The Balance of Nature: Food Chains & Webs

Food Chains and Food Webs
In the living world, every form of life is food for another. Food chains and webs show how food and energy are passed between species. 

Food Chain
A food chain is a food pathway that links different species in a community. In a food chain, energy and nutrients are passed from one organism to another. Food Chains rarely contain more than six species because amount of energy passed on diminishes at each stage, or trophic level. The longest chains usually involve aquatic animals. 

In a food chain, an animal passes on only about 10 percent of the energy it receives. The rest is used up in maintaining it's body, or in movement, or it escapes as heat. The amount of available energy decreases at every trophic level, and each level supports fewer individuals than the one before. This results in a pyramid of numbers with many organisms at the bottom and few at the top. 

Food Web
A community of living things may contain hundreds or even thousands of different species. Each species is usually involved in several different food chains. Therefore different food chains often interconnect to form a large network, called a food web. Even in a small ecosystem, such as a pond, food webs can be extremely complicated. 

Trophic Level
In a food chain, each species occupies a certain position in the chain. This position is called a trophic level. For example, owls eat mice, so if a food chain contains an owl and a mouse, the owls will be at a higher level. The number of trophic levels is the same as the number of species in the food chain. The same species may occupy different trophic levels in different food chains. 

A producer is an autotroph, which means that it can make its own food. Producers form the first trophic level of a food chain, because they make the food that supports the other species in the chain. Green plants, and some kinds of bacteria, are the most important producers. They harness the Sun's energy to make food by photosynthesis. A few species of bacteria make food by chemosynthesis, which uses the energy in chemicals. 

Consumers are hetrotrophs, or living things that cannot make food for themselves. They survive by taking in good that has been made by other living things. A food chain contains several kinds of consumers, each of which occupies a different trophic level.

* Primary consumers eat producers
* Secondary consumers eat primary consumers
* Terciary consumers eat secondary consumers
* and so on........... 

Decomposers, or saprotrophs, are a vital part of food webs. During the process of decay, they break down the organic compounds in dead remains and release their raw materials, such as carbon dioxide, back into the environment. Bacteria and fungi are the most important decomposers. 

Predator / Prey
A predator is carnivorous. This means that it lives by eating other animals, which are known as it's prey. The term predator usually refers to animals that catch and kill. Most predators are larger that their prey; they have special adaptations to help them find and catch their food. These include good vision, a keen sense of smell, or strong legs for rapid movement. Prey also have special adaptations to help them survive the attack of their predators. Camouflage helps them blend in with their environment and hide, sharp senses warn them of attack, and speed allows them to escape.

In the absence of predators, a species quickly increases in numbers. Eventually, a shortage of resources forces the population to level out. If a predator is introduced, the numbers of it's prey fall. Both populations soon decrease, but in nature, the prey rarely dies out altogether. 

from Pennsylvania State University
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