ecologists identify feeding relations and organize species into trophic species through extensive gut content analysis of different species. The technique has been improved through the use of stable isotopes to better trace energy flow through the web.  It was once thought that omnivory was rare, but recent evidence suggests otherwise. This realization has made trophic classifications more complex.  Energy flow and biomass Main article: Energy flow (ecology) See also: Ecological efficiency
The Law of Conservation of Mass dates from Antoine Lavoisier’s 1789 discovery that mass is neither created nor destroyed in chemical reactions. In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. :11 Left: Energy flow diagram of a frog. The frog represents a node in an extended food web. The energy ingested is utilized for metabolic processes and transformed into biomass. The energy flow continues on its path if the frog is ingested by predators, parasites, or as a decaying carcass in soil.
This energy flow diagram illustrates how energy is lost as it fuels the metabolic process that transform the energy and nutrients into biomass. Right: An expanded three link energy food chain (1. plants, 2. herbivores, 3. carnivores) illustrating the relationship between food flow diagrams and energy transformity. The transformity of energy becomes degraded, dispersed, and diminished from higher quality to lesser quantity as the energy within a food chain flows from one trophic species into another. Abbreviations: I=input, A=assimilation, R=respiration, NU=not utilized, P=production, B=biomass. 25] Food webs depict energy flow via trophic linkages. Energy flow is directional, which contrasts against the cyclic flows of material through the food web systems.  Energy flow “typically includes production, consumption, assimilation, non-assimilation losses (feces), and respiration (maintenance costs). “:5 In a very general sense, energy flow (E) can be defined as the sum ofmetabolic production (P) and respiration (R), such that E=P+R. The mass (or biomass) of something is equal to its energy content. Mass and energy are closely intertwined.
However, concentration and quality of nutrients and energy is variable. Many plant fibers, for example, are indigestible to many herbivores leaving grazer community food webs more nutrient limited than detrital food webs where bacteria are able to access and release the nutrient and energy stores. ”Organisms usually extract energy in the form of carbohydrates, lipids, and proteins. These polymers have a dual role as supplies of energy as well as building blocks; the part that functions as energy supply results in the production of nutrients (and carbon dioxide, water, and heat).
Excretion of nutrients is, therefore, basic to metabolism. “:1230-1231 The units in energy flow webs are typically a measure mass or energy per m2 per unit time. Different consumers are going to have different metabolic assimilation efficiencies in their diets. Each trophic level transforms energy into biomass. Energy flow diagrams illustrate the rates and efficiency of transfer from one trophic level into another and up through the hierarchy.  ————————————————-
It is 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 waste. 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. 32] 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.  Food Web A food web is a graphical description of feeding relationships among species in an ecological community, that is, of who eats whom (Fig. 1). It is also a means of showing how energy and materials (e. g. , carbon) flow through a community of species as a result of these feeding relationships.
Typically, species are connected by lines or arrows called “links”, and the species are sometimes referred to as “nodes” in food web diagrams. Relationships between soil food web, plants, organic matter, and birds and mammals. “The herbivores are usually preyed upon by carnivores, which get the energy of the sunlight at third-hand, and these again may be preyed upon by other carnivores, and so on, until we reach an animal which has no enemies, and which forms, as it were, a terminus on this food cycle. There are, in fact, chains of animals linked together by food, and all dependent in the long run upon plants.
We refer to these as ‘food-chains’, and to all the food chains in a community as the ‘food-cycle. ‘” A food web differs from a food chain in that the latter shows only a portion of the food web involving a simple, linear series of species (e. g. , predator, herbivore, plant) connected by feeding links. A food web aims to depict a more complete picture of the feeding relationships, and can be considered a bundle of many interconnected food chains occurring within the community. All species occupying the same position within a food chain comprise a trophic level within the food web.
For instance, all of the plants in the foodweb comprise the first or “primary producer” tropic level, all herbivores comprise the second or “primary consumer” trophic level, and carnivores that eat herbivores comprise the third or “secondary consumer” trophic level. Additional levels, in which carnivores eat other carnivores, comprise a tertiary trophic level. Elton emphasized early on that food chains tend to show characteristic patterns of increasing body size as one moves up the food chain, for example from phytoplankton to invertebrate grazers to fishes, or from insects to rodents to larger carnivores like foxes.
Because individuals of small-bodied species require less energy and food than individuals of larger-bodied species, a given amount ofenergy can support a greater number of individuals of the smaller-bodied species. Hence, ecological communities typically show what Elton called a pyramid of numbers (later dubbed the Eltonian pyramid), in which the species at lower trophic levels in the food web tend to be more numerous than those at higher trophic levels.
A second reason for the pyramid of numbers is low ecological efficiency: some energy is lost at each transfer between consumer and prey, such that theenergy that reaches top predators is a very small fraction of that available in the plants at the base of the food web. Although there is wide variation among types of organisms and types of ecosystems, a general rule of thumb is that available energydecreases by about an order of magnitude at each step in the food chain.
That is, only about 10% of theenergy harvested by plants is consumed and converted into herbivore biomass, only 10% of that makes it into biomass of primary carnivores, and so on. Thus, the structure of food webs is dictated in part by basic constraints set by thermodynamics. The predictable dissipation of energy at each step in food chains is one of the factors thought to limit the length of most food chains to a maximum of four or five steps. Cohen et al. (2003) emphasized that the correlations mong body size, abundance, and trophic level produce a characteristic trivariate structure to (pelagic) food webs (Fig. 2). The pyramid of numbers is less obvious at the most basal levels in terrestrial communities based on trees, which are typically much larger than theherbivores that feed on them. Pyramids of numbers or biomass may even be inverted in cases where the microscopic plants that support the web show very rapid turnover, that is, where they grow and are eaten so rapidly that there is less plant biomass than herbivore biomass present at a given time. ————————————————-
Decomposers are an assemblage of small organisms, including invertebrates, fungi, and bacteria, that do not fit neatly into any trophic level because they consume dead biomass of organisms from all trophic levels. Decomposers are a critical component of the food web, however, because they recycle nutrients that otherwise would become sequestered in accumulating detritus. All food chains in a community constitute a food web. A food web is simply the total set of feeding relationship amongst and between the species composing a biotic community. These relationships may achieve considerable complexity.
With many food chains and cross connecting links, there is greater opportunity for the prey and predator population in an ecosystem to adjust to the changes. ————————————————- The producer-consumer arrangement is one kind of structure known as trophic structure(trophic = food) and each food (nutritional) level in the food chain is called trophic level or energy level. The first trophic level in an ecosystem is occupied by the plants-producers (green plant-primary producers), because they utilize solar energy which is transformed to chemical form during photosynthesis.
The energy stored in food or green plants is consumed by the plant eaters (herbivores) which make the second trophic level. Herbivores are also called primaryconsumers. Primary consumers in turn are eaten by carnivores (also known as secondary consumers) which occupy third trophic level. Secondary consumers (Primary carnivores) may be eaten by other carnivores (secondary or top carnivores) which are known as tertiary consumers and occupy fourth trophic level. Decomposer occupy fifth trophic level in an ecosystem.
Food Web- In nature, food chain relationships are very complex. They never operate as isolated sequences, as one organism may form the food source of many organisms and so on. Thus, instead of a food chain, a number of food chains are interconnected with each other and form a web-like structure known as ‘food web’. For example, grass may be grazed by grasshoppers as well as cattle, rabbits and each of these may be eaten by different type of carnivores, such as birds, toads, snakes, foxes, depending on their food habit.
Thus, a particular organism may not occupy the same tropic level in every food chain; it may simultaneously behave as secondary, tertiary or a top consumer. Organisms, whose food is obtained from plants by the same number of steps are said to belong to the same tropic level. Thus, green plants occupy the first tropic level or the producer level. The plant grazers occupy the second tropic level or primary consumer or herbivore level (all plant-grazing insects, cattle, deer, rabbits, etc. ).
Flesh-eaters, that eat herbivores, form the third tropic level or the secondary consumer or carnivore level-1 (frogs, small fish, etc. ). The third tropic level is the tertiary consumer or carnivore level-2, which eats the flesh of herbivores and secondary consumers. In a similar fashion, tropic levels can be expanded based on the food habits of organisms. Charles Elton, a British ecologist, however, concluded that the number of links in a food chain rarely exceeds five, because in the process of energy transfer there is always the loss of energy to the environment.
It is the energy transfer mechanism which determines the number of links in a food chain. Man and many other animals who are omnivores occupy different tropic levels in food chains in relation to pure carnivores. The food web maintains the stability of the eco-system. For example, green land can be grazed by different organisms like insects, rabbits, rodents, etc. The insects then can be eaten by frogs which can be eaten by snakes. Snakes can either be eaten by hawks