Реферат: Food Chain Essay Research Paper A food
Food Chain Essay, Research Paper
A food chain is an idea developed by a scientist named Charles Elton in 1927. He described the way plants get energy from sunlight, plant-eating animals get their energy from eating plants, and meat-eating animals get their energy from eating other animals. The idea of a chain means that all these animals are linked together, so anything that affects one link in the chain affects everything in the chain.
]The key concept that we need to learn from food chains, food webs and food pyramids is the transfer of energy. For all life on earth there is only one available source of energy, the sun. Furthermore, only plants can change this energy into a form that can be used by animals. All plants rely either directly or indirectly on plants for their energy. Plants feed upon sunlight. Only plants have the ability to convert sunlight into food that they and animals can use.
Feeding levels are also called trophic levels. The word “trophic” is a Greek word for nourishment. To determine the trophic level, we count the number of energy transfers. Green plants are at the first trophic, or nourishment, level because there has been one transfer of energy, from the sun to the plants.
The first link in the chain, the plant, is called the producer, while all the links above it are called consumers. For example, look at a simple chain in which grass uses sunlight to produce sugars and proteins so it can grow. Rabbits eat the grass, and get energy from it. Foxes eat rabbits and get energy from them. Nothing eats foxes, so they are said to be at the top of that particular food chain. If something happens to the grass, suppose a farmer plows up some of the grass to plant a field. Then the rabbits have less food and some of them will die. Then because there are fewer rabbits, some of the foxes will die, too, even though they don’t eat the grass directly.
Of course, in the real world, there are no simple food chains like this. Rabbits eat many things besides grass, and foxes eat many things besides rabbits and other things also eat grass and rabbits. When talking about the real world, it is more common to think of food webs.
Food chains are still an important concept to understand. In the 1960s, a pesticide called DDT became popular for its effectiveness in killing insect pests. It was sprayed in small concentrations, so as not to affect larger animals, but it never went away once it was sprayed. Eventually, rain washed it into rivers and lakes, where the concentration was still very small, but this is where the food chain took effect. DDT was taken up by plankton in the water, and accumulated in them. They were eaten by small fish, so each small fish accumulated all the DDT from the plankton it ate. Then the small fish were eaten by larger fish, and the larger fish were eaten by birds such as ospreys. Each large fish the osprey ate contained all the DDT from everything below it in the food chain, and that magnified the concentration of the DDT in the water by about 10 million times. Ospreys that were affected by it laid eggs with unusually thin and soft shells, and often their chicks didn’t hatch properly. When ecologists noticed this and other similar problems, they knew that DDT was causing damage to the environment, and as a result, it is now banned.
An ecosystem is a living community which depends on each member and its surrounding environment Every participant in an ecosystem has an important part to play and if one becomes more dominant than the others, the ecosystem can develop problems.
The grazing food chain shows the general flow of energy in communities. For most ecosystems the model begins with the photosynthetic fixation of light, carbon dioxide, and water by plant autotrophs or primary producers who make sugars and other organic molecules. Once produced, these compounds can be used to create the various types of plant tissues. Primary consumers or herbivores form the second link in the grazing food chain. They gain their energy by consuming primary producers. Secondary consumers or primary carnivores, the third link in the chain, gain their energy by consuming herbivores. Tertiary consumers or secondary carnivores are animals that receive their organic energy by consuming primary carnivores.
The various levels in the grazing food chain are linked to each other like links in a chain. The levels suggest a particular order for the passage of energy through the food chain. Like many very simple models, the idea of a food chain only provides a simple abstraction of the nature of energy flow through communities. The ultimate disposition of the energy assimilated by consumers is by four routes: respiration, biomass accumulation, decay of organic matter by bacteria and other decomposer organisms, and consumption by consumers.
The actual amount of energy incorporated in the tissues of consumers at each tropic level is not determined by the gross amounts consumed. Rather it is the amount of organic energy converted into actual biomass. Consumers lose significant amounts of consumed energy due to assimilation inefficiencies, morphological and physiological maintenance, reproduction, and the process of finding and capturing food. The energy to perform the latter three processes is supplied by respiration.
Therefore the number of trophic levels that can be maintained in any ecosystem is infinite. The limit is reached when consumers can no longer consume enough energy to balance the energy lost in the assimilation process, morphological and physiological maintanence, growth, reproduction, and in finding the food. Normally, ecosystems have about four or five trophic levels.
The detritius food chain differs from the grazing food chain in several significant ways. First, the organisms making it up are in general physically smaller. Second, the functional roles of the different organisms do not fall as neatly into categories like the grazing food chain’s trophic levels. Finally, detritivores live in environments that are rich in scattered food particles such as the soil or a sea bed. As a result, decomposers are less mobile than herbivores or carnivores.
The organisms of the detritus food chain include members of many different species of animals and plants, such as algae, bacteria, slime molds, fungi, protozoa, insects, mites, crustaceans, centipedes, mollusks, worms, sea cucumbers, and even some vertebrates. These organisms consume organic wastes and the dead bodies of both plants and animals.
Earthworms are one of the most important soil decomposers. These organisms consume vast amounts of organic matter and mineral soil. As the organic matter passes through their digestive system, it is subjected to digestive enzymes and the grinding action of mineral soil particles. The amount of material consumed per day is often equal to their body weight. Decomposers tend to always be active, processing large amounts of organic matter and releasing a great deal of energy mostly as heat from metabolic activities. Decomposers in most ecosystems process large amounts of organic matter, converting it back into its inorganic nutrient form.
In mature forest and grassland soils, the decomposition process will establish an equilibrium over time where litterfall additions equal the amount of organic decomposition. The rate of decomposition in a detritus food chain is controlled by many factors. In most terrestrial ecosystems, temperature and soil oxygen and moisture content tend to be the primary variables controlling the activities of decomposers. In some ecosystems, oxygen may not be readily available. In these cases aerobic respiration cannot take place, and the breakdown of organic compounds and energy extraction must then proceed by ananaerobic means like fermentation. Organisms involved in fermentation include bacteria and yeast. Anaerobic decomposition releases much less energy from organic matter than does aerobic respiration. The end products of anaerobic decomposition include molecules such as carbon dioxide, water, and humus. They also include small-molecule alcohols, organic acids, ptomaines, amines, and other products, as well as gaseous substances such as methane.
Respiration is so much more efficient at releasing the energy contained in organic molecules therefore the activity of the detritus food chain is much higher in an aerobic environment, and the breakdown of materials more complete. Organic matter breakdown is substantially slower and less complete in anaerobic environments. It also results in the accumulation of undegraded organic matter in the form of peat, organic soils, and highly organic sediments.
In conclusion, there is no waste in a properly functioning natural food chain. Everything once living or alive will be consumed by decomposers at some time and returned to inorganic form.