All organisms, dead or alive, are potential food sources for other organisms. A caterpillar eats a leaf, a robin eats the caterpillar, a hawk eats the robin. Eventually, the tree and the hawk also die and are consumed by decomposers.
Organisms in an ecological community are related to each other through their dependence on other organisms for food. In a food chain a producer is eaten by a herbivore that is in turn eaten by a carnivore. Eventually, the carnivore dies and is eaten by a decomposer. For example, in a lake, phytoplankton are eaten by zooplankton and zooplankton are eaten by small fish. The small fish are eaten by large fish. The large fish eventually die and decompose. Nothing goes to waste. Food chains are channels for the oneway flow of solar energy captured by photosynthesis through the living components of ecosystems. Food chains are also pathways for the recycling of nutrients from producers, through herbivores, carnivores, omnivores, and decomposers, finally returning to the producers.
The perfectly linear relations represented by food chains are almost never found in natural ecosystems. Although all organisms have somewhat specialized diets, most can eat a variety of different foods. Thus, each trophic level appears as part of several different interconnected food chains. These food chains combine into highly complex food webs.
As with food chains, a food web's source of energy is the sun. The solar energy is harvested by producers such as green plants or algae. These producers are known as autotrophs or photosynthesizing autotrophs. Almost all other organisms obtain their energy, directly or indirectly, from the sun. The exceptions are the communities found around deep ocean thermal vents, which are supported by various bacteria that convert heat energy into stored chemical energy. These bacteria are known as chemotrophs or chemosynthetic autotrophs.
Autotrophs are always found at the first trophic level. In an ecosystem this trophic level may include monerans, protists, and several different phyla of plants. They can all be placed at the first trophic level because they all have the same source of energy, and the entire food web depends on the energy harvested by them. For example, in a grazing food web, a herbivore eats living plant tissue and is eaten in turn by an array of carnivores and omnivores. Herbivores and the carnivores that prey on them are known as heterotrophs. In contrast, a detrivore (also a heterotroph) harvests energy from dead organic material and provides energy for a separate food chain.
Each step in a food web or food chain involves a transfer of matter and energy (in the form of chemical bonds stored in food) from organism to organism. Thus food webs are energy webs because the relationships represented by connections in the web represent the flow of energy from a group of organisms at one trophic level to another group of organisms at a different level. Because energy is lost (as waste heat) at each step, food chains rarely involve more than four or five steps or trophic levels.
At each level the organisms waste much energy in the form of heat generated by normal activity. Only a fraction is stored as food or used for growth. Only about 10 percent of the food entering a link is available for the next organism in the chain. After about five links, there is insufficient energy to support a population of organisms (other than decomposers). For example, in the food chain starting with diatoms and ending with killer whales, only about 0.01 percent of the initial energy stored by the diatoms is delivered to the killer whales.
Energy flow through a food web depends greatly on the nature of the producers at the first trophic level. These are usually photosynthetic plants, phytoplankton, or algae. In forest ecosystems, trees are the largest and most abundant organism. They determine the physical structure of the ecosystem, and they can be eaten directly by small or even very large animals. However, much of the matter and energy harvested by the trees goes to build a supporting structure. These supporting structures are composed of cellulose and other wood fibers that are poor sources of energy (although they may be good sources of valuable minerals and other nutrients).
In contrast, grasses do not invest much energy in supporting structures, so more energy is available per kilogram of plant material present to the grazers that obtain energy from plants. Consequently, all of the aboveground parts of the grass plants are eaten by herbivores.
Energy spreads out through the food web, from the lowest trophic level to the highest. At the "top of the food chain," large carnivores harvest the remaining energy. However, all things eventually die, no matter where they are in the food web, and the dead organic matter accumulates in the soil, lake bottom, or forest floor. This detritus becomes the basis for a completely different ecosystem, the detritus food web.
Detritus feeders and decomposers harvest solar energy from the detritus by breaking down the organic material into simpler organic compounds and inorganic compounds. By this process, the matter is recycled and made available for reuse by plants. The detritus food web is vitally important to all ecosystems on Earth. Without it, dead organic matter would accumulate and bury everything.
Humans are omnivores. They can operate on several trophic levels, eating plants, insects, mammals, birds, fish, mollusks, and many other organisms. Humans can also shorten the food chain when resources are scarce. In areas of the world where the population may be straining resources, people commonly increase the total food supply by eliminating one or more steps in the food chain. For example, to obtain more energy humans can switch from eating herbivores that obtain their energy from cereal grains to eating the cereal grains themselves.
The food web does not tell us everything there is to know about the complex biological communities called ecosystems. Not all relationships are equally important in these dynamic, evolving communities. Food webs contain both strong and weak links. Weak links can often be broken with little impact on the community. On the other hand, some species have a disproportionately large effect on the community in which they occur. Called key-stone species, they help to maintain diversity by controlling populations of species that would otherwise come to dominate the community. Or they may provide critical resources for a wide range of species.
For example, in the intertidal communities of the Northwest Pacific coast of North America, the starfish Pisaster ochraceus feeds on the small mussel Mytilus californius. Experiments have shown that when the starfish is artificially removed, the population of mussels explodes, soon covering all available space. Other species are crowded out. The interaction between Pisaster and Mytilus helps to maintain the species diversity of these intertidal communities.
Research has shown that ecological communities with complex feeding relationships have greater long-term stability and are less affected by external stresses. This suggests an evolutionary basis for the diverse and complex ecological relationships found in many communities of organisms. However, humans often violate this sound ecological principle in order to increase agricultural productivity by creating artificial ecosystems that contain only one plant, such as corn. These systems are called monocultures. While greater agricultural productivity is possible with monoculture crops, they are very unstable ecosystems. Disease, drought, or a new insect pest can easily destroy an entire year's harvest.
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